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Version 1: Decembeer 2008
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WEEK 1- BASIC CONCEPTS OF GENETICS………………………………………..3
WEEK 2-CHROMOSOMES AND HEREDITY………………………………………..6
WEEK 3- GENETIC TERMS AND MENDEL’S EXPERIMENTS………………….11
WEEK 4 –MENDELIAN LAWS OF INHERITANCE………………………………..19
WEEK 5 - MONOHYBRID INHERITANCE IN FRUIT FLY……………………….22
WEEK 6 –DIHYBRID INHERITANCE………………………………………………..26
WEEK 7 –CONCEPT OF DOMINANCE………………………………………………30
WEEK 8 – DEVIATIONS BY LINKAGE, MULTPLE ALLELES,……………….33
AND LETHAL GENES IN MICE
WEEK 9 - SEX DETERMINATION……………………………………………………44
WEEK 10 –SEX LINKAGE……………………………………………………………..52
WEEK 11 – GENETIC VARIATION ………………………………………………..68
WEEK 12 –MUTATION………………………………………………………………..73
WEEK 13 – GENETIC SYNDROMES…………………………………………………82
WEEK 14- BIOTECHNOLOGY AND NUCLEIC ACIDS……………………………85
WEEK 15 –GENETIC MANIPULATION TECHNIQUES AND…………………….91
IMPORTANCE OF BIOTECHNOLOGY
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WEEK 1- BASIC CONCEPTS OF GENETICS
1.1-Definition&Importance of Genetics
All living organisms reproduce. Reproduction results in the formation of offspring of the same
kind. A pea plant produces only pea plants each time it reproduces. A rat produces only rats.
Humans produce only humans. However, the resulting offspring need not and most often do not
totally resemble the parent. Several characteristic differences may occur between individuals
belonging to the same species. The similarities and differences among the members of a species
are not coincidental. Both the similarities and differences have been received from their parents.
The mechanism of transmission of characters, resemblances as well as differences, from the
parental generation to the offspring, is called as heredity. The differences shown by individuals
within the same species and in the offspring are described as variations. The scientific study of
heredity, variations and the environmental factors responsible for these, is known as genetics.
Genetics, study of the function and behavior of genes. Genes are bits of biochemical instructions
found inside the cells of every organism from bacteria to humans. Offspring receive a mixture of
genetic information from both parents. This process contributes to the great variation of traits
that we see in nature, such as the color of a flower’s petals, the markings on a butterfly’s wings,
or such human behavioral traits as personality or musical talent. Geneticists seek to understand
how the information encoded in genes is used and controlled by cells and how it is transmitted
from one generation to the next. Geneticists also study how tiny variations in genes can disrupt
an organism’s development or cause disease. Increasingly, modern genetics involves genetic
engineering, a technique used by scientists to manipulate genes. Genetic engineering has
produced many advances in medicine and industry, but the potential for abuse of this technique
has also presented society with many ethical and legal controversies.
Genetic information is encoded and transmitted from generation to generation in
deoxyribonucleic acid (DNA). DNA is a coiled molecule organized into structures called
chromosomes within cells. Segments along the length of a DNA molecule form genes. Genes
direct the synthesis of proteins, the molecular laborers that carry out all life-supporting activities
in the cell. Although all humans share the same set of genes, individuals can inherit different
forms of a given gene, making each person genetically unique
• Genetics is central to the life of every individual: it influences our physical features,
susceptibility to numerous diseases,personality, and intelligence.
• Genetics plays important roles in agriculture, the pharmaceutical industry, and medicine. It is
central to the study of biology.
• Genetic variation is the foundation of evolution and is critical to understanding all life.
• The study of genetics can be divided into transmission genetics, molecular genetics, and
population genetics.
• The use of genetics by humans began with the domestication of plants and animals
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1.2
Genes
A gene is the fundamental unit of heredity- The precise way in which a gene is defined often
varies. At the simplest level, we can think of a gene as a unit of information that encodes a
genetic characteristic. We will enlarge this definition as we learn more about
what genes are and how they function.
Genes come in multiple forms called alleles- A gene that specifies a characteristic may exist in
several forms, called alleles. For example, a gene for coat color in cats may exist in alleles that
encode either black or orange fur.
Genes encode phenotypes- One of the most important concepts in genetics is the distinction
between traits and genes. Traits are not inherited directly. Rather, genes are inherited and, along
with environmental factors, determine the expression of traits. The genetic information that an
individual organism possesses is its genotype; the trait is its
phenotype. For example, the A blood type is a
phenotype; the genetic information that encodes the blood type A antigen is the genotype.
Genetic information is carried in DNA and RNA Genetic information is encoded in the
molecular structure of nucleic acids, which come in two types:
deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are polymers consisting
of repeating units called nucleotides; each nucleotide consists of a sugar, a phosphate, and a
nitrogenous base. The nitrogenous bases in DNA are of four types
(abbreviated A, C, G, and T), and the sequence of these bases encodes genetic information. Most
organisms carry their genetic information in DNA, but a few viruses carry it in RNA. The four
nitrogenous bases of RNA are abbreviated A, C, G,
and U.
Genes are located on chromosomes- The vehicles of genetic information within the cell are
chromosomes ( FIGURE 1), which consist of DNA and associated proteins. The cells of each species have
a characteristic number of chromosomes; for example, bacterial cells normally possess a single
chromosome; human cells possess 46; pigeon cells possess 80. Each chromosome carries a large number
of genes.
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FIG. 1
Genes are located on chromosomes.
Study Questions
1 Explain the term genetics
2. Give at least three examples of the role of genetics in society today.
3. Briefly explain why genetics is crucial to modern biology.
4. What role did genetics play in the development of the first domesticated plants and animals?
5. Briefly define the following terms: (a) gene; (b) chromosome
6. Outline the relations between genes, DNA, and chromosomes.
7. Explain the importance of chromosomes and genes in heredity.
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WEEK 2-CHROMOSOMES AND HEREDITY
2.1 Chromosomes
Chromosomes (chroma = colour, soma = body) are tiny thread-like structures found in the
nucleus of a cell. In a non-dividing cell they appear as a chromatin network while during cell
division they become condensed to form short and thick chromosomes. Chromosomes are unique
cell structures which are capable of replication. They store and transmit the coded information
which is responsible for all the life processes of an organism. Hence, chromosomes are
commonly described as carriers of heredity. The term "chromosome" was coined by Waldeyer in
1888 since these structures easily take up dye stains. Chromosomes store and transmit the coded
information which is responsible for all the life processes of an organism. Hence, chromosomes
are commonly described as carriers of heredity
2.1.1 Bacterial Chromosome
In bacteria, which being prokaryotic organisms, the entire hereditary material is packed into a
single, irregularly folded compact mass called nucleoid or genophore or bacterial chromosome. It
is short and simple consisting of a single DNA molecule. The DNA is in the form of a double
helix which forms a closed ring or circle with no free ends. It is permanently attached to a
mesosome, an infolding of the plasma membrane. The bacterial chromosome lacks a protein coat
and it is in direct contact with the cytoplasm, since a nuclear membrane is absent. A small
amount of protein, mainly in the form of an enzyme called RNA polymerase, may be found
associated with the bacterial chromosome.
2.1.2 Eukaryotic Chromosome
The eukaryotic cells show a varied number of chromosomes. It is presumed to be the result of
breaking up of a single long chromosome into several short units, in the course of evolution, to
accommodate the increase in the amount of genetic information. The eukaryotic chromosomes
occur inside the nucleus of a cell, separated from the cytoplasm by a distinct nuclear membrane.
2.2 Structure of Chromosomes
The most ideal stage to study the structure of chromosomes is the metaphase of mitosis. A
metaphase chromosome consist of two identical components called chromatids, which are held
together at a specific region called primary constriction. It is usually found in the centre and
hence it is commonly described as centromere. It shows a plate-like proteinaceous structure
called kinetochore where the microtubules of the spindle become attached during cell division.
The portions of a chromatid found on either
2.2.1 Chromosomes and Centromere
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Centromere is the primary constriction in the chromatid of a chromosome where the spindle
tubules become attached during cell division. A chromosome generally has only one centromere
and such a chromosome is described as monocentric. Sometimes two centromeres may occur in
the chromosome as in the case of maize plant. Such a chromosome is described as dicentric. In
such a chromosome each chromatid will have three arms. In the round worm Parascaris equorum
the chromosomes have more than two centromeres and are hence described as polycentric. Very
rarely, a centromere may be absent and such a chromosome may be described as acentric or
holocentric. In such chromosomes the entire surface of the chromatid functions as the
centromere.
2.2.2 Giant Chromosomes
There are chromosomes which are extremely large compared to normal chromosomes. Such
chromosomes, called giant chromosomes occur in some animal cells. In unisexual organisms, the
chromosomes can be distinguished into autosomes and allosomes. Autosomes or somatic
chromosomes carry genes which determine the somatic characteristics and do not have any
influence on determining the sex of the organism. Allosomes are sex chromosomes which carry
genes responsible for sexual characteristics and as such have a significant role in the
determination of sex. Generally, one specific pair of chromosomes in the diploid number is
identified as the allosomes
2.3 Structure of a eukaryotic chromosome
The chromosomes of eukaryotic cells are larger and more complex than those found in
prokaryotes, but each unreplicated chromosome nevertheless consists of a single molecule of
DNA. Although linear, the DNA molecules in eukaryotic chromosomes are highly folded and
condensed; if stretched out, some human chromosomes would be several centimeters long—
thousands of times longer than the span of a typical nucleus. To package such a tremendous
length of DNA into this small volume, each DNA molecule is coiled again and again and tightly
packed around histone proteins, forming the rod-shaped chromosomes. Most of the time the
chromosomes are thin and difficult to observe but, before cell division, they condense further
into thick, readily observed structures; it is at this stage that chromosomes
are usually studied
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( FIGURE 2)
FIG. 2 Eukaryotic chromosome structure
A functional chromosome has three essential elements: a centromere, a pair of telomeres, and
origins of replication. The centromere is the attachment point for spindle microtubules, which
are the filaments responsible for moving chromosomes during cell division. The centromere
appears as a constricted region that often stains less strongly than does the rest of the
chromosome. Before cell division, a protein complex called the kinetochore assembles on the
centromere, to which spindle microtubules later attach. Chromosomes without a centromere
cannot be drawn into the newly formed nuclei; these chromosomes are lost, often with
catastrophic consequences to the cell. On the basis of the location of the centromere,
chromosomes are classified into four types: metacentric, submetacentric, acrocentric, and
telocentric ( FIGURE 3). One of the two arms of a chromosome (the short arm of a
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submetacentric or acrocentric chromosome) is designated by the letter p and the other arm is
designated by q.
FIG. 3 Four major types of Eukaryotic chromosomes
Telomeres are the natural ends, the tips, of a linear chromosome (see Figure 2.7); they serve to
stabilize the chromosome ends. If a chromosome breaks, producing new ends, these ends have a
tendency to stick together, and the chromosome is degraded at the newly broken ends. Telomeres
provide chromosome stability. The results of research suggest that telomeres also participate in
limiting cell division and may play
important roles in aging and cancer.
Origins of replication are the sites where DNA synthesis begins; they are not easily observed by
microscopy. Their structure and function will be discussed in more detail in
catastrophic consequences to the cell.
2.4 Chromosomal Basis of Inheritance
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Heredity depends upon the protoplasmic continuity between the parent and the offspring. It is the
gametes that establish the continuity between parent and offspring and hence, the mechanism of
inheritance operates across this exceedingly slender protoplasmic bridge. The machinery of
inheritance lies mainly in the nucleus or more particularly the chromosomes of the cells which
form the gametes. The chromosomes in fact are the only entities that are always passed on in
equal quantities from parents to offspring, during sexual reproduction. The rediscovery of
Mendelism in 1900 and the subsequent developments in the field of genetics, very clearly
established the importance of chromosomes in the inheritance of characters. This led to the
establishment of the chromosomal theory of inheritance.
Study Questions:
1) Distinguish between the following pairs of terms: a) chromatids and chromosomes,
b) prokaryotic and eukaryotic
chromosomes.
2) With the aid of a suitable diagram, show the structure of a typical eukaryotic chromosome.
3)Explain the four major types of eukaryotic chromosomes based on centromere position.
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WEEK 3- GENETIC TERMS AND MENDEL’S EXPERIMENTS
3.1 Genetic terms
Phenotype- this is the physical expression of genes in an organism. It is denoted with
words such as Tall, Dwarf or Short, Smooth, Wrinkled, Long wing, Vestigial wing, etc.
Genotype- the genetic constitution of a cell or an organism, as distinguished from its
physical and behavioral characteristics, i.e., its phenotype. It is denoted with letters such
as TT or Tt for Tall, SS or Ss for Smooth, etc. The genotype of an organism may be
homozygous or heterozygous.
Allele or Allelomorphic pair- a pair of contrasting genes controlling a character. The pair could
be identical- T&T or different-T&t
Homozygous-individual with identical alleles(TT , tt, SS, ss) controlling same character or trait.
Such organisms always produce identical gametes during meiosis and are thus said to be pure
breeding or true breeding or breeds true.
Heterozygous- individual with non-identical or dissimilar alleles(Tt, Ss,Rr) controlling same
character or trait. Such organisms produce different or non-identical gametes during meiosis and
are thus said not to be pure breeding or true breeding or does not breed true.
Dominant trait or character -this is controlled by dominant genes that can express itself in all
generations in the presence of the contrasting gene. It suppresses the effect of the contrasting
gene. It is denoted with capital letters such as TT , SS, etc
Recessive trait or character -this is controlled by recessive genes that can not express itself in
all generations, but only in certain generations in the absence of the dominant gene. It is denoted
with small letters such as tt, ss .
Filial generations - these are the series of offspring produced from genetic crossings. It is
denoted with letter F and a subscript to show the particular generation, e g F1, F2 , F3 for first ,
second and third Filial generations respectively.
Hybrid - a product of crossing between two contrasting parents. For example, Tall x Short to
produce All Tall F1 offspring. That is TT x tt to give Tt.
Monohybrid cross- a cross between a pair of contrasting characters, e.g. Tall x Short, Smooth
pod x Wrinkled pod, etc. At F1 the dominant character is expressed in all the offspring while the
recessive character is masked .At F2 , the genotypic ratio of the offspring is 1: 2:1 while the
phenotypic ratio is 3:1 in complete dominance condition.
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Dihybrid cross- a cross between two pairs of contrasting characters, e.g. Tall &Smooth x Short
&Wrinkled. At F1 , the dominant character is expressed in all the offspring, while the recessive
character is masked. However , at F2, both the dominant and recessive characters are expressed
in the offspring in varying genotypic ratios and phenotypic ratio of 9:3:3:1.
3.2 Brief history of genetics
The biggest name in the entire history of genetics beyond any doubt is that of Gregor Johann
Mendel (1822-1884). It was Mendel, more than any other scientist, who synthesized the basic
principles of heredity into a body of knowledge that has formed the very core of modern
genetics. It must be emphasized here that Mendel was not the original pioneer in the field of
genetics. As with all other scientific achievements, many scientists before the period of Mendel
had laid the foundation. However, it was Mendel who combined the ideas put forth by other
scientists into a definite set of working principles that are acceptable even today. Like so many
other discoveries by famous scientists, Mendel's ideas on the mechanism of inheritance did not
gain any importance and his principle temporality died with him. Fortunately for science, three
scientists, Hugo De Vries, Tschermarck and Correns rediscovered the ideas, when they obtained
the same results in the experiments conducted by each one of them independently. The
rediscovery of Mendelism brought new emphasis to the field of heredity and the modern science
of genetics was born. The basic ideas and the conclusions drawn by scientists
after this rediscovery came to be known as Mendelian Genetics.
Gregor Johann Mendel
Mendel (1822-1884) was born in to the family of a poor peasant in Moravia, Austria. He
received his school education with utmost difficulty due to poverty in the family. In 1843 he
joined a church as a monk where in 1847 he became the abbot (head) of the monastery at Brunn,
Austria (now called Bruno in Czechoslovakia). In 1851 he went to University of Vienna to study
natural history and mathematics, for two years. After his return he worked as a teacher in natural
history and mathematics between 1856 and 1865. It was during this period that Mendel
developed curiosity over the pattern of inheritance of characters from parent organisms to
offspring. After careful thought, he designed breeding experiments in the pea plants. He carefully
analyzed the results, gave a mathematical interpretation and published them in 1866. However,
unfortunately for Mendel, his results and conclusions could not convince the contemporary
biologists. Mendel died in 1884 without knowing that he had laid the foundation for modern
genetics. It was only in 1900, that Mendel's work was rediscovered and its significance was
made known to the scientific world. Nevertheless, Mendel is now regarded as the father of
modern genetics' for his significant and pioneering contributions to the field of genetics
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3.3 Mendel's Experiments
Mendel was primarily a monk in a monastery. In addition to his normal duties of preaching
in the church, Mendel evinced a keen interest in the maintenance of the garden in the premises of
the church. In the course of his routine rounds in the garden, Mendel was keenly observing the
pattern of inheritance of certain characters in some of the plants. He became interested in
investigating the mechanism by which the characters are transferred from the parent plants to
their offspring. He decided to conduct some experiments in this direction. After careful
examination and thinking, Mendel selected the pea plants (Pisum sativum) for his experiments.
In the pea plants, Mendel found certain clear advantages such as:
- Under natural conditions the pea plants exhibited only self-pollination. This
is because the flowers exhibit a condition called cliestogamy (petals remain
closed)
- Every pea plant produced a large number of seeds
- The duration of life cycle in the pea plants was very short
- It was possible to conduct cross pollination by transferring pollen
.
grains from one plant to another
The following table represents the contrasting characters, which Mendel was able to identify in
the pea plants.
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FIG. 4 Monohybrid Traits of Mendel
During his observation on the pattern of inheritance in the pea plants, Mendel had identified
three principles, which are today known as Mendelian laws or laws of inheritance. It was
because of his significant contribution in the field of heredity, Mendel is commonly described as
"Father of Modern Genetics
3.3.1 Monohybrid Inheritance
In the initial set of experiments, Mendel concentrated only on the pattern of inheritance of
a single pair of contrasting characters. This pattern of inheritance involving only one pair of
contrasting characters is known as monohybrid inheritance.
In the first set of experiments, Mendel conducted cross-pollination between a pure breeding tall
plant and a pure breeding dwarf plant. He collected the seeds from this cross pollination and
allowed them to germinate. All the resulting plants were found to be tall.
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FIG.5 Mendel's Experiments with Tall and Short Plants
In a similar pollination between a pure breeding plant with axial flowers and a
pure breeding plant with terminal flowers, all the resulting plants of the next generation produced
only axial flowers
FIG.6 Axial and Terminal Flowers of plants
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Similar results were obtained with reference to all the pairs of contrasting characters. Based on
these results, Mendel came to the conclusion that in a cross-involving two contrasting characters,
only one character expresses itself in the next generation. Mendel called the character, which
expressed as dominant character and the character, which failed to express, as recessive
character. This idea came to be known as the principle of dominance.
At this stage, Mendel wanted to know whether the tall plants resulting from a cross between tall
and dwarf plants, were similar to the tall plants of the P1 generation. Hence, he allowed the tall
plants of the F1 generation to undergo self-pollination. In the next generation, Mendel found both
tall plants and dwarf plants, approximately in the ratio 3:1. The results were most surprising
since the recessive character dwarfness had reappeared in the next generation. (F2 generation)
FIG.7
Results of Monohybrid Inheritance crosses
From the results, it was clear that the tall plants of the F1 generation were different genetically
from the tall plants of the P1 generation. Similar results were obtained by Mendel for the other
contrasting characters also. Based on these results, Mendel came to the conclusion that certain
factors are involved in the expression of each of these contrasting characters. He presumed that if
the F1 tall plants on self pollination could give rise to both tall and dwarf plants, this plant should
have contained two factors, one responsible for tallness and the other responsible for dwarfness.
Similarly, he presumed that the P1 tall plants also should have contained two factors, both
responsible for tallness. He represented these ideas by using the letters of English alphabet to
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represent the factors. He represented the factor for dominant character by a capital letter and the
factor for recessive character by a small letter.
Eg : Factor for tallness ------------T
Factor for dwarfness -------- t
Based on these results. Mendel arrived at another conclusion, which is known as the first law
of inheritance or law of segregation or the law of purity of gametes. The law states that: In a
cross involving a pair of contrasting characters, the factors responsible for the two opposite
characters stay together in the F1 generation but segregate (separate) during the formation of
gametes.
Certain specific terminologies are now used to describe the various conditions in any cross
involving opposite characters
Homozygous condition is a condition, which describes the nature of a plant based on its genetic
composition. A homozygous plant is one, which has both the factors responsible for the same
characters. Such a plant produces only one type of gametes
Example 1:
A pure breeding tall plant of the P1 generation can be described as Homozygous tall
plant : It carries two factors both of which are responsible for tallness (TT). Similarly, the dwarf
plant of the P1 generation can be described as homozygous dwarf plant. It carries two factors,
both responsible for dwarfness (tt)
Heterozygous condition is a condition in which a plant carries two factors, one responsible
for the dominant character and the other responsible for the recessive character. A heterozygous
plant produces more than one type of gamete.
Example 2:
Tall plant of F1 generation (Tt). It carries two factors, one responsible for tallness (T)
and the other responsible for dwarfism (t)
- Phenotype is the term used to describe the external expression of a particular character.
-Genotype is the term used to describe the actual genetic composition.
-The phenotypes of two plants may be similar but their genotypes may be different.
Example 3:
The tall plants of both P1 and F1 generation have the same phenotype, both are tall plants.
However, their genotypes are different. The tall plant of the P1 generation is a homozygous tall
plant whereas the tall plant of the F1 generation is heterozygous tall plant.
17
Thus, in the monohybrid inheritance, the F2 generation ratio is of two types phenotypic ratio
(3:1) for dominant and recessive characters and genotypic ratio (1:2:1) for the homozygous
dominant, heterozygous dominant and homozygous recessive conditions.
3.3.2 Punnet Square
The genotypes and phenotypes resulting from various combination of gametes can be
easily determined by Punnet squares, devised by Reginald C. Punnet (1875 1967). Hence each of
the possible gametes is placed in an individual column or a row, with vertical column
representing the female and horizontal row the male parent. The gametes are then arranged in all
possible combinations and the resulting genotypes are entered in the boxes along with the
phenotypes.
Example 4:
Study Questions:
1) Give reasons why Mendel chose the pea plant for his experiments.
2) Explain each of the following genetic terms: a) monohybrid, b) dihybrid,
c) genotype, d) phenotype, e) alleles, f) recessive gene, g) dominant gene
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WEEK 4 –MENDELIAN LAWS OF INHERITANCE
4.1 The two Mendelian Laws of inheritance
Mendel’s First Law of inheritance:
This first law is also called the law of segregation of genes. The law states that, genes are
responsible for the development of the individual and that, they are independently transmitted
from one generation to another without undergoing any alteration.
Mendel’s second Law of Inheritance
This second law is also called the law of independent assortment. This law states that gametes
produced by segregation of genes as explained by Mendel’ s first law behaves as a separate unit
and are inherited independently of any other gametes. That is, each of the gametes have equal
chances of mating with one another.
4.2 Test Cross and Back Cross
Mendel devised a system of conducting verification for the results obtained by him. It is
known as test cross. It is a cross between F1 plant and the recessive parent. A test crossconducted for the monohybrid inheritance results in the two opposite characters expressing in a
ratio of 1:1.
19
FIG.8 Test Cross to explain Mendelian laws
Similarly, a test cross-conducted for the dihybrid inheritance results in the
expression of the two parental combinations and the two recombinations appear in the ratio 1:1:1
Significance of Test Cross
- Test cross can be used to determine the genotype of the F1 plant.
- The test cross can be used to support the idea that the reappearance of the
recessive
character in the F2 generation is due to the heterozygous condition of the F1 plant.
-The test can be used to verify whether any given pair of characters can be alleles (contrasting
characters).
If an F1 individual or an individual of F2 or F3 generations is crossed with any one of the
parents it is called a back cross
4.3 Explanation of Mendelian laws in relation to meiosis
With the rediscovery of Mendelism, several scientists focused on identifying the basis for
inheritance. William Sutton recognized a close parallelism between the transmission of 'factors'
identified by Mendel and the behaviour of chromosomes during meiosis and fertilization. These
ideas can be summarized as follows.
-Mendel assumed that hereditary factors exist in pairs. It is now established that chromosomes
occur in pairs in the body cells
-Mendel proposed the idea of separation of paired factors during the formation of gametes, in the
law of segregation (law of purity of gametes). It is now established that the paired homologous
chromosomes separate during meiosis in such a way that each gamete receives one chromosome
of each homologous pair.
-Mendel proposed the idea that the factors responsible for traits of each pair are independent of
every other pair in the process of their distribution into the gametes (law of independent
assortment). It is now known that during meiosis, the chromosomes of various homologous pairs
assort at random so that the chromosomes of each pair
segregate independently of the chromosomes of every other pair.
-Mendel firmly believed that the parental characters mix and express in the offspring. It is now
known that the homologous chromosomes from the two parents come together in the zygote as
result of fusion of male and female gamete
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-Mendel held the view that characters are not lost completely, even when they do not express.
The chromosomes retain their structure and identity throughout the life of an organism,
irrespective of whether they are noticeable or not..
Thus, the behaviour of chromosomes particularly during meiosis and fertilization provides a firm
basis for explaining Mendel's laws of inheritance
Study Questions:
.
1) State and explain the two Mendelian laws of inheritance
2) Explain the behaviour of chromosomes during meiosis in relation to Mendelian laws of
inheritance. .
21
WEEK 5 - MONOHYBRID INHERITANCE IN FRUIT FLY
5.1 Monohybrid inheritance in fruit fly (Drosophila melanogaster)
The fruit fly Drosophila melanogaster, has eight chromosomes: three pairs of autosomes and one pair of sex
chromosomes
Drosophila melanogaster, the fruit fly, can easily be cultured and at 25°C it completes its
development in two weeks. This means that several generations can be studied in a short time.
The wild fly can he crossed to various mutant forms. E.g. flies with vestigial wings.
Let the alleles be represented by symbols
VG = dominant gene for long wing
vg = recessive gene for vestigial wing
Reciprocal crosses may be made
In the above monohybrid experiment two alleles for wing length are involved. The female parent
is homozygous for the gene responsible for the long wing while the male parent is homozygous
for the recessive character and exhibits vestigial wings as its phenotype.
During gamete formation, the genes separate and each gamete carries only one of the two genes.
After fertilization the resulting zygote possesses one gene of each type and they are therefore
termed heterozygous. Since the gene for long wing is dominant they all manifest this character.
22
FIG.9 Adult fruit flies (Drosophila ,melanogaster)
The F1 flies will exactly resemble the parental long-winged flies phenotypically, but their
genotypes differ, the parent having the constitution VG VG and the F1 offspring, having the
constitution VG vg.
The F1 flies can therefore produce two kinds of eggs (VG and vg) and two kinds of sperm (VG
and vg) and if we suppose random fusion between the gametes during mating, there are four
possible combinations: VG VG, VG vg, VG vg. And vgvg. in statistically equal numbers. These
F2 individuals therefore show three different genotypes in the ratio 1:2: 1, but only two
phenotypes. Long and vestigial wing.
The occurrence of the heterozygous condition in the F1 generation can be shown by what is
known as a back cross. An F1 fly is mated with the homozygous parent fly showing the recessive
character.
Phenotype
F1 long-winged fly x P vestigial-winged fly
Genotype
Gametes
Fertilization
Genotype
phenotype
5.2 Albinism
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Albinism is the lack of normal pigmentation occurs in all races. A rare condition, albinism
occurs when a person inherits a recessive allele, or group of genes, for pigmentation from each
parent. In this case, production of the enzyme tyrosinase is defective. Tyrosinase is necessary to
the formation of melanin, the normal human skin pigment. Without melanin, the skin lacks
protection from the sun and is subject to premature aging and skin cancer. The eyes, too,
colorless except for the red blood vessels of the retina that show through, cannot tolerate light.
Albinos tend to squint even in normal indoor lighting and frequently have vision problems
FIG.1O Albinism in a Nigerian child
Study Questions:
1) List examples of monohybrid inheritance in fruit fly(Drosophila melanogaster).
24
2) What is albinism? Is it controlled by dominant or recessive genes?
3) Differentiate between monohybrid cross and dihybrid cross
25
WEEK 6 –DIHYBRID INHERITANCE
6.1 Dihybrid inheritance
To determine the set of alleles an organism has for a given trait just by visual observation can
often be difficult. In the pea plant example, for instance, plants with smooth peas might be
carrying two dominant alleles for that characteristic (RR) or one dominant and one recessive
allele (Rr). Geneticists use the term genotype to refer to the combination of genes that code for a
trait, while the term phenotype describes the physical manifestation of that trait. Therefore, the
presence of two dominant alleles for pea texture (RR) would reflect the genotype while a smooth
pea indicates the phenotype.
Mendel did not limit his experiments to testing the rules of inheritance of single traits. He also
studied plant traits involving multiple pairs of genes, breeding plants that have round, yellow
seeds with plants that produce wrinkled, green seeds. Such experiments demonstrated that the
patterns of inheritance he observed in his experiments with single traits also apply to cases
involving more complex gene combinations.
We will now extend Mendel’s principle of segregation to more complex crosses for alleles at
multiple loci. Understanding the nature of these crosses will require an additional principle, the
principle of independent assortment In addition to his work on monohybrid crosses, Mendel also
crossed varieties of peas that differed in two characteristics (dihybrid crosses). For example, he
had one homozygous variety of pea that produced round seeds and yellow endosperm; another
homozygous variety produced wrinkled seeds and green endosperm. When he crossed the two,
all the F1 progeny had round seeds and yellow endosperm He then self-fertilized the F1 and
obtained the following progeny in the F2: 315 round, yellow seeds; 101 wrinkled, yellow seeds;
108 round, green seeds; and 32 wrinkled, green seeds. Mendel recognized that these traits
appeared approximately in a 9:3:3:1 ratio; that is, of the progeny were round and yellow, were
wrinkled and yellow, were round and green, and were wrinkled and green.
6.2 The Principle of Independent Assortment
Mendel carried out a number of dihybrid crosses for pairs of characteristics and always obtained
a 9:3:3:1 ratio in the F2. This ratio makes perfect sense in regard to segregation and dominance if
we add a third principle, which Mendel recognized in his dihybrid crosses: the principle of
independent assortment (Mendel’s second law). This principle states that alleles at different
loci separate independently of one another.
A common mistake is to think that the principle of segregation and the principle of independent
assortment refer to two different processes. The principle of independent assortment is really an
extension of the principle of segregation. The principle of segregation states that the two alleles
of a locus separate when gametes are formed; the principle of independent assortment states that,
when these two alleles separate, their separation is independent of the separation of alleles at
other loci.
Let’s see how the principle of independent assortment explains the results that Mendel obtained
in his dihybrid cross. Each plant possesses two alleles coding for each characteristic so the
parental plants must have had genotypes RRYY and rryy
26
FIG.11 Dihybrid cross
27
6.3 Exceptions to Mendel’s Rules or Deviations from Mendelian ratio
Mendel published his studies in a science journal in 1865, at which time no other scientist
commented on his work. Since that time, geneticists have learned that sometimes genes do not
easily conform to so-called Mendelian patterns of inheritance
(a) Incomplete Dominance
In cases of incomplete dominance, the inheritance of a dominant and a recessive allele results in
a blending of traits to produce intermediate characteristics. For example, four-o’clock paint
plants may have red, white, or pink flowers. Plants with red flowers have two copies of the
dominant allele R for red flower color (RR). Plants with white flowers have two copies of the
recessive allele r for white flower color (rr). Pink flowers result in plants with one copy of each
allele (Rr), with each allele contributing to a blending of colors.
(b) Quantitative Inheritance
Mendel focused his studies on traits determined by a single pair of genes, and the resulting
phenotype was easy to distinguish. A tall plant can be markedly different from a short one, and a
green pea can easily be distinguished from a yellow one. There are some traits, however, that are
not easy to distinguish. Human skin color, for example, may be any of a wide variety of shades.
Traits such as skin color differ from the ones Mendel studied because they are determined by
more than one pair of genes. In this form of inheritance, known as quantitative inheritance, each
pair of genes has only a slight effect on the trait, while the cumulative effect of all the genes
determines the physical characteristics of the trait. At least four pairs of genes control human
skin color. Multiple genes also control many traits important in agriculture, such as milk
production in cows and ear length in corn.
(c) Multiple Alleles
Another exception to Mendelian genetics involves genes with multiple alleles. Certain traits are
controlled by multiple alleles that have complex rules of dominance. In humans, for example, the
gene for blood type has three alleles: IA, IB, and i. With three alternatives for each member of a
gene pair, there are six possible combinations of these genes (IAIA, IBIB, ii, IAi, IBi, IAIB). Although
there are six possible combinations, humans have only four major blood types: A, B, AB, and O.
This results because both IA and IB dominate over i, but not over each other, so a person with a
gene combination of IAIA or IAi has blood type A. The gene combinations IBIB and IBi both
produce blood type B. IAIB results in a blood type AB, and ii results in blood type O.
Study Questions:
1) Differentiate between monohybrid cross and dihybrid cross
28
2) State the genotypic and phenotypic ratios of offspring produced at F2, in a monohybrid
cross where there is complete dominance.
3) In rabbit, brown hair is dominant over white hair. When a pure-breeding brown hair
rabbit is crossed with a pure-breeding white hair rabbit, all the F1 offspring produced had
brown hair
i)
ii)
indicate the genotypes of the parents,
indicate with the aid of labeled diagrams how the F1 and F2 offspring are
produced
4) Illustrate with the aid of a Punnet square the F2 offspring produced in a dihybrid cross
between Tall and Smooth podded plants X Short and Wrinkled podded plants. Tall is
dominant over Short and Smooth pod over wrinkled pod.
5) Explain the deviations from Mendelian ratio
29
.
WEEK 7 –CONCEPT OF DOMINANCE
7.1 Complete Dominance as explained in Mendel’s experiments:
In a cross between two heterozygous Tall (Tt) parents, the offspring in a complete
dominance situation would segregate in a genotypic ratio of 1:2:1 and a phenotypic ratio of
3:1 as shown below:
Parent phenotype
Tall X Tall
Parent genotype
Tt
Gametes
T
Fertilization
TT
Tt
t
T
Tt
Genotypic ratio
1
Offspring
Phenotype
Tall
Phenotypic ratio
X
:
t
tT
2
Tall
tt
:
Tall
1
Short
( 3 Tall : 1 Short)
7.2 Gene Linkage
In his experiments, Mendel was careful to study traits in pea plants where one trait did not appear
to influence another, such as the plant’s height or the pea’s texture. These two phenotypes
(height and texture) occur randomly with respect to one another in a manner known as
independent assortment. Today scientists understand that independent assortment occurs when
the genes affecting the phenotypes are found on different chromosomes.
An exception to independent assortment develops when genes appear near one another on the
same chromosome. When genes occur on the same chromosome, they are inherited as a single
30
unit. Genes inherited in this way are said to be linked. For example, in fruit flies the genes
affecting eye color and wing length are inherited together because they appear on the same
chromosome.
But in many cases, genes on the same chromosome that are inherited together produce offspring
with unexpected allele combinations. This results from a process called crossing over.
Sometimes at the beginning of meiosis, a chromosome pair (made up of a chromosome from the
mother and a chromosome from the father) may intertwine and exchange sections of
chromosome. The pair then breaks apart to form two chromosomes with a new combination of
genes that differs from the combination supplied by the parents. Through this process of
recombining genes, organisms can produce offspring with new combinations of maternal and
paternal traits that may contribute to or enhance survival.
7.3 Multiple Alleles
The term multiple allele is a condition where more than two genes occupy the same locus, on
the same pair of homologous chromosomes, in different organisms. Each of these genes
expresses a totally different character. The inheritance of A B O blood groups in man is an
example of multiple alleles. The four blood groups A, B, AB and O are due to the presence of
three genes occupying the same locus on the same pair of chromosomes in different human
beings. The discovery of blood groups dates back to the year 1900. A German doctor by name
Carl Land Steiner discovered the four blood groups in man. He was examining the reason for the
instant death of many persons immediately after a blood transfusion. He isolated the plasma and
RBC from the blood samples of different persons. He found that whenever the plasma was mixed
with the RBC of the same person, the mixture was smooth. However, when the plasma and RBC
belonged to different persons, the mixture was found to be smooth in some cases and clumped in
others. Detailed analysis conducted by Land Steiner showed that the human blood contained two
specific substances called antigens, which are responsible for either smooth mixing or clumping.
He named these antigens as antigen A and antigen B. Based on the presence or absence of these
antigens; he classified the human blood into four groups namely A, B, AB and O.
The following table represents the antigens and the corresponding antibodies found in
the four blood groups of man.
31
Blood Transfusion
The transfer of blood from one person to another is called blood transfusion. In all
cases of blood transfusion, it is necessary to match the blood group of the recipient with the
blood group of donor. The following table represents the blood group matching.
ü = Smooth X = Clump
From the table it is clear that persons with blood group AB can receive blood
from any other person. Hence they are commonly described as universal recipients persons with
blood group O can donate blood to any other person. Hence, they are commonly described as
universal donors.
The fruit-fly, Drosophila melanogaster has 15 alleles for eye colour. In rabbits,
there are 4 alleles for colour. In all these cases, at any given time, only two of the alleles can
occupy the same locus on a pair of homologous chromosomes.
Study Questions
1) What is the concept of dominance?
2) How does complete dominance differ from incomplete dominance?
3) State the four blood phenotypes of man and their possible genotypes/
32
WEEK 8 -- DEVIATIONS BY LINKAGE, MULTPLE ALLELES ,
AND LETHAL GENES IN MICE
8.1 Co dominance
Co-dominance represents a situation where two allelic genes when present together in
an individual, express their traits independently instead of showing a typical dominant recessive
relationship. As a result the heterozygous progeny of the F2 generation show a phenotype that is
different from both the homozygous parent.
A classical example of co dominance is the expression of blood group AB. The
genotype for this blood group is IA Ib. Each of the two genes produces the respective antigen and
neither of them checks the expression of the other
S
A
Sickle-cell is an inherited condition in which two co-dominant alleles exist – Hb and Hb . This
condition is caused by a single point substitution mutation, resulting in a single amino-acid being
changed. With 2 alleles, three phenotypes therefore exist:
S
1)
S
Hb Hb . These individuals have sickle-cell anaemia, and normally die young.
S
A
2) Hb Hb . These individuals have sickle-cell trait, have only a few sickle-cells and are
resistant to malaria
A
A
3) Hb Hb . These individuals are normal, and are susceptible to malaria.
FIG.12 Result of a cross between a Normal (AA) and Carrier (AS) persons for the sickle
cell trait
33
8.2
Incomplete Dominance
It is a type of intragenic (or interallellic) interaction where both the alleles of a given
trait express as a blend (mixture) as against a normal Mendelian pattern where one allele is
dominant over the other. As a resulting of this blending, an intermediate character is expressed.
This situation occurs due to the fact that the dominant gene is not in a position to completely
suppress the expression of recessive gene. With the result, the heterozygous offspring will be
phenotypically and genotypically different from either of the homozygous parent. Following are
the two familiar examples of incomplete dominance.
Flower Colour in Four 'O Clock Plant
In the plant Mirabilis jalapa, commonly called as four'o clock plant, the inheritance
of flower colour is an example for incomplete dominance.
The plant produces two types of flowers red coloured and white coloured. This condition is
an example for a pair of contrasting characters. When a plant which is homozygous for red
flowers (AA) is crossed with a plant which is homozygous for white flowers (aa), the plants of
the F1 generation produce pink flowers which is a blend of red and white condition. This result
clearly indicates that neither red flowered condition nor white flowered condition is dominant.
However, when two hybrid plants with pink flowers (Aa) are crossed, the F2 generation plants
show red flowered, pink flowered and white flowered condition in the ratio 1:2:1. This ratio is
very much in accordance with the law of segregation.
34
FIG.13 Flower Colour in Four O Clock Plant
This example very clearly indicates
- The phenomenon of incomplete dominance
-That the genes responsible for red and white flowers do not actually mix, since
the pure characters reappear in the F2 generation
both
- That there is no specific gene responsible for producing pink flowers
-That the homozygous white flowered plants have genes aa which is unable to produce the
colouring pigment
- That the heterozygous pink flowered plants have genes Aa and hence can produce only
half the amount of colouring pigment that is normally produced in a red flowered plant (AA)
The seven characters in pea plants that Mendel chose to study extensively all exhibited
dominance, but Mendel did realize that not all characters have traits that exhibit dominance. He
conducted some crosses concerning the length of time that pea plants take to flower. When he
crossed two homozygous varieties that differed in their flowering time by an average of 20 days,
the length of time taken by the F1 plants to flower was intermediate between those of the two
parents. When the heterozygote has a phenotype intermediate between the phenotypes of the two
homozygotes, the trait is said to display incomplete dominance.
35
Incomplete dominance is also exhibited in the fruit color of eggplants. When a homozygous
plant that produces purple fruit (PP) is crossed with a homozygous plant that produces white
fruit (pp), all the heterozygous F1 (Pp) produce violet fruit When the F1 are crossed with each
other, of the F2 are purple (PP), are violet (Pp), and are white (pp), as shown. This 1:2:1 ratio is
different from the 3:1 ratio that we would observe if eggplant fruit color exhibited dominance
36
FIG.14 Incomplete dominant trait in fruit colour of egg plant
37
When a trait displays incomplete dominance, the genotypic ratios and phenotypic ratios of the
offspring are the same, because each genotype has its own phenotype. It is impossible to obtain
eggplants that are pure breeding for violet fruit, because all plants with violet fruit are
heterozygous. Another example of incomplete dominance is feather color in chickens. A cross
between a homozygous black chicken and a homozygous white chicken produces F1 chickens
that are gray. If these gray F1 are intercrossed, they produce F2 birds in a ratio of 1 black: 2 gray:
1 white. Leopard white spotting in horses is incompletely dominant over unspotted horses: LL
horses are white with numerous dark spots, heterozygous Ll horses have fewer spots, and ll
horses have no spots.
.
FIG.15 Incomplete dominance of Leopard spotting in horses
.
Andalusian Fowl
Blue Andalusian poultry. This is an example of a monohybrid inheritance in which there is
interaction of function between the alleles. Mendel’s hybrids all displayed the presence of a
dominant gene but in some crosses there is an apparent blending due to the interaction of the
alleles when they are both present. The Andalusian fowl illustrates this phenomenon well. The
feathers show an attractive blue sheen which makes them popular with fanciers, but when they
are bred they always produce some offspring which are either black, or white with black
38
splashes. It is evident that the Blue Andalusians are heterozygous for two alleles neither of which
is truly dominant. Let B= gene for black colour, and BW=gene for splashed white. It is
conventional to use capital symbols for dominant alleles and small symbols for recessive alleles.
In this case, incomplete dominance is indicated by the use of a capital for each allele. It should
be noted that the genes segregate unchanged in gamete formation and show their true identity in
the F2, when pure black and pure splashed white birds appear.
The Andalusian fowl has three varieties splashed white, black and blue. A cross between a pure
splashed white male fowl and a pure black female fowl yields, hybrid fowls with blue colour, in
the F1 generation. When the blue hybrid fowls are crossed, with each other, the F2 generation
shows fowls with splashed white, blue and black colour in the ratio 1:2:1.
8.3
Non-Mendelian Inheritances
The rediscovery of Mendelism and the subsequent idea of chromosomal basis of
inheritance, paved way for several significant discoveries in the field of heredity. Many other
patterns of inheritance which cannot be explained on the basis of Mendel's laws alone, were
discovered in plant and animals. Such patterns of inheritance are described as non-Mendelian
inheritance
It is now clearly established that the expression of specific traits is due to the expression of any
one gene. It can be sometimes altered by the influence of other genes. Such a phenomenon where
certain genes bring about a modification of the normal phenotypic expression of a given gene is
known as gene interaction. Gene interactions are of two types intragenic and intergenic.
- Intragenic interactions, also known as inter-allelic interactions occur between the
two alleles of the same gene. Such an interaction results in a phenotype which is different from
the typical dominant recessive phenotype. The common examples of intragenic interactions are
incomplete dominance and co-dominance.
- Intergenic interactions, also known as non-allelic interactions occur between alleles of
different genes located on the same or different chromosomes. Such an interaction also results in
a change in the phenotypic expression. The familiar examples are complementary genes,
supplementary genes, epitasis, plieotropy and lethal genes.
8.4 Lethal genes in mice
A lethal allele is one that causes death at an early stage of development— often before birth—
and so a some genotypes may not appear among the progeny. A lethal allele causes death,
frequently at an early developmental stage, and so one or more genotypes are missing from the
progeny of a cross. Lethal alleles may modify the ratio of progeny resulting from a cross.
In 1905, Lucien Cuenot reported a peculiar pattern of inheritance in mice. When he mated two
yellow mice, approximately of their offspring was yellow and were non yellow. When he testcrossed the yellow mice, he found that all were heterozygous; he was never able to obtain a
yellow mouse that bred true. There was a great deal of discussion about Cuenot’s results among
his colleagues, but it was eventually realized that the yellow allele must be lethal when
homozygous
39
FIG.16 Progeny of a cross resulting from the segregation of a lethal allele in mice
. Cuenot originally crossed two mice heterozygous for yellow: Yy _ Yy. Normally, this cross
would be expected to produce YY, Yy, and yy (see Figure The homozygous YY mice are
conceived but never complete development, which leaves a 2 : 1 ratio of Yy (yellow) to yy
(nonyellow) in the observed offspring; all yellow mice are heterozygous (Yy)
Another example of a lethal allele, originally described by Erwin Baur in 1907, is found in
snapdragons. The aurea strain in these plants has yellow leaves. When two plants with yellow
leaves are crossed, of the progeny have yellow leaves and have green leaves. When green is
crossed with green, all the progeny have green leaves; however, when yellow is crossed with
green, of the progeny are green and are yellow, confirming that all yellow-leaved snapdragons
are heterozygous. A 2 : 1 ratio is almost always produced by
a recessive lethal allele; so observing this ratio among the progeny of a cross between individuals
with the same phenotype is a strong clue that one of the alleles is lethal In both of these
examples, the lethal alleles are recessive because they cause death only in homozygotes. Unlike
its effect on survival, the effect of the allele on color is dominant; in both mice and snapdragons,
a single copy of the allele in the heterozygote produces a yellow color. Lethal alleles also can be
40
dominant; in this case, homozygotes and heterozygotes for the allele die. Truly dominant lethal
alleles cannot be transmitted unless they are expressed after the onset of reproduction, as in
Huntington disease.
8.5 The genetic basis of ABO Blood Group
Another multiple-allele system is at the locus for the ABO blood group. This locus determines
your ABO blood type and, like the MN locus, codes for antigens on red blood cells. The three
common alleles for the ABO blood group locus are: IA, which codes for the A antigen; IB,
which codes for the B antigen; and i, which codes for no antigen (O).We can represent the
dominance relations among the ABO alleles
as follows: IA _ i, IB _ i, IA _ IB. The IA and IB alleles are both dominant over i and are co
dominant with each other; the AB phenotype is due to the presence of an IA allele and an IB
allele, which results in the production of A and B antigens on red blood cells. An individual with
genotype ii produces neither antigen and has blood type O. The six common genotypes at this
locus and their phenotypes are shown in FIG.17.
Antibodies are produced against any foreign antigens For instance, a person having blood type A
produces B antibodies, because the B antigen is foreign. A person having blood type B produces
A antibodies, and someone having blood type AB produces neither A nor B antibodies because
neither A nor B antigen is foreign. A person having blood type O possesses no A or B antigens;
consequently that person produces both A antibodies and B antibodies. The presence of
antibodies against foreign ABO antigens means that successful blood transfusions are possible
only between persons with certain compatible blood types.
The inheritance of alleles at the ABO locus can be illustrated by a paternity suit involving the
famous movie actor Charlie Chaplin. In 1941, Chaplin met a young actress named Joan Barry,
with whom he had an affair. The affair ended in February 1942 but, 20 months later, Barry gave
birth to a baby girl and claimed that Chaplin was the father. Barry then sued for child support. At
this time, blood typing had just come into widespread use, and Chaplin’s attorneys had Chaplin,
Barry, and the child blood typed. Barry had blood pe A, her child had blood type B, and Chaplin
had blood type O. Could Chaplin have been the father of Barry’s child? Your answer should be
no. Joan Barry had blood type A, which can be produced by either genotype IAIA or IAi her
baby possessed blood type B, which can be produced by either genotype IBIB or IBi. The baby
could not have inherited the IB allele from Barry (Barry could not carry an IB allele if she were
blood type A); therefore the baby must have inherited the i allele from her. Barry must have had
genotype IAi, and the baby must have had genotype IBi. Because the baby girl inherited her i
allele from Barry, she must have inherited the IB allele from her father. With blood type O,
produced only by genotype ii, Chaplin could not have been the father of Barry’s child. In the
course of the trial to settle the paternity suit, three pathologists came to the witness stand and
declared that it was genetically impossible for Chaplin to have fathered the child. Nevertheless
the jury ruled that Chaplin was the father and ordered him to pay child support and Barry’s legal
expenses
41
FIG. 17 ABO blood phenotype, genotype, antigen type and antibodies made by the body
42
FIG. 18 Safe blood transfusions.
Study Questions
1) Explain each of the following with suitable examples : i) linkage ii) multiple alleles
iii) co dominance iv) lethal genes.
2) Explain the genetic basis of ABO blood group
3) Which of the blood groups are referred to as Universal donor and Universal recipient?
4) A man heterozygous and in blood group A is married to an heterozygous woman in
blood group B. What are the possible blood groups of their offspring?
State the blood phenotype of the following genotypes: i) IAi, ii) IAIB , iii) ii .
43
WEEK 9 - SEX DETERMINATION
9.1 Sex determination
Sexual reproduction is the formation of offspring that are genetically distinct from their parents;
most often, two parents contribute genes to their offspring. Among most eukaryotes, sexual
reproduction consists of two processes that lead to an alternation of haploid and diploid cells:
meiosis produces haploid gametes, and fertilization produces diploid zygotes. The term sex
refers to sexual phenotype. Most organisms have only two sexual phenotypes: male and female.
The fundamental difference between males and females is gamete size: males produce small
gametes; females produce relatively large gametes. The mechanism by which sex is established
is termed Sex Determination. We define the sex of an individual in terms of the individual’s
phenotype—ultimately, the type of gametes that it produces. Sometimes an individual has
chromosomes or genes that are normally associated with one sex but a morphology
corresponding to the opposite sex. For instance, the cells of female humans normally have two X
chromosomes, and the cells of males have one X chromosome and one Y chromosome. A few
rare persons have male anatomy, although their cells each contain two X chromosomes. Even
though these people are genetically female, we refer to them as male because their sexual
phenotype is male.
There are many ways in which sex differences arise. In some species, both sexes are present in
the same individual, a condition termed hermaphroditism; organisms that bear both male and
female reproductive structures are said to be monoecious (meaning “one house”). Species in
which an individual has either male or female reproductive structures are said to be dioecious
(meaning “two houses”). Humans are dioecious. Among dioecious species, the sex of an
individual may be determined chromosomally, genetically, or environmentally.
9.2 Chromosomal Sex-Determining Systems
The chromosome theory of inheritance states that genes are located on chromosomes, which
serve as the vehicles for gene segregation in meiosis. Definitive proof of this theory was
provided by the discovery that the sex of certain insects is determined by the presence or absence
of particular chromosomes.
In 1891, Hermann Henking noticed a peculiar structure in the nuclei of cells from male insects.
Understanding neither its function nor its relation to sex, he called this structure the X body.
Later, Clarence E.McClung studied Henking’s X body in grasshoppers and recognized that it
was a chromosome. McClung called it the accessory chromosome, but eventually it became
known as the X chromosome, from Henking’s original designation. McClung observed that the
cells of female grasshoppers had one more chromosome than the cells of male grasshoppers and
he concluded that accessory chromosomes played a role in sex determination. In 1905, Nettle
Stevens and Edmund Wilson demonstrated that, in grasshoppers and other insects, the cells of
females have two X chromosomes, whereas the cells of males have a single X. In some insects,
they counted the same number of chromosomes in
cells of males and females but saw that one chromosome pair was different: two X chromosomes
were found in female cells, whereas a single X chromosome plus a smaller chromosome, which
they called Y, was found in male cells.
44
Stevens and Wilson also showed that the X and Y chromosomes separate into different cells in
sperm formation; half of the sperm receive an X chromosome and half receive a Y. All egg cells
produced by the female in meiosis receive one X chromosome. A sperm containing a Y
chromosome unites with an X-bearing egg to produce an XY male, whereas a sperm containing
an X chromosome unites with an X-bearing egg to produce an XX female. This accounts for he
50:50 sex ratio observed in most dioecious organisms. Because sex is inherited like other
genetically determined characteristics, Stevens and Wilson’s discovery that sex was associated
with the inheritance of a particular chromosome also demonstrated that genes are on
chromosomes. As Stevens and Wilson found for insects, sex is frequently determined by a pair
of chromosomes, the sex chromosomes, which differ between males and females. The nonsex
chromosomes, which are the same for males and females, are called autosomes.We think of sex
in these organisms as being determined by the presence of the sex chromosomes, but in fact the
individual genes located on the sex chromosomes are usually responsible for the sexual
phenotypes.
9.3 XX-XO sex determination
The mechanism of sex determination in the grasshoppers studied by McClung is one of the
simplest mechanisms of chromosomal sex determination and is called the XX-XO system. In this
system, females have two X chromosomes (XX), and males possess a single X chromosome
(XO). There is no O chromosome; the letter O signifies the absence of a sex chromosome. In
meiosis in females, the two X chromosomes pair and then separate, with one X chromosome
entering each haploid egg. In males, the single X chromosome segregates in meiosis to half the
sperm cells—the other half receive no sex chromosome. Because males produce two different
types of gametes with respect to the sex chromosomes, they are said to be the heterogametic sex.
Females, which produce gametes that are
all the same with respect to the sex chromosomes, are the homogametic sex. In the XX-XO
system, the sex of an individual is therefore determined by which type of male gamete fertilizes
the egg.
X-bearing sperm unite with X-bearing
eggs to produce XX zygotes, which eventually develop as females. Sperm lacking an X
chromosome unite with X-bearing eggs to produce XO zygotes, which develop into males.
9.4 XX-XY sex determination
In many species, the cells of males and females have the same number of chromosomes, but the
cells of females have two X chromosomes (XX) and the cells of males have a single X
chromosome and a smaller sex chromosome called the Y chromosome (XY). In humans and
many other organisms, the Y chromosome is acrocentric not Y shaped as is commonly assumed.
In this type of sex-determining system, the male is the heterogametic sex—half of his gametes
have an X chromosome and half have a Y chromosome. The female is the homogametic sex—all
her egg cells contain a single X chromosome.
Many organisms, including some plants, insects, and reptiles, and all mammals (including
humans), have the XX-XY sex-determining system. Although the X and Y chromosomes are not
generally homologous, they do pair and segregate into different cells in meiosis. They can pair
45
because these chromosomes are homologous at small regions called the pseudoautosomal
regions in which they carry the same genes. Genes found in these regions will display the same
pattern of inheritance as that of genes located on autosomal chromosomes. In humans, there are
pseudoautosomal regions at both tips of the X and Y chromosomes. ZZ-ZW sex determination In
this system, the female is heterogametic and the male is homogametic. To prevent confusion
with the XX-XY system, the sex chromosomes in this system are labeled Z and W, but the
chromosomes do not resemble Zs and Ws. Females in this system are ZW; after meiosis, half of
the eggs have a Z chromosome and the other half have a W.Males are ZZ; all sperm contain a
single Z chromosome. The ZZ-ZW system is found in birds, moths, some amphibians, and some
fishes. Receiving the same allele from their mother and a 100% chance of receiving the same
allele from their father; the average relatedness between sisters is therefore 75%. Brothers have a
50% chance of receiving the same copy of each of their mother’s two alleles at any particular
locus; so their average relatedness is only 50%. The greater genetic relatedness among female
siblings in insects with haplodiploid sex determination may contribute to the high degree of
social cooperation that exists among females (the workers) of these insects.
9.5 Haplodiploidy
Some insects in the order Hymenoptera (bees, wasps, and ants) have no sex chromosomes;
instead, sex is based on the number of chromosome sets found in the nucleus of each cell. Males
develop from unfertilized eggs, and females develop from fertilized eggs. The cells of male
hymenopterans possess only a single set of chromosomes (they are haploid) inherited from the
mother. In contrast, the cells of females possess two sets of chromosomes (they are diploid), one
set inherited from the mother and the other set from the father.
The haplodiploid method of sex determination produces some odd genetic relationships. When
both parents are diploid, siblings on average have half their genes in common because they have
a 50% chance of receiving the same allele from each parent. In these insects, males produce
sperm by mitosis (they are already haploid); so all offspring receive the same set of paternal
genes. The diploid females produce eggs by normal meiosis. Therefore, sisters have a 50%
chance of
9.6 Genic Sex-Determining Systems
In some plants and protozoan, sex is genetically determined, but there are no obvious differences
in the chromosomes of males and females—there are no sex chromosomes These organisms
have genic sex determination; genotypes at one or more loci determine the sex of an individual.
It is important to understand that, even in chromosomal sex-determining systems, sex is actually
determined by individual genes. For example, in mammals, a gene located on the Y chromosome
determines the male phenotype. In both genetic sex determination and chromosomal sex
determination, sex is controlled by individual genes; the difference is that, with chromosomal sex
determination, the chromosomes that carry those genes appear different in males and females.
46
9.7 Environmental Sex Determination
Genes have had a role in all of the examples of sex determination discussed thus far, but sex is
determined fully or in part by environmental factors in a number of organisms. One fascinating
example of environmental sex determination is seen in the marine mollusk Crepidula fornicata,
also known as the common slipper limpet.
Slipper limpets live in stacks, one on top of another. Each limpet begins life as a swimming
larva. The first larva to settle on a solid, unoccupied substrate develops into a female limpet. It
then produces chemicals that attract other larvae, which settle on top of it. These larvae develop
into males, which then serve as mates for the limpet below. After a period of time, the males on
top develop into females and, in turn, attract additional larvae that settle on top of the stack,
develop into males, and serve as mates for the limpets under them. Limpets can form stacks of a
dozen or more animals; the uppermost animals are always male. This type of sexual development
is called sequential hermaphroditism; each individual animal can be both male and female,
although not at the same time. In Crepidula fornicata, sex is determined environmentally by the
limpet’s position in the stack.
Environmental factors are also important in determining sex in many reptiles. Although most
snakes and lizards have sex chromosomes, in many turtles, crocodiles, and alligators,
temperature during embryonic development determines sexual phenotype. In turtles, for
example, warm temperatures produce females during certain times of the year, whereas cool
temperatures produce males. In alligators, the reverse is true.
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9.8 Sex Determination in Drosophila
The fruit fly Drosophila melanogaster, has eight chromosomes: three pairs of autosomes and one
pair of sex chromosomes. Normally, females have two X chromosomes and males have an X
chromosome and a Y chromosome. However, the presence of the Y chromosome does not
determine maleness in Drosophila; instead, each fly’s sex is determined by a balance between
genes on the autosomes and genes on the X chromosome. This type of sex determination is
called the genic balance system. In this
system, a number of genes seem to influence sexual development. The X chromosome contains
genes with female producing effects, whereas the autosomes contain genes with male-producing
effects. Consequently, a fly’s sex is determined by the X:A ratio, the number of X chromosomes
divided by the number of haploid sets of autosomal chromosomes.
47
An X:A ratio of 1.0 produces a female fly; an X:A ratio of 0.5 produces a male. If the X:A ratio
is less than 0.5, a male phenotype is produced, but the fly is weak and sterile—such flies are
sometimes called metamales. An X:A ratio between 1.0 and 0.50 produces an intersex fly, with a
mixture of male and female characteristics. If the X:A ratio is greater than 1.0, a female
phenotype is produced,
but these flies (called metafemales) have serious developmental problems and many never
emerge from the pupas case. Presents some different chromosome complements in Drosophila
and their associated sexual phenotypes. Flies with two sets of autosomes and XXY sex
chromosomes (an X:A ratio of 1.0) develop as fully fertile females, in spite of the presence of a
Y chromosome. Flies with only a single X (an X:A ratio of 0.5), develop as males, although they
are sterile. These observations confirm that the Y chromosome does not determine sex in
Drosophila.
Mutations in genes that affect sexual phenotype in Drosophila have been isolated. For example,
the transformer mutation converts a female with an X:A ratio of 1.0 into a phenotypic male,
whereas the doublesex mutation transforms normal males and females into flies with intersex
phenotypes. Environmental factors, such as the temperature of the rearing conditions, also can
affect the development of sexual characteristics.
9.9
Sex Determination in Humans
Humans, like Drosophila, have XX-XY sex determination, but in humans the presence of a gene
on the Y chromosome determines maleness. The phenotypes that result from abnormal numbers
of sex chromosomes, which arise when the sex chromosomes do not segregate properly in
meiosis or mitosis, illustrate the importance of the Y chromosome in human sex determination.
a) Poly-X females
In about 1 in 1000 female births, the child’s cells possess three X chromosomes, a condition
often referred to as triplo-X syndrome. These persons have no distinctive features other than a
tendency to be tall and thin. Although a few are sterile, many menstruate regularly and are
fertile. The incidence of mental retardation among
triple-X females is slightly greater than in the general population, but most XXX females have
normal intelligence.
Much rarer are women whose cells contain four or five X chromosomes. These women usually
have normal female anatomy but are mentally retarded and have a number of physical problems.
The severity of mental retardation increases as the number of X chromosomes increases beyond
three.
Further information about sex-chromosomal abnormalities in humans
The role of sex chromosomes
The phenotypes associated with sex-chromosome anomalies allow us to make several
inferences about the role of sex chromosomes in human sex determination.
48
1. The X chromosome contains genetic information essential for both sexes; at least one copy of
an X chromosome is required for human development.
2. The male-determining gene is located on the Y chromosome. A single copy of this
chromosome, even in the presence of several X chromosomes, produces a male phenotype.
3. The absence of the Y chromosome results in a female phenotype.
4. Genes affecting fertility are located on the X and Y chromosomes. A female usually needs at
least two copies of the X chromosome to be fertile.
5. Additional copies of the X chromosome may upset normal development in both males and
females, producing physical and mental problems that increase
as the number of extra X chromosomes increases.
b)
The male-determining gene in humans
The Y chromosome in humans and all other mammals is of paramount importance in producing
a male phenotype. However, scientists discovered a few rare XX males whose cells apparently
lack a Y chromosome. For many years, these males presented a real enigma: How could a male
phenotype exist without a Y chromosome? Close examination eventually revealed a small part of
the Y chromosome attached to another chromosome. This finding indicates that it is not the
entire Y chromosome that determines maleness in humans; rather, it
is a gene on the Y chromosome. Early in development, all humans possess undifferentiated
gonads and both male and female reproductive ducts.
Then, about 6 weeks after fertilization, a gene on the Y chromosome
becomes active. By an unknown mechanism, this gene causes the neutral gonads to develop into
testes, which begin to secrete two hormones: testosterone and Mullerian inhibiting substance.
Testosterone induces the development of male characteristics, and Mullerian-inhibiting
substance causes the degeneration of the female reproductive ducts. In the absence of this maledetermining gene, the neutral
Gonads become ovaries, and female features develop. In 1987, David Page and his colleagues at
the Massachusetts Institute of Technology located what appeared to
be the male-determining gene near the tip of the short arm of the Y chromosome. They had
examined the DNA of several XX males and XY females. The cells of one XX male that they
studied possessed a very small piece of a Y chromosome
attached to one of the Xs. This piece came from a section, called 1A, of the Y chromosome.
Because this person had a male phenotype, they reasoned that the male determining gene must
reside within the 1A section of the
Y chromosome.
Examination of the Y chromosome of a 12 year-old XY girl seemed to verify this conclusion. In
spite of the fact that she possessed more than 99.8% of a Y chromosome, this XY person had a
female phenotype. Page and his colleagues
assumed that the male-determining gene must reside within the 0.2% of the Y chromosome that
she was missing. Further examination showed that this Y chromosome was indeed missing part
of section 1A. They then sequenced the DNA
within section 1A of normal males and found a gene called ZFY, which appeared to be the testisdetermining factor.
Within a few months, however, results from other laboratories suggested that ZFY might not in
fact be the male determining gene. Marsupials (pouched mammals), which also have XX-XY sex
49
determination, were found to possess a ZFY gene on an autosomal chromosome, not on the Y
chromosome. Furthermore, several human XX males were found who did not possess a copy of
the ZFY gene. A new candidate for the male-determining gene, called the sex-determining region
Y (SRY) gene, was discovered in 1990 This gene is found in XX males and is missing from all
XY females; it is also found on the Y chromosome of all mammals examined to date. Definitive
proof that SRY is the male-determining gene came when scientists placed a copy of this gene into
XX mice by means of genetic engineering. The XX mice that received this gene, although sterile,
developed into anatomical males.
The SRY gene encodes a protein that binds to DNA and causes a sharp bend in the molecule.
This alteration of DNA structure may affect the expression of other genes that encode testis
formation. Although SRY is the primary determinant of maleness in humans, other genes (some
X linked, others Y linked, and still others autosomal) also play a role in fertility and the
development of sex differences. Androgen-insensitivity syndrome illustrates several important
points about the influence of genes on a person’s sex. First, this condition demonstrates that
human sexual
development is a complex process, influenced not only by the SRY gene on the Y chromosome,
but also by other genes found elsewhere. Second, it shows that most people carry genes for both
male and female characteristics, as illustrated
by the fact that those with androgen-insensitivity syndrome have the capacity to produce female
characteristics, even though they have male chromosomes. Indeed, the genes for most male and
female secondary sex characteristics are present not on the sex hromosomes but on autosomes.
The key to maleness and femaleness lies not in the genes but in the control of their expression.
Study Questions
1) What is the most defining difference between males and females?
2) How do monoecious organisms differ from dioecious organisms?
3) Describe the XX-XO system of sex determination. In this
system, which is the heterogametic sex and which is the homogametic sex?
4) How is sex determined in insects with haplodiploid sex determination?
5) What is meant by genic sex determination?
6) How does sex determination in Drosophila differ from sex determination in humans?
7) What characteristics are exhibited by an X-linked trait?
8) Explain how Bridges’s study of nondisjunction in Drosophila helped prove the chromosome
theory of inheritance.
9) What is the sexual phenotype of fruit flies having the following chromosomes?
Sex chromosomes Autosomal chromosomes
(a) XX all normal
(b) XY all normal
(c) XO all normal
(d) XXY all normal
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(e) XYY all normal
(f) XXYY all normal
(g) XXX all normal
(h) XX four haploid sets
(i) XXX four haploid sets
(j) XXX three haploid sets
(k) X three haploid sets
(l) XY three haploid sets
(m) XX three haploid sets
51
WEEK 10 –SEX LINKAGE
10.1
Sex-Linked Characteristics
Sex-linked characteristics are determined by genes located on the sex chromosomes. Genes on
the X chromosome determine X-linked characteristics; those on the Y chromosome determine Ylinked characteristics. Because little genetic information exists on the Y chromosome in many
organisms, most sex-linked characteristics are X linked. Males and females differ in their sex
chromosomes; so the pattern of inheritance for sex-linked characteristics differs from that
exhibited by genes located on autosomal chromosomes.
Sex-linked linked inheritance can be seen in eye colour in Drosophila melanogaster , colour
blindness and haemophilia in man:
a) X-Linked White Eyes in Drosophila
The first person to explain sex-linked inheritance was the American biologist Thomas Hunt
Morgan.
Morgan began his career as an embryologist, but the discovery of Mendel’s principles inspired
him to begin conducting genetic experiments, initially on mice and rats.
In 1909, Morgan switched to Drosophila melanogaster; a year later, he discovered among the
flies of his laboratory colony a single male that possessed white eyes, in stark
contrast with the red eyes of normal fruit flies. This fly had a tremendous effect on the future of
genetics and on Morgan’s career as a biologist. With his white-eyed male, Morgan unraveled the
mechanism of X-linked inheritance, ushering in the “golden age” of Drosophila genetics that
lasted from 1910 until 1930. Morgan’s laboratory, located on the top floor of Schermerhorn Hall
at Columbia University, became known as the
Fly Room To say that the Fly Room was unimpressive is an understatement. The cramped room,
only about 16 _ 23 feet, was filled with eight desks, each occupied by a student and his
experiments. The primitive laboratory equipment consisted of little more than milk bottles for
rearing the flies and hand-held lenses for observing their traits. Later, microscopes replaced the
hand-held lenses, and crude incubators were added to maintain the fly by women with androgeninsensitivity syndrome. These persons have female external sexual characteristics and
psychological orientation. Indeed, most are unaware of their condition until they reach puberty
and fail to menstruate.
Examination by a gynecologist reveals that the vagina ends blindly and that the uterus, oviducts,
and ovaries are absent. Inside the abdominal cavity lies a pair of testes,
which produce levels of testosterone normally seen in males. The cells of a woman with
androgen-insensitivity syndrome contain an X and a Y chromosome.
How can a person be female in appearance when her cells contain a Y chromosome and she has
testes that produce testosterone? The answer lies in the complex relation between genes and sex
in humans. In a human embryo with a Y chromosome, the SRY gene causes the gonads to
develop into testes, which produce testosterone. Testosterone stimulates
embryonic tissues to develop male characteristics. But, for testosterone to have its effects, it must
bind to an androgen receptor. This receptor is defective in females with androgen-insensitivity
syndrome; consequently, their cells are insensitive to testosterone, and female characteristics
52
develop. The gene for the androgen receptor is located on the X chromosome; so persons with
this condition always inherit it from their mothers. (All XY persons inherit the X chromosome
from their mothers.) cultures, but even these additions did little to increase the physical
sophistication of the laboratory. Morgan and his
students were not tidy: cockroaches were abundant (living off spilled Drosophila food), dirty
milk bottles filled the sink, ripe bananas—food for the flies—hung from the ceiling,
and escaped fruit flies hovered everywhere. In spite of its physical limitations, the Fly Room was
the source of some of the most important research in the history of biology. There was daily
excitement among the students, some of whom initially came to the laboratory as
undergraduates. The close quarters facilitated informality and the free flow of ideas. Morgan and
the Fly Room illustrate the tremendous importance of “atmosphere” in producing good science.
To explain the inheritance of the white-eyed characteristic
in fruit flies ,Morgan systematically carried out a series of genetic crosses). First, he crossed pure
breeding, red-eyed females with his white-eyed male, producing F1 progeny that all had red
eyes. (In fact, Morgan found three white-eyed males among the 1237 progeny, but he assumed
that the white eyes were due to new mutations.) Morgan’s results from this initial cross were
consistent with Mendel’s principles: a cross between a homozygous dominant individual and a
homozygous recessive individual produces heterozygous offspring exhibiting the dominant trait.
His results suggested that white eyes were a simple recessive trait. However, when Morgan
crossed the F1 flies with one another, he found that all the female F2 flies possessed red eyes but
that half the male F2 flies had red eyes and the other half had white eyes. This finding was
clearly not the expected result for a simple recessive rait, which should appear in of both male
and female F2 offspring.
To explain this unexpected result, Morgan proposed that the locus affecting eye color was on the
X chromosome (that eye color was X linked). He recognized that the eye colour
alleles were present only on the X chromosome—no homologous allele was present on the Y
chromosome.
Because the cells of females possess two X chromosomes, females could be homozygous or
heterozygous for the eye colour alleles. The cells of males, on the other hand, possess
only a single X chromosome and can carry only a single eye-color allele. Males therefore cannot
be either homozygous or heterozygous but are said to be hemizygous for X-linked loci.
To verify his hypothesis that the white-eye trait is X linked, Morgan conducted additional
crosses. He predicted that a cross between a white-eyed female and a red eyed male would
produce all red-eyed females and all white-eyed males When Morgan performed
this cross, the results were exactly as predicted. Note that this cross is the reciprocal of the
original cross and that the two reciprocal crosses produced different results in the
F1 and F2 generations. Morgan also crossed the F1 heterozygous females with their white-eyed
father, the red-eyed F2 females with white-eyed males, and white-eyed females with white-eyed
males. In all of these crosses, the results were consistent with Morgan’s conclusion that white
eyes is an X linked characteristic.
b) Nondisjunction and the Chromosome Theory of Inheritance
53
When Morgan crossed his original white-eyed male with homozygous red-eyed females, all 1237
of the progeny had red eyes, except for three white-eyed males. As already
mentioned, Morgan attributed these white-eyed F1 males to the occurrence of further mutations.
However, flies with these unexpected phenotypes continued to appear in
his crosses. Although uncommon, they appeared far too often to be due to mutation. Calvin
Bridges, one of Morgan’s students, set out to investigate the genetic basis of
these exceptions
Bridges found that, when he crossed a white-eyed female (XwXw) with a red-eyed male (X_Y),
about 2.5% of the male offspring had red eyes and about 2.5% of the
female offspring had white eyes. In this cross, every male fly should inherit its mother’s X
chromosome and should be XwY with white eyes. Every female fly should inherit a dominant
red-eye allele on its father’s X chromosome, along with a white-eyed allele on its mother’s X
chromosome; thus, all the female progeny should be X_Xw and have red eyes. The appearance
of red eyed males and white-eyed females in this cross was therefore unexpected.
To explain this result, Bridges hypothesized that, occasionally, the two X chromosomes in
females fail to separate during anaphase I of meiosis. Bridges termed this failure
of chromosomes to separate nondisjunction. When nondisjunction occurs, some of the eggs
receive two copies of the X chromosome and others do not receive an X
chromosome If these eggs are fertilized by sperm from a red-eyed male, four combinations of
sex chromosomes are produced. When an egg carrying two X
chromosomes is fertilized by a Y-bearing sperm, the resulting zygote is XwXwY. Sex in
Drosophila is determined by the X:A ratio); in this case the X:A ratio is 1.0, so the XwXwY
zygote develops into a white-eyed female.
An egg with two X chromosomes that is fertilized by an X-bearing sperm produces XwXwX_,
which usually dies. An egg with no X chromosome that is fertilized by an X bearing sperm
produces X_O, which develops into a red eyed male. If the egg with no X chromosome is
fertilized by a Y-bearing sperm, the resulting zygote with only a Y
chromosome and no X chromosome dies. Rare nondisjunction of the X chromosomes among
white-eyed females therefore produces a few red-eyed males and white eyed
females, which is exactly what Bridges found in his crosses. Bridges’s hypothesis predicted that
the white-eyed females would possess two X chromosomes and one Y and
that red-eyed males would possess a single X chromosome. To verify his hypothesis, Bridges
examined the chromosomes of his flies and found precisely what he predicted.
the appearance of an occasional odd fly in his culture but that he was able to predict a fly’s
chromosomal makeup on the basis of its eye-color genotype. This association between genotype
and chromosomes gave unequivocal evidence that sex-linked genes were located on the X
chromosome and confirmed the chromosome theory of inheritance.
c) X-Linked Color Blindness in Humans
To further examine X-linked inheritance, let’s consider another X-linked characteristic: red–
green color blindness in humans. Within the human eye, color is perceived in
54
light-sensing cone cells that line the retina. Each cone cell contains one of three pigments
capable of absorbing light of
a particular wavelength; one absorbs blue light, a second
absorbs red light, and a third absorbs green light. The human eye actually detects only three
colors—red, green, and blue—but the brain mixes the signals from different
cone cells to create the wide spectrum of colors that we perceive. Each of the three pigments is
encoded by a separate locus; the locus for the blue pigment is found on chromosome 7, and those
for green and red pigments lie close together on the X chromosome. The most common types of
human color blindness are caused by defects of the red and green pigments; we will refer to these
conditions as red–green color blindness. Mutations
that produce defective color vision are
generally recessive
and, because the genes coding for the red and green pigment are located on the X chromosome,
red–green color blindness is inherited as an X-linked recessive characteristic.
We will use the symbol Xc to represent an allele for red–green color blindness and the symbol
X_ to represent an allele for normal color vision. Females possess two
X chromosomes; so there are three possible genotypes among females: X_X_ and X_Xc, which
produce normal vision, and XcXc, which produces color blindness. Males have only a single X
chromosome and two possible genotypes:
X_Y, which produces normal vision, and Xc Y which produces color blindness.
If a color-blind man mates with a woman homozygous for normal color vision, all of the gametes
produced by the woman will contain an allele for normal color vision. Half of the man’s gametes
will receive the X chromosome with the color-blind allele, and the other half will receive the Y
chromosome, which carries no alleles affecting color vision. When an Xc-bearing sperm unites
with the X_-bearing egg, a heterozygous female with normal vision (X_Xc) is produced. When a
Y-bearing sperm unites with the X_-bearing egg, an heterozygous male with normal vision
(X_Y) is produced.
In the reciprocal cross between a color-blind woman and a man with normal color vision the
woman produces only Xc-bearing gametes. The man produces some gametes that contain the X_
chromosome and others that contain the Y chromosome. Males inherit the X chromosome. from
their mothers; because both of the mother’s X chromosomes bear the Xc allele in this case, all
the male offspring will be color blind. In contrast, females inherit an X chromosome from both
parents; thus the female offspring of this reciprocal
cross will all be heterozygous with normal vision. Females are color blind only when color-blind
alleles have been inherited from both parents, whereas a color-blind male need inherit a colorblind allele from his mother only; for this reason, color blindness and most other rare X-linked
recessive characteristics are more common in males.
In these crosses for color blindness, notice that an affected woman passes the X-linked recessive
trait to her sons but not to her daughters, whereas an affected man passes the trait to his
grandsons through his daughters but never to his sons. X-linked recessive characteristics seem to
alternate between the sexes, appearing in females one generation
and in males the next generation; thus, this pattern of inheritance exhibited by X-linked recessive
characteristics is sometimes called crisscross inheritance.
.
d) Symbols for X-Linked Genes
There are several different ways to record genotypes for X-linked traits. Sometimes the
genotypes are recorded in the same fashion as for autosomal characteristics—the
55
hemizygous males are simply given a single allele: the genotype of a female Drosophila with
white eyes would be ww, and the genotype of a white-eyed hemizygous male would be w.
Another method is to include the Y chromosome, designating it with a diagonal slash (/). With
this
method, the white-eyed female’s genotype would still be ww and the white-eyed male’s
genotype would be w/. Perhaps the most useful method is to write the X and Y chromosomes in
the genotype, designating the X-linked alleles with superscripts, as we have done in this chapter.
With this method, a white-eyed female would be XwXw and a white-eyed male XwY. Using Xs
and Ys in the genotype has
the advantage of reminding us that the genes are X linked and that the male must always have a
single allele, inherited from the mother.
e) Recognizing Sex-linked Inheritance
What features should we look for to identify a trait as sex linked? A common misconception is
that any genetic characteristic in which the phenotypes of males and females differ must be sex
linked. In fact, the expression of many autosomal characteristics differs between males and
females. The genes that code for these characteristics are the same in both sexes, but their
expression is influenced by sex hormones. The different sex hormones of males and females
cause the same genes to generate different phenotypes in males and females. Another
misconception is that any characteristic that is found more frequently in one sex is sex linked. A
number of
autosomal traits are expressed more commonly in one sex than in the other, because the
penetrance of the trait differs in the two sexes; these traits are said to be sex influenced.
For some autosomal traits, the penetrance in one sex is so low that the trait is expressed in only
one sex; these traits are said to be sex limited. Several features of sex-linked characteristics
make them easy to recognize. Y-linked traits are found only in males, but this fact does not
guarantee that a trait is Y linked, because some autosomal characteristics are expressed only in
males. A Y-linked trait is unique, however, in that all the male offspring of an affected male will
express the father’s phenotype, provided the penetrance of the trait is 100%. This need not be the
case for autosomal traits that are sex-limited to males. Even when the penetrance is less than
100%, a Y-linked trait can be inherited only from the father’s side of the family. Thus, a Ylinked trait can be inherited only from the paternal grandfather (the father’s father), never from
the maternal grandfather (the mother’s father). X-linked characteristics also exhibit a distinctive
pattern of inheritance. X linkage is a possible explanation when the results of reciprocal crosses
differ. If a characteristic is X linked, a cross between an affected male and an
unaffected female will not give the same results as a cross between an affected female and an
unaffected male. For almost all autosomal characteristics, the results of reciprocal
crosses are the same.We should not conclude, however, that, when the reciprocal crosses give
different results, the characteristic is X linked. Other sex-associated forms of
inheritance, , also produce different results in reciprocal crosses. The key to recognizing
X-linked inheritance is to remember that a male always inherits his X chromosome from his
mother, not from his father. Thus, an X-linked characteristic is not passed directly from father to
son; if a male clearly inherits
56
- Sex chromosomes differ in number and appearance between males and females; other, nonsex
chromosomes are termed autosomes. In organisms with chromosomal sex-determining systems,
the homogametic sex produces gametes that are all identical with regard to sex chromosomes;
the heterogametic sex produces two types of gametes, which differ in their sexchromosome
composition.
- In the XX-XO system, females possess two X chromosomes, and males possess a single X
chromosome.
--In the XX-XY system, females possess two X chromosomes, and males possess a single X and
a single Y chromosome. The X and Y chromosomes are not homologous,
- Sexual reproduction is the production of offspring that are genetically distinct from the parents.
Among diploid eukaryotes, sexual reproduction consists of two processes:
meiosis, which produces haploid gametes, and fertilization, in which gametes unite to produce
diploid zygotes.
- Most organisms have two sexual phenotypes—males and females. Males produce small
gametes; females produce large gametes. The sex of an individual normally refers to the
individual’s sexual phenotype, not its genetic makeup.
- The mechanism by which sex is specified is termed sex determination. Sex may be determined
by differences in specific chromosomes, ploidy level, genotypes, or environment.mechanisms.
The discussion of sex determination lays the foundation for an understanding of sex-linked
inheritance, covered in the last part of the chapter. Because males and females
differ in sex chromosomes, which are not homologous, they do not possess the same number of
alleles at sex-linked loci, and the patterns of inheritance for sex-linked characteristics are
different from those for autosomal characteristics.The chromosome
theory of inheritance, which states that genes are located on chromosomes, was first elucidated
through the study of sex-linked traits. This theory provided the first
clues about the physical basis of heredity
.
- In Drosophila melanogaster, sex is determined by a balance between genes on the X
chromosomes and genes on the autosomes, the X:A ratio. An X:A ratio of 1.0 produces a
female; an X:A ratio of 0.5 produces a male; and an X:A ratio between 1.0 and 0.5 produces an
intersex.
-In humans, sex is ultimately determined by the presence or absence of the SRY gene located on
the Y chromosome.
- Sex-linked characteristics are determined by genes on the sex chromosomes; X-linked
characteristics are encoded by genes on the X chromosome, and Y-linked characteristics are
encoded by genes on the Y chromosome.
- A female inherits X-linked alleles from both parents; a male inherits X-linked alleles from his
female parent only.
. Color blindness in humans is most commonly due to an X-linked
recessive allele. Betty has normal vision, but her mother is color
57
blind. Bill is color blind. If Bill and Betty marry and have a child
together, what is the probability that the child will be color blind?
3. Chickens, like all birds, have ZZ-ZW sex determination. The
bar-feathered phenotype in chickens results from a Z-linked allele
that is dominant over the allele for nonbar feathers. A barred
female is crossed with a nonbarred male. The F1 from this cross
are intercrossed to produce the F2. What will the phenotypes and
their proportions be in the F1 and F2 progeny?
.
. In Drosophila melanogaster, forked bristles are caused by an
allele (Xf) that is X linked and recessive to an allele for normal
bristles (X_). Brown eyes are caused by an allele (b) that is autosomal
and recessive to an allele for red eyes (b_). A female fly that is
homozygous for normal bristles and red eyes mates with a male
fly that has forked bristles and brown eyes. The F1 are intercrossed
to produce the F2. What will the phenotypes and proportions of
the F2 flies be from this cross?
f)Haemophilia ( bleeding disease) :
Haemopilia is an abnormality controlled by a recessive gene located on the X
chromosome. Bleeding from a puncture or an open wound takes an abnormally long time
to stop or fails to stop because clotting of blood would not occur. Small injuries like
puncture , extraction of tooth and the like can cause such persons to bleed to death.
10.2 Relevance of Genetics in disputed paternity
The pattern of blood groups is sometimes used to settle legal cases involving disputed paternity.
The following worked examples illustrates this;
1) Suppose a child is of blood group AB and the mother of blood group A, Can a man of
blood group O be the father of such a child?
Solution:
58
The child of blood group AB will have the genotype IAIB
The mother of blood group A will be of genotype IAIA or IAi
A man of blood group O will have the genotype ii
The possible crossings are:
Phenotypes O x A
Genotypes ii x IAIA or IAi
i)
ii x IAIA
IA
Gametes i i
Fertilization iIA
Phenotypes -
ii)
IA
iIA
iIA
iIA
all of blood group A
ii x IAi
IA
Gametes i i
Fertilization iIA
Phenotypes
A
i
iIA
ii
O
A
ii
O
From these crossings, a man of blood group O cannot produce a child in blood group AB, but
children in blood groups A and O. Hence, it is highly unlikely that the child of blood group AB
is his.
59
2) Two men in blood groups A and O respectively were in dispute over a child in blood
group AB produced from a mother of blood group B. Who is likely to be the father of the
child out of the two men?
Solution:
Blood groups
genotypes
1st Man - A
IAIA
2nd Man-O
ii
Child - AB
IAIB
Mother – B
IBIB or IB i
Possible crossings of 1st man:
Phenotypes A x B
Genotypes IAIA or IAi
i) phenotypes
genotypes
fertilization
offspring
phenotypes
I BI B
x
or IBi
IAIA x IBIB
IA
IA
IAIB
AB
IB
IAIB
AB
IB
IAIB
AB
ii Relevance of Genetics in disputed paternity
60
IAIB
AB
or IAi
The pattern of blood groups is sometimes used to settle legal cases involving disputed paternity.
The following worked examples illustrates this;
3) Suppose a child is of blood group AB and the mother of blood group A, Can a man of
blood group O be the father of such a child?
Solution:
The child of blood group AB will have the genotype IAIB
The mother of blood group A will be of genotype IAIA or IAi
A man of blood group O will have the genotype ii
The possible crossings are:
Phenotypes O x A
Genotypes ii x IAIA or IAi
iii)
ii x IAIA
IA
Gametes i i
Fertilization iIA
Phenotypes -
iv)
IA
iIA
iIA
iIA
all of blood group A
ii x IAi
IA
Gametes i i
Fertilization iIA
Phenotypes
A
i
iIA
ii
O
A
61
ii
O
From these crossings, a man of blood group O cannot produce a child in blood group AB, but
children in blood groups A and O. Hence, it is highly unlikely that the child of blood group AB
is his.
4) Two men in blood groups A and O respectively were in dispute over a child in blood
group AB produced from a mother of blood group B. Who is likely to be the father of the
child out of the two men?
Solution:
Blood groups
genotypes
1st Man - A
IAIA
2nd Man-O
ii
Child - AB
IAIB
Mother – B
IBIB or IB i
a)Possible crossings of 1st man:
Phenotypes A x B
Genotypes IAIA or IAi
x
I BI B
or IBi
i) phenotypes
A x B
genotypes
IAIA x IBIB
gametes
fertilization
IA
IA
IAIB
IB
IAIB
IB
IAIB
62
IAIB
or IAi
offspring
phenotype
ii)
AB
AB
IAIA x IBi
IA
gametes
IA
IB
i
fertilization
IAIB
IAi
IAIB
IAi
offspring
phenotype
AB
A
AB
A
phenotype
A
x
B
genotype
IAi
x
I BI B
i
IB
IB
gametes
iv)
AB
phenotype A x B
genotype
iii)
AB
IA
fertilization
IAIB
IAIB
iIB
iIB
offspring
phenotype
AB
AB
B
B
phenotype
A
x
B
63
genotype
IAi x
IBi
gametes
IA i
IB
fertilization
IAIB
ii
iIB
ii
offspring
phenotype
AB
O
B
O
b)
possible crossings of 2nd man:
phenotype
genotype
i)
i
O x B
I BI B
ii
phenotype
O
x
genotype
ii
x
gametes
i
i
I Bi
or
B
I Bi
IB
i
fertilization
iIB
ii
iIB
ii
offspring
phenotype
B
O
B
O
64
ii)
phenotype
O
x
genotype
ii
gametes
i
B
I BI B
x
IB
i
IB
fertilization
iIB
iIB
iIB
offspring
phenotype
B
B
B
iIB
B
From the results of the possible crossings, it could be seen that the second man in blood group O
is highly unlikely to be the father as his children are either of blood group B or O. The 1st man in
blood group A is likely to be the father as his children are of blood groups A, AB and O.
The table below shows the possible and impossible phenotypes of children in ABO mating
Mating parents
Possible phenotypes
of children
A
X
Impossible phenotypes
of children
A
A and O
B and AB
None
A
X
B
A, B, O and AB
A
X
AB
A, B and AB
A
X
O
A and O
B
X
B
B and O
B
X
AB
O
B and AB
A and AB
A, B and AB
O
65
B
X
O
B and O
A and AB
AB X
AB
AB X
O
A and B
AB and O
O
O
O
A, B and AB
X
A, B and AB
O
Study Questions
1) In Drosophila, yellow body is due to an X-linked gene that is recessive to the gene for gray
body.
(a) A homozygous gray female is crossed with a yellow male. The F1 are intercrossed to produce
F2. Give the genotypes and phenotypes, along with the expected proportions, of the F1 and F2
progeny.
(b) A yellow female is crossed with a gray male. The F1 are intercrossed to produce the F2. Give
the genotypes and phenotypes, along with the expected proportions, of the
F1 and F2 progeny.
(c) A yellow female is crossed with a gray male. The F1 females are backcrossed with gray
males. Give the genotypes and phenotypes, along with the expected proportions, of
the F2 progeny.
(d) If the F2 flies in part b mate randomly, what are the expected phenotypic proportions of flies
in the F3?
2) Both John and Cathy have normal color vision. After 10 years of marriage to John, Cathy
gave birth to a color-blind daughter. John filed for divorce, claiming he is not the
father of the child. Is John justified in his claim of nonpaternity? Explain why. If Cathy had
given birth to a color-blind son, would John be justified in claiming nonpaternity?
3) Red–green color blindness in humans is due to an X-linked recessive gene. A woman whose
father is color blind possesses one eye with normal color vision and one eye with color
blindness.
(a) Propose an explanation for this woman’s vision pattern.
(b) Would it be possible for a man to have one eye with normal color vision and one eye with
color blindness?
.13) Color blindness in humans is most commonly due to an X-linked recessive allele. Betty has
normal vision, but her mother is color blind. Bill is color blind. If Bill and Betty marry and have
a child together, what is the probability that the child will be color blind?
66
4) . Chickens, like all birds, have ZZ-ZW sex determination. The bar-feathered phenotype in
chickens results from a Z-linked allele that is dominant over the allele for nonbar feathers. A
barred female is crossed with a nonbarred male. The F1 from this cross are intercrossed to
produce the F2. What will the phenotypes and their proportions be in the F1 and F2 progeny?
5) In Drosophila melanogaster, forked bristles are caused by an allele (Xf) that is X linked and
recessive to an allele for normal bristles (X_). Brown eyes are caused by an allele (b) that is
autosomal and recessive to an allele for red eyes (b_). A female fly that is
homozygous for normal bristles and red eyes mates with a male fly that has forked ristles and
brown eyes. The F1 are intercrossed to produce the F2. What will the phenotypes and
proportions of the F2 flies be from this cross?
6.) Color blindness in humans is most commonly due to an X-linked recessive allele. Betty has
normal vision, but her mother is color blind. Bill is color blind. If Bill and Betty marry and have
a child together, what is the probability that the child will be color blind?
7) Chickens, like all birds, have ZZ-ZW sex determination. The bar-feathered phenotype in
chickens results from a Z-linked allele that is dominant over the allele for nonbar feathers. A
barred female is crossed with a nonbarred male. The F1 from this cross
are intercrossed to produce the F2. What will the phenotypes and their proportions be in the F1
and F2 progeny?
.
8) In Drosophila melanogaster, forked bristles are caused by an allele (Xf) that is X linked and
recessive to an allele for normal bristles (X_). Brown eyes are caused by an allele (b) that is
autosomal and recessive to an allele for red eyes (b_). A female fly that is
homozygous for normal bristles and red eyes mates with a male fly that has forked bristles and
brown eyes. The F1 are intercrossed to produce the F2. What will the phenotypes and
proportions of the F2 flies be from this cross?
9) What are sex –linked characters? List three examples
10) A woman produced a child of blood group O for her first husband of blood group A and
another child of blood group AB for her second husband of blood group B. what is the woman’s
blood genotype and blood group?
67
WEEK 11 – GENETIC VARIATION
11.1 Genetic variations
Variations are the small differences that exist between individuals. It can be described as being
either discontinuous or continuous
These are characters or traits that can be found amongst various species of organisms. These
variations can either be heritable or non-heritable.
The heritable characters or variations are those that can be passed through the germ line
(sperms or eggs) during the process of sexual reproduction or through attributes passed down
through the soma such as leaves, stems and roots that occasionally serve as reproductive
units(asexual reproduction).
In sexually-reproducing organisms, the sperm, pollen and eggs constitute the germ line
containing the attributes in parents (biological variations) which are subsequently passed on to
the offspring. Thus, the germ lines are the bearers of information that controls the development
of the offspring in the direction of the information content of the zygote which results from the
union of the sperm and the egg.
Developmental errors that do not involve the germ line may be manifested in the parents, but
are not, in general transmissible to the offspring. Such errors and post-natal accidents are referred
to as non-heritable characters e.g. amputation of limbs, or blindness that results from accidents,
weight lifting, a blacksmith or professional skills. A weight lifter or a blacksmith develop extra
muscles but this muscle development cannot be passed on to the offspring unless the offspring is
subjected to the same physical exercises as the parents.
It is to be however noted that the environment does contribute to observable biological
variations. Culture traits such as language and mannerisms are passed on from parents to
offspring, but unlike heritable traits, cultural traits depend on cultural environment rather than
parentage. Thus, cultural traits are also environmentally induced and are not transmitted through
the germ line. Some traits are partly heritable and partly cultural e.g. body movement of a
woman is usually different from that of a man. Part of the difference is due to the muscle
structure of the female and the distribution of sexual organs such as breasts and ovary in the
females, but a large part of the seductive movements is learnt from other females.
11.2 Heritable variations may either be continuous (indiscrete) or discontinuous (discrete)
11.2.1 Continuous or indiscrete variations
These are variations that has intermediary values, which are not exact, i.e. indiscrete.
Such variations are mainly morphological or physical variations. Examples are height, weight,
length, skin pigmentation (colour), colour of eyes, fingerprints, etc.
In continuous variation there is a complete range of measurements from one extreme to the other.
Height is an example of continuous variation - individuals can have a complete range of heights, for
example, 1.6, 1.61, 1.62, 1.625 etc metres high.
68
Other examples of continuous variation include:
• Weight; Hand span; Shoe size; Milk yield in cows
Continuous variation is the combined effect of many genes (known as polygenic inheritance) and
is often significantly affected by environmental influences. Milk yield in cows, for example, is
determined not only by their genetic make-up but is also significantly affected by environmental
factors such as pasture quality and diet, weather, and the comfort of their surroundings.
When plotted as a histogram, these data show a typical bell-shaped normal distribution curve, with
the mean (= average), mode (= biggest value) and median (= central value) all being the same.
FIG.19 a and b Height and weight as examples of continuous variation
69
11.2.2 Discontinuous or discrete variations
These are variations that do not have intermediary values. That is, the values are exact or
discrete. Such variations are mainly physiological variations. Examples are sickling character or
trait, blood group, tasters, tongue rolling, haemophilia, etc
This is where individuals fall into a number of distinct classes or categories, and is based on features
that cannot be measured across a complete range. You either have the characteristic or you don't.
Blood groups are a good example: you are either one blood group or another - you can't be in
between. Such data is called discrete (or categorical) data. Chi-squared statistical calculations work
well in this case.
Discontinuous variation is controlled by alleles of a single gene or a small number of genes. The
environment has little effect on this type of variation.
FIG.20 Blood groups as an example of discontinuous variation
11.3 Causes of Variation and Role of meiosis in causing variation
Variation in the phenotype is caused either by the environment, by genetics, or by a combination of
the two. Meiosis and sexual reproduction introduces variation through Independent assortment of
the parental chromosomes; through Crossing-over during Prophase I; and through the random
fertilization that forms the zygote.
Meiosis is a major source of genetic variation within a population. Though chromosome sets of
the four resulting haploid cells look identical, they may be quite different genetically.
Homologous chromosomes of the original diploid cell may have subtle mutational differences
between their genes, and since each pair of chromosomes separates independently during
Anaphase, different haploid cells may have quite different combinations of genes derived from
the two parents of this haploid organism
70
FIG.21 Meiosis as a major source of genetic variation
71
FIG .22
Cross Over as a source of genetic variation
An additional source of variation lies in the formation and resolution of Chiasmata, because in
this process , homologous chromatids regularly cross over , exchanging segments of their DNA.
Crossing over makes new combinations of genes on a single chromosome.
72
Study Questions:
1) What are genetic variations? State two causes of variation in nature
2) Distinguish with examples between heritable and non heritable characters
3) Explain each of the following terms , giving four examples of each:
i)
Continuous variation
ii)
Discontinuous variation
4) Classify the following list of variations into Continuous and Discontinuous
Variations, using the format below: eye colour, tongue rolling, ability to taste PTC,
height, blood groups, intelligence, mass/ weight, hemophilia
Continuous variation
Discontinuous variation
Size
Sex of a child
5) Explain the role of meiosis in causing variation
73
WEEK 12 -MUTATION
12.1 Mutation
This is a sudden, random alteration in the genotype of an organism, involving qualitative or
quantitative alterations in the genetic material itself. It may involve a single gene (point
mutation) or a whole chromosome (chromosomal aberrations). Mutation may either affect the
gene number or chromosome structure.
In biology, mutations are changes to the nucleotide sequence of the genetic material of an
organism. Mutations can be caused by copying errors in the genetic material during cell division,
by exposure to ultraviolet or ionizing radiation, chemical mutagens, or viruses, or can be induced
by the organism, itself, by cellular processes such as hyper mutation. In multicellular organisms
with dedicated reproductive cells, mutations can be subdivided into germ line mutations, which
can be passed on to descendants through the reproductive cells, and somatic mutations, which
involve cells outside the dedicated reproductive group and which are not transmitted to
descendants, usually. If the organism can reproduce asexually through mechanisms such as
cuttings or budding the distinction can become blurred. For example, plants can sometimes
transmit somatic mutations to their descendants asexually or sexually where flower buds develop
in somatically mutated parts of plants. A new mutation that was not inherited from either parent
is called a de novo mutation. The source of the mutation is unrelated to the consequence,
although the consequences are related to which cells are affected.
Mutations create variation within the gene pool. Less favorable (or deleterious) mutations can be
reduced in frequency in the gene pool by natural selection, while more favorable (beneficial or
advantageous) mutations may accumulate and result in adaptive evolutionary changes. For
example, a butterfly may produce offspring with new mutations. The majority of these mutations
will have no effect; but one might change the color of one of the butterfly's offspring, making it
harder (or easier) for predators to see. If this color change is advantageous, the chance of this
butterfly surviving and producing its own offspring are a little better, and over time the number
of butterflies with this mutation may form a larger percentage of the population.
Neutral mutations are defined as mutations whose effects do not influence the fitness of an
individual. These can accumulate over time due to genetic drift. It is believed that the
overwhelming majority of mutations have no significant effect on an organism's fitness. Also,
DNA repair mechanisms are able to mend most changes before they become permanent
mutations, and many organisms have mechanisms for eliminating otherwise permanently
mutated somatic cells.
Mutation is generally accepted by the scientific community as the mechanism upon which
natural selection acts, providing the advantageous new traits that survive and multiply in
offspring or disadvantageous traits that die out with weaker organisms
74
These changes to the genetic make-up of an individual which cannot be accounted for by the
normal processes (above) may (rarely) involve chromosomal mutations (e.g. Down’s Syndrome,
where an individual has trisomy (= 3 copies) of chromosome 21; they therefore have 47
chromosomes), but more commonly, these mutations are confined to a single gene and are
known as gene mutations. Since DNA replication is not perfect, with an error rate of about 1 in
12
10 bases, we gradually acquire more of these during our lives. Certain chemicals and radiation
increase this error rate, but note that 99% of our DNA is ‘junk DNA’ and so the vast majority of
mutations are unlikely to have any effect whatsoever.
FIG.23
Gene mutation in man
Base deletion means a single base is omitted. This will affect all subsequent amino-acids in
that protein and the effects are therefore likely to be severe.
Base substitution means that one base is replaced by another. This will only affect a single
amino-acid, and normally this has little effect unless the active site of an enzyme is affected.
One example where the effects are dramatic is sickle-cell anaemia, which is caused by a single
amino-acid substitution
Occasionally mistakes occur during DNA replication and protein synthesis. Any alteration in
the structure of a gene results in a mutation. Mutations occur during DNA replication when the
chemical structure of genes undergoes random modifications. Once a change has occurred, the
altered genes continue to replicate in their changed form unless another mutation occurs.
Sometimes mutations occur during transcription or translation, causing protein synthesis to go
75
awry. Although mutations may occur in any living cell, they are most important when they occur
in gametes because then the change affects the traits of following generations.
Most mutations harm an organism. If a mutation occurs in a gene sequence that codes for a
particular protein, the mutation may result in a change in the amino acid sequence directed by the
gene. This change, in turn, may affect the function of the protein. The implications can be
significant: The amino acid sequence distinguishing normal hemoglobin from the altered form of
hemoglobin responsible for sickle-cell anemia differs by only a single amino acid.
Some mutations may be neutral or silent and do not affect the function of a protein. Occasionally
a mutation benefits an organism. Over the course of evolutionary time, however, mutations serve
the crucial role of providing organisms with previously nonexistent proteins. In this way,
mutations are a driving force behind genetic diversity and the rise of new or more competitive
species better able to adapt to changes, such as climate variations, depletion of food sources, or
the emergence of new types of disease (see Evolution).
Mutations can produce a change in any region of a DNA molecule. In a point mutation, for
example, a single nucleotide replaces another nucleotide. Although a point mutation produces a
small change to the DNA sequence, it may cause a change in the amino acid sequence, and thus
the function, of a protein.
Far more serious are mutations that involve the addition or deletion of one or more bases from a
DNA molecule. Adding or subtracting even a single base from a normal sequence during
transcription can disrupt translation by shifting the “reading frame” of every subsequent codon.
For example, an mRNA strand may include two codons in the following sequence: AUG UGA.
The addition of a cytosine base at the beginning of this sequence shifts the “spelling” of these
codons so that they read: CAU GUG. This may result in an incorrect amino acid sequence during
translation, or the protein may be truncated. Known as frameshift mutation, this type of alteration
could result in the production of a protein with no real function or one with a harmful effect.
Sometimes mutations are caused by transposition, in which long stretches of DNA (containing
one or more genes) move from one chromosome to another. These jumping genes, called
transposons, can disrupt transcription and change the type of amino acids inserted into a protein.
Transposons rearrange and interrupt genes in a way that generally improves the genetic variation
of a species.
While mutations can occur spontaneously, some can be caused by exposure to physical or
chemical agents in the environment called mutagens. Common environmental mutagens include
ultraviolet rays from the sun and various chemicals, such as asbestos, cigarette smoke, and
nitrous acid. High-energy radiation, such as medical X rays, can cause DNA strands to break,
leading to the deletion of potentially important genetic information.
Radiation damage can also affect an entire chromosome, disrupting the function of many genes.
In chromosomal translocation, a piece of one chromosome breaks off and merges with another
chromosome. In some cases, large sections of chromosomes may break off and be lost.
76
The cell has highly effective self-repair mechanisms that can correct the harmful changes made
by mutations and prevent some mutations from being passed on. Some 50 specialized enzymes
locate different types of faulty sequences in the DNA and clip out those flaws. Another repair
mechanism scans DNA after replication and marks mismatched base pairs for repair.
. 12.2 Mutations: Types and Causes
The development and function of an organism is in large part controlled by genes. Mutations can
lead to changes in the structure of an encoded protein or to a decrease or complete loss in its
expression. Because a change in the DNA sequence affects all copies of the encoded protein,
mutations can be particularly damaging to a cell or organism. In contrast, any alterations in the
sequences of RNA or protein molecules that occur during their synthesis are less serious because
many copies of each RNA and protein are synthesized.
Geneticists often distinguish between the genotype and phenotype of an organism. Strictly
speaking, the entire set of genes carried by an individual is its genotype, whereas the function
and physical appearance of an individual is referred to as its phenotype. However, the two terms
commonly are used in a more restricted sense: genotype usually denotes whether an individual
carries mutations in a single gene (or a small number of genes), and phenotype denotes the
physical and functional consequences of that genotype.
12.3 Mutations Are Recessive or Dominant
A fundamental genetic difference between organisms is whether their cells carry a single set of
chromosomes or two copies of each chromosome. The former are referred to as haploid; the
latter, as diploid. Many simple unicellular organisms are haploid, whereas complex multicellular
organisms (e.g., fruit flies, mice, humans) are diploid.
Different forms of a gene (e.g., normal and mutant) are referred to as alleles. Since diploid
organisms carry two copies of each gene, they may carry identical alleles, that is, be homozygous
for a gene, or carry different alleles, that is, be heterozygous for a gene. A recessive mutation is
one in which both alleles must be mutant in order for the mutant phenotype to be observed; that
is, the individual must be homozygous for the mutant allele to show the mutant phenotype. In
contrast, the phenotypic consequences of a dominant mutation are observed in a heterozygous
individual carrying one mutant and one normal allele
Recessive mutations inactivate the affected gene and lead to a loss of function. For instance,
recessive mutations may remove part of or all the gene from the chromosome, disrupt expression
of the gene, or alter the structure of the encoded protein, thereby altering its function.
Conversely, dominant mutations often lead to a gain of function. For example, dominant
mutations may increase the activity of a given gene product, confer a new activity on the gene
product, or lead to its inappropriate spatial and temporal expression. Dominant mutations,
however, may be associated with a loss of function. In some cases, two copies of a gene are
required for normal function, so that removing a single copy leads to mutant phenotype. Such
77
genes are referred to as haplo-insufficient. In other cases, mutations in one allele may lead to a
structural change in the protein that interferes with the function of the wild-type protein encoded
by the other allele. These are referred to as dominant negative mutations.
Some alleles can be associated with both a recessive and a dominant phenotype. For instance,
fruit flies heterozygous for the mutant Stubble (Sb) allele have short and stubby body hairs rather
than the normal long, slender hairs; the mutant allele is dominant in this case. In contrast, flies
homozygous for this allele die during development. Thus the recessive phenotype associated
with this allele is lethal, whereas the dominant phenotype is not.
12.4 Inheritance Patterns of Recessive and Dominant Mutations Differ
Recessive and dominant mutations can be distinguished because they exhibit different patterns of
inheritance. To understand why, we need to review the type of cell division that gives rise to
gametes (sperm and egg cells in higher plants and animals). The body (somatic) cells of most
multicellular organisms divide by mitosis whereas the germ cells that give rise to gametes
undergo meiosis. Like body cells, premeiotic germ cells are diploid, containing two of each
morphologic type of chromosome. Because the two members of each such pair of homologous
chromosomes are descended from different parents, their genes are similar but not usually
identical. Single-celled organisms (e.g., the yeast S. cerevisiae) that are diploid at some phase of
their life cycle also undergo meiosis
. One round of DNA replication, which makes the cell 4n, is followed by two separate cell
divisions, yielding four haploid (1n) cells that contain only one chromosome of each homologous
pair. The apportionment, or segregation, of homologous chromosomes to daughter cells during
the first meiotic division is random; that is, the maternally and paternally derived members of
each pair, called homologs, segregate independently, yielding germ cells with different mixes of
paternal and maternal chromosomes. Thus parental characteristics are reassorted randomly into
each new germ cell during meiosis. The number of possible varieties of meiotic segregants is 2n,
where n is the haploid number of chromosomes. In the case of a single chromosome, , meiosis
gives rise to two types of gametes; one type carries the maternal homolog and the other carries
the paternal homolog.
Now, let's see what phenotypes are generated by mating of wild-type individuals with mutants
carrying either a dominant or a recessive mutation., Half the gametes from an individual
heterozygous for a dominant mutation in a particular gene will have the wild-type allele, and half
will have the mutant allele. Since fertilization of female gametes by male gametes occurs
randomly, half the first filial (F1) progeny resulting from the cross between a normal wild-type
individual and a mutant individual carrying a single dominant allele will exhibit the mu-tant
phenotype. In contrast, all the gametes produced by a mutant homozygous for a recessive
mutation will carry the mutant allele. Thus, in a cross between a normal individual and one who
is homozygous for a recessive mutation, none of the F1 progeny will exhibit the mutant
phenotype However, one-fourth of the progeny from parents both heterozygous for a recessive
mutation will show the mutant phenotype.
78
12.5 Mutations Involve Large or Small DNA Alterations
A mutation involving a change in a single base pair, often called a point mutation, or a deletion
of a few base pairs generally affects the function of a single gene. Changes in a single base pair
may produce one of three types of mutation:
- Missense mutation, which results in a protein in which one amino acid is substituted for another
- Nonsense mutation, in which a stop codon replaces an amino acid codon, leading to premature
termination of translation
- Frame shift mutation, which causes a change in the reading frame, leading to introduction of
unrelated amino acids into the protein, generally followed by a stop codon
Small deletions have effects similar to those of frame shift mutations, although one third of these
will be in-frame and result in removal of a small number of contiguous amino acids.
The second major type of mutation involves large-scale changes in chromosome structure and
can affect the functioning of numerous genes, resulting in major phenotypic consequences. Such
chromosomal mutations (or abnormalities) can involve deletion or insertion of several
contiguous genes, inversion of genes on a chromosome, or the exchange of large segments of
DNA between nonhomologous chromosomes).
12.6 Mutations Occur Spontaneously and Can Be Induced
Mutations arise spontaneously at low frequency owing to the chemical instability of purine and
pyrimidine bases and to errors during DNA replication. Natural exposure of an organism to
certain environmental factors, such as ultraviolet light and chemical carcinogens (e.g., aflatoxin
B1), also can cause mutations.
A common cause of spontaneous point mutations is the deamination of cytosine to uracil in the
DNA double helix. Subsequent replication leads to a mutant daughter cell in which a T·A base
pair replaces the wild-type C·G base pair. Another cause of spontaneous mutations is copying
errors during DNA replication. Although replication generally is carried out with high fidelity,
errors occasionally occur. In order to increase the frequency of mutation in experimental
organisms, researchers often treat them with high doses of chemical mutagens or expose them to
ionizing radiation. Mutations arising in response to such treatments are referred to as induced
mutations. Generally, chemical mutagens induce point mutations, whereas ionizing radiation
gives rise to large chromosomal abnormalities.
Ethyl methane sulfonate (EMS), a commonly used mutagen, alkylates guanine in DNA, forming
O6-ethylguanine. During subsequent DNA replication, O6-ethylguanine directs incorporation of
deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in which a G·C
base pair is replaced with an A·T base pair).
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12.7 Some Human Diseases Are Caused by Spontaneous Mutations
Many common human diseases, often devastating in their effects, are due to mutations in single
genes. Genetic diseases arise by spontaneous mutations in germ cells (egg and sperm), which are
transmitted to future generations. For example, sickle-cell anemia, which affects 1 in 500
individuals of African descent, is caused by a single missense mutation at codon 6 of the βglobin gene; as a result of this mutation, the glutamic acid at position 6 in the normal protein is
changed to a valine in the mutant protein. This alteration has a profound effect on hemoglobin,
the oxygen-carrier protein of erythrocytes, which consists of two α-globin and two β-globin
subunits The deoxygenated form of the mutant protein is insoluble in erythrocytes and forms
crystalline arrays. The erythrocytes of affected individuals become rigid and their transit through
capillaries is blocked, causing severe pain and tissue damage. Because the erythrocytes of
heterozygous individuals are resistant to the parasite causing malaria, which is endemic in
Africa, the mutant allele has been maintained. It is not that individuals of African descent are
more likely than others to acquire a mutation causing the sickle-cell defect, but rather the
mutation has been maintained in this population by interbreeding.
Spontaneous mutation in somatic cells (i.e., non-germline body cells) also is an important
mechanism in certain human diseases, including retinoblastoma, which is associated with retinal
tumors in children (see. The hereditary form of retinoblastoma, for example, results from a germline mutation in one Rb allele and a second somatically occurring mutation in the other Rb allele.
When an Rb heterozygous retinal cell undergoes somatic mutation, it is left with no normal
allele; as a result, the cell proliferates in an uncontrolled manner, giving rise to a retinal tumor. A
second form of this disease, called sporadic retinoblastoma, results from two independent
mutations disrupting both Rb alleles Since only one somatic mutation is required for tumor
development in children with hereditary retinoblastoma, it occurs at a much higher frequency
than the sporadic form, which requires acquisition of two independently occurring somatic
mutations. The Rb protein has been shown to play a critical role in controlling cell division).
SUMMARY
- Diploid organisms carry two copies (alleles) of each gene, whereas haploid organisms carry
only one copy.
- Mutations are alterations in DNA sequences that result in changes in the structure of a gene.
Both small and large DNA alterations can occur spontaneously. Treatment with ionizing
radiation or various chemical agents increases the frequency of mutations.
- Recessive mutations lead to a loss of function, which is masked if a normal copy of the gene is
present. For the mutant phenotype to occur, both alleles must carry the mutation.
- Dominant mutations lead to a mutant phenotype in the presence of a normal copy of the gene.
The phenotypes associated with dominant mutations may represent either a loss or a gain of
function.
- In meiosis, a diploid cell undergoes one DNA replication and two cell divisions, yielding four
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haploid cells. The members of each pair of homologous chromosomes segregate independently
during meiosis, leading to the random reassortment of maternal and paternal alleles in the
gametes.
- Dominant and recessive mutations exhibit characteristic segregation patterns in genetic
crosses.
Two classes of mutations are spontaneous mutations (molecular decay) and induced mutations
caused by mutagens
12.8 Induced mutations
Mutagen or mutagenic agent
This is any agent that brings about mutation. It may be physical or chemical.
a) Physical mutagenic agents
These include (1) temperature and (2) radiations. The radiation may be ionized e.g.
x rays and cosmic rays or non-ionizing, e.g. ultraviolet (UV) rays.
b) Chemical mutagenic agents
(1) Reactants – These reacts with the purine and pyrimidine bases, e.g. nitrous
acid, formaldehyde.
(2) Base analogues- These mimics a pyrimidine or purine base, e.g. 5-bromo
uracil (a pyrimidine analog), 2-amino-purine (a purine analogue).
(3) acridine dyes e.g. acridine orange, acridine yellow and proflavin.
(4) Alkylating agents e.g. mustard gases
(5) Carcinogens e.g. methyl cholanthrene
(6) Acids e.g. phenols
Study Questions:
1) What is mutation?
2) Distinguish between the various types of mutation.
3) When is mutation considered to be beneficial?
4) Differentiate, with examples, between physical and chemical mutagens.
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WEEK 13 – GENETIC SYNDROMES
13.1 Down syndrome
This was formerly called Mongolism. It is a syndrome as it consists of a set of
abnormalities. It occurs as a result of nondisjunction (failure of human
chromosomes to separate during meiotic anaphase. People with Down syndrome
have subnormal intelligence and a characteristic facial appearance, including a
folding of the eyelid reminiscent of the epicanthic fold of Asiatic people. A
karyotype of the chromosomes a person with Down syndrome shows that they
have three copies of chromosome 21, so the condition is also known as trisomy 21. Down syndrome children have slight to severe mental retardation.
FIG.24
Down syndrome due to trisomy for chromosome 21
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13.2 Turner syndrome
Persons who have Turner syndrome are female; they do not undergo puberty and their female
secondary sex characteristics remain immature: menstruation is usually absent, breast
development is slight, and pubic hair is sparse. This syndrome is seen in 1 of 3000 female births.
Affected women are frequently short and have a low hairline, a relatively broad chest with
widely spaced nipples, and folds of skin on the neck. Their intelligence is usually normal. Most
women who have Turner syndrome are sterile. In 1959, C. E. Ford used new techniques to study
human chromosomes and discovered that cells from a 14-year-old girl with Turner syndrome had
only a single X chromosome; this chromosome complement is usually referred to as XO with a
karyotype of 45 XO. There are no known cases in which a person is missing both X
chromosomes, an indication that at least one X chromosome is necessary for human
development. Presumably, embryos missing both Xs are spontaneously aborted in the early
stages of development.
FIG.25 Turner syndrome (45 XO)
13.3 Klinefelter syndrome
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Persons who have Klinefelter syndrome, which occurs with a frequency of about 1 in 1000 male
births, have cells with one or more Y chromosomes and multiple X chromosomes. The cells of
most males having this condition are XXY, but cells of a few Klinefelter males are XXXY,
XXXXY, or XXYY. Persons with this condition, though male, frequently have small testes,
some breast enlargement, and reduced facial and pubic hair. They are often taller than normal
and sterile; most have normal intelligence.
FIG.26. Kninefelter syndrome (XXY-47)
Study Questions
:
1) List the three main forms of syndromes found in human chromosomes.
2) How many chromosomes would you expect to find in the karyotypes of persons with Down
syndrome, Turner syndrome and Klinefelter syndrome? What are their sex chromosome
genotypes?
3) Describe the physical characteristics of persons with Down syndrome and Turner syndrome.
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WEEK 14- BIOTECHNOLOGY AND NUCLEIC ACIDS
14.1 Biotechnology
Biotechnology, the manipulation of biological organisms to make products that benefit human
beings. Biotechnology contributes to such diverse areas as food production, waste disposal,
mining, and medicine.
Although biotechnology has existed since ancient times, some of its most dramatic advances
have come in more recent years. Modern achievements include the transferal of a specific gene
from one organism to another (by means of a set of genetic engineering techniques known as
transgenics); the maintenance and growth of genetically uniform plant- and animal-cell cultures,
called clones; and the fusing of different types of cells to produce beneficial medical products
such as monoclonal antibodies, which are designed to attack a specific type of foreign substance
The first achievements in biotechnology were in food production, occurring about 5000 BC.
Diverse strains of plants or animals were hybridized (crossed) to produce greater genetic variety.
The offspring from these crosses were then selectively bred to produce the greatest number of
desirable traits (see Genetics). Repeated cycles of selective breeding produced many present-day
food staples. This method continues to be used in food-production programs.
The modern era of biotechnology had its origin in 1953 when American biochemist James
Watson and British biophysicist Francis Crick presented their double-helix model of DNA. This
was followed by Swiss microbiologist Werner Arber's discovery in the 1960s of special
enzymes, called restriction enzymes, in bacteria. These enzymes cut the DNA strands of any
organism at precise points. In 1973 American geneticist Stanley Cohen and American biochemist
Herbert Boyer removed a specific gene from one bacterium and inserted it into another using
restriction enzymes. This event marked the beginning of recombinant DNA technology,
commonly called genetic engineering. In 1977 genes from other organisms were transferred to
bacteria. This achievement eventually led to the first transfer of a human gene, which coded for a
hormone, to Escherichia coli bacteria. Although the transgenic bacteria (bacteria to which a gene
from a different species has been transferred) could not use the human hormone, they produced it
along with their own normal chemical compounds.
In the 1960s an important project used hybridization followed by selective breeding to increase
food production and quality of wheat and rice crops. American agriculturalist Norman Borlaug,
who spearheaded the program, was awarded the Nobel Peace Prize in 1970 in recognition of the
important contribution that increasing the world's food supply makes to the cause of peace.
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14.2
Nucleic Acids
Nucleic Acids, extremely complex molecules produced by living cells and viruses. Their name
comes from their initial isolation from the nuclei of living cells. Certain nucleic acids, however,
are found not in the cell nucleus but in cell cytoplasm. Nucleic acids have at least two functions:
to pass on hereditary characteristics from one generation to the next, and to trigger the
manufacture of specific proteins. How nucleic acids accomplish these functions is the object of
some of the most intense and promising research currently under way. The nucleic acids are the
fundamental substances of living things, believed by researchers to have first been formed about
3 billion years ago, when the most elementary forms of life began on earth. The origin of the socalled genetic code they carry has been accepted by researchers as being very close in time to the
origin of life itself (see Evolution; Genetics). Biochemists have succeeded in deciphering the
code, that is, determining how the sequence of nucleic acids dictates the structure of proteins.
The two classes of nucleic acids are the deoxyribonucleic acids (DNA) and the ribonucleic acids
(RNA). The backbones of both DNA and RNA molecules are shaped like helical strands. Their
molecular weights (see Molecule) are in the millions. To the backbones are connected a great
number of smaller molecules (side groups) of four different types (see Amino Acids). The
sequence of these molecules on the strand determines the code of the particular nucleic acid. This
code, in turn, signals the cell how to reproduce either a duplicate of itself or the proteins it
requires for survival.
All living cells contain the genetic material DNA. The cells of bacteria may have but one strand
of DNA, but such a strand contains all the information needed by the cell in order to reproduce
an identical offspring. The cells of mammals contain scores of DNA strands grouped together in
chromosomes. In short, the structure of a DNA molecule or combination of DNA molecules
determines the shape, form, and function of the offspring. Some viruses, called retroviruses,
contain only RNA rather than DNA, but viruses in themselves are generally not considered true
living organisms (see Virus).
The pioneering research that revealed the general structure of DNA was performed by the British
biophysicists Francis Crick and Maurice Wilkins and by the American biochemist James Dewey
Watson. Using an X-ray diffraction picture of the DNA molecule obtained by Wilkins in 1951,
Crick and Watson were able to construct a model of the DNA molecule that was completed in
1953. For their work, the three scientists received the 1962 Nobel Prize in physiology or
medicine. The American biochemist Arthur Kornberg synthesized DNA from “off-the-shelf”
substances, for which he was awarded, with the American biochemist Severo Ochoa (for
research on RNA), the 1959 Nobel Prize in physiology or medicine. The DNA that he
synthesized, although structurally similar to natural DNA, was not biologically active. In 1967,
however, Kornberg and a team of researchers at Stanford University succeeded in producing
biologically active DNA from relatively simple chemicals.
Certain kinds of RNA have a slightly different function from that of DNA. They take part in the
actual synthesis of the proteins a cell produces. This is of particular interest to virologists
86
because many viruses reproduce by “forcing” the host cells to manufacture more viruses. The
virus injects its own RNA into the host cell, and the host cell obeys the code of the invading
RNA rather than that of its own. Thus the cell produces proteins that are, in fact, viruses instead
of the proteins required for cell function. The host cell is destroyed, and the newly formed
viruses are free to inject their RNA into other host cells.
The structure of two types of RNA and their function in protein production have been
determined, one type by a team of Cornell University and U.S. Department of Agriculture
investigators led by Robert W. Holley of Cornell, and the other type by James T. Madison and
George A. Everett of the Department of Agriculture. Important research into the interpretation of
the genetic code and its role in protein synthesis was also performed by the Indian-born
American chemist Har Gobind Khorana at the University of Wisconsin Enzyme Institute and the
American biochemist Marshall W. Nierenberg of the National Heart Institute. In 1970 Khorana
achieved the first complete synthesis of a gene and repeated his feat in 1973. Since then one type
of RNA has been synthesized. Also, in the early 1980s, American biochemists Thomas Robert
Cech and Sidney Altman independently proved that certain types of RNA, called ribozymes, can
function as true catalysts (see Catalysis). See also Heredity.
14.3 DNA (Deoxyribonucleic acid)
.
14.3.1 The structure of part of a DNA double helix
Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the
development and functioning of all known living organisms and some viruses. The main role of
DNA molecules is the long-term storage of information. DNA is often compared to a set of
blueprints or a recipe, or a code, since it contains the instructions needed to construct other
components of cells, such as proteins and RNA molecules. The DNA segments that carry this
genetic information are called genes, but other DNA sequences have structural purposes, or are
involved in regulating the use of this genetic information.
Chemically, DNA consists of two long polymers of simple units called nucleotides, with
backbones made of sugars and phosphate groups joined by ester bonds. These two strands run in
opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of
four types of molecules called bases. It is the sequence of these four bases along the backbone
that encodes information. This information is read using the genetic code, which specifies the
sequence of the amino acids within proteins. The code is read by copying stretches of DNA into
the related nucleic acid RNA, in a process called transcription.
Within cells, DNA is organized into structures called chromosomes. These chromosomes are
duplicated before cells divide, in a process called DNA replication. Eukaryotic organisms
(animals, plants, fungi, and protists) store their DNA inside the cell nucleus, while in prokaryotes
(bacteria and archae) it is found in the cell's cytoplasm. Within the chromosomes, chromatin
proteins such as histones compact and organize DNA. These compact structures guide the
interactions between DNA and other proteins, helping control which parts of the DNA are
transcribed.
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14.3.2 Properties of DNA
DNA is a long polymer made from repeating units called nucleotides. The DNA chain is 22 to
26 Ångströms wide (2.2 to 2.6 nanometers), and one nucleotide unit is 3.3 Å (0.33 nm) long.
Although each individual repeating unit is very small, DNA polymers can be enormous
molecules containing millions of nucleotides. For instance, the largest human chromosome,
chromosome number 1, is approximately 220 million base pairs long. In living organisms, DNA
does not usually exist as a single molecule, but instead as a tightly-associated pair of molecules.
These two long strands entwine like vines, in the shape of a double helix. The nucleotide repeats
contain both the segment of the backbone of the molecule, which holds the chain together, and a
base, which interacts with the other DNA strand in the helix. In general, a base linked to a sugar
is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a
nucleotide. If multiple nucleotides are linked together, as in DNA, this polymer is called a
polynucleotide.
The backbone of the DNA strand is made from alternating phosphate and sugar residues. The
sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined
together by phosphate groups that form phosphodiester bonds between the third and fifth carbon
atoms of adjacent sugar rings. These asymmetric bonds mean a strand of DNA has a direction. In
a double helix the direction of the nucleotides in one strand is opposite to their direction in the
other strand. This arrangement of DNA strands is called antiparallel. The asymmetric ends of
DNA strands are referred to as the 5′ (five prime) and 3′ (three prime) ends, with the 5' end being
that with a terminal phosphate group and the 3' end that with a terminal hydroxyl group. One of
the major differences between DNA and RNA is the sugar, with 2-deoxyribose being replaced by
the alternative pentose sugar ribose in RNA
The DNA double helix is stabilized by hydrogen bonds between the bases attached to the two
strands. The four bases found in DNA are adenine (abbreviated A), cytosine (C), guanine (G)
and thymine (T). These four bases are attached to the sugar/phosphate to form the complete
nucleotide, as shown for adenosine monophosphate.
These bases are classified into two types; adenine and guanine are fused five- and six-membered
heterocyclic compounds called purines, while cytosine and thymine are six-membered rings
called pyrimidines.] A fifth pyrimidine base, called uracil (U), usually takes the place of thymine
in RNA and differs from thymine by lacking a methyl group on its ring. Uracil is not usually
found in DNA, occurring only as a breakdown product of cytosine. The double helix is a righthanded spiral. As the DNA strands wind around each other, they leave gaps between each set of
phosphate backbones, revealing the sides of the bases inside (see animation). There are two of
these grooves twisting around the surface of the double helix: one groove, the major groove, is
22 Å wide and the other, the minor groove, is 12 Å wide.[10] The narrowness of the minor groove
means that the edges of the bases are more accessible in the major groove. As a result, proteins
like transcription factors that can bind to specific sequences in double-stranded DNA usually
make contacts to the sides of the bases exposed in the major groove.
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14.3.3 Base pairing in DNA
Each type of base on one strand forms a bond with just one type of base on the other strand. This
is called complementary base pairing. Here, purines form hydrogen bonds to pyrimidines, with A
bonding only to T, and C bonding only to G. This arrangement of two nucleotides binding
together across the double helix is called a base pair. As hydrogen bonds are not covalent, they
can be broken and rejoined relatively easily. The two strands of DNA in a double helix can
therefore be pulled apart like a zipper, either by a mechanical force or high temperature. As a
result of this complementarity, all the information in the double-stranded sequence of a DNA
helix is duplicated on each strand, which is vital in DNA replication. Indeed, this reversible and
specific interaction between complementary base pairs is critical for all the functions of DNA in
living organisms
The two types of base pairs form different numbers of hydrogen bonds, AT forming two
hydrogen bonds, and GC forming three hydrogen bonds (see figures, left). DNA with high GCcontent is more stable than DNA with low GC-content, but contrary to popular belief, this is not
due to the extra hydrogen bond of a GC base pair but rather the contribution of stacking
interactions (hydrogen bonding merely provides specificity of the pairing, not stability). [13] As a
result, it is both the percentage of GC base pairs and the overall length of a DNA double helix
that determine the strength of the association between the two strands of DNA. Long DNA
helices with a high GC content have stronger-interacting strands, while short helices with high
AT content have weaker-interacting strands.[14] In biology, parts of the DNA double helix that
need to separate easily, such as the TATAAT Pribnow box in some promoters, tend to have a
high AT content, making the strands easier to pull apart.[15] In the laboratory, the strength of this
interaction can be measured by finding the temperature required to break the hydrogen bonds,
their melting temperature (also called Tm value). When all the base pairs in a DNA double helix
melt, the strands separate and exist in solution as two entirely independent molecules. These
single-stranded DNA molecules have no single common shape, but some conformations are
more stable than others.
14.3.4 Sense and antisense DNA sequence
A DNA sequence is called "sense" if its sequence is the same as that of a messenger RNA copy
that is translated into protein.[17] The sequence on the opposite strand is called the "antisense"
sequence. Both sense and antisense sequences can exist on different parts of the same strand of
DNA (i.e. both strands contain both sense and antisense sequences). In both prokaryotes and
eukaryotes, antisense RNA sequences are produced, but the functions of these RNAs are not
entirely clear.[18] One proposal is that antisense RNAs are involved in regulating gene expression
through RNA-RNA base pairing.]
A few DNA sequences in prokaryotes and eukaryotes, and more in plasmids and viruses, blur the
distinction between sense and antisense strands by having overlapping genes In these cases,
89
some DNA sequences do double duty, encoding one protein when read along one strand, and a
second protein when read in the opposite direction along the other strand. In bacteria, this
overlap may be involved in the regulation of gene transcription, while in viruses, overlapping
genes increase the amount of information that can be encoded within the small viral genome.
Study Question:
1)
2)
3)
4)
5)
Define Biotechnology.
Name the two types of Nucleic acids.
State the main components of DNA.
Name the four nitrogenous bases found in DNA and show their specific pairing.
What is meant by sense and antisense sequence of DNA?
90
WEEK 15 –GENETIC MANIPULATION TECHNIQUES AND IMPORTANCE
OF BIOTECHNOLOGY
15.1 Gene Manipulation (Genetic Engineering
Genetic engineering can be described as an in vitro manipulation of genes. It refers to
artificial synthesis, modification, removal, addition and repair of the genetic material (DNA) to
alter the genotype at will. It has evoked great interest because it may enable the geneticists in the
near future to correct the disease-causing defective genes for the improvement of human race,
and may be to even create life.
.
Genetic engineering, recombinant DNA technology, genetic modification/manipulation
(GM) and gene splicing are terms that apply to the direct manipulation of an organism's genes.
Genetic engineering is different from traditional breeding, where the organism's genes are
manipulated indirectly; genetic engineering uses the techniques of molecular cloning and
transformation to alter the structure and characteristics of genes directly. Genetic engineering
techniques have found some successes in numerous applications. Some examples are in
improving crop technology, the manufacture of synthetic human insulin through the use of
modified bacteria, the manufacture of erythropoietin in hamster ovary cells, and the production
of new types of experimental mice such as the oncomouse (cancer mouse) for research.
91
Kenyans examining insect-resistant transgenic Bt corn.
15.2 Five steps involved in genetic manipulation
1.
2.
3.
4.
5.
Isolation of the genes of interest
Insertion of the genes into a transfer vector
Transfer of the vector to the organism to be modified
Transformation of the cells of the organism
Separation of the genetically modified organism (GMO) from those that have not been
successfully modified
Isolation is achieved by identifying the gene of interest that the scientist wishes to insert into the
organism, usually using existing knowledge of the various functions of genes. DNA information
can be obtained from cDNA or gDNA libraries, and amplified using PCR techniques. If
necessary, i.e. for insertion of eukaryotic genomic DNA into prokaryotes, further modification
may be carried out such as removal of introns or ligating prokaryotic promoters.
Insertion of a gene into a vector such as a plasmid can be done once the gene of interest is
isolated. Other vectors can
ones such as liposomes, or even direct insertion using gene guns. Restriction enzymes and
ligases are of great use in this crucial step if it is being inserted into
Werner Arber and Hamilton Smith received the 1978 Nobel Prize in Physiology or Medicine for
their isolation of restriction endonucleases.
Once the vector is obtained, it can be used to transform the target organism. Depending on the
vector used, it can be complex or simple. For example, using raw DNA with DNA guns is a
fairly straightforward process but with low success rates, where the DNA is coated with
molecules such as gold and fired directly into a cell. Other more complex methods, such as
bacterial transformation or using viruses as vectors have higher success rates.
After transformation, the GMO can be isolated from those that have failed to take up the vector
in various ways.
92
Although there has been a revolution in the biological sciences in the past twenty years, there is
still a great deal that remains to be discovered. The completion of the sequencing of the human
genome, as well as the genomes of most agriculturally and scientifically important animals and
plants, has increased the possibilities of genetic research immeasurably. Expedient and
inexpensive access to comprehensive genetic data has become a reality with billions of
sequenced nucleotides already online and annotated.
Knockout mice
•
•
•
•
Loss of function experiments, such as in a gene knockout experiment, in which an
organism is engineered to lack the activity of one or more genes. This allows the
experimenter to analyze the defects caused by this mutation, and can be considerably
useful in unearthing the function of a gene. It is used especially frequently in
developmental biology. A knockout experiment involves the creation and manipulation
of a DNA construct in vitro, which, in a simple knockout, consists of a copy of the
desired gene which has been slightly altered such as to cripple its function. The construct
is then taken up by embryonic stem cells, where the engineered copy of the gene replaces
the organism's own gene. These stem cells are injected into blastocysts, which are
implanted into surrogate mothers. Another method, useful in organisms such as
Drosophila (fruitfly), is to induce mutations in a large population and then screen the
progeny for the desired mutation. A similar process can be used in both plants and
prokaryotes.
Gain of function experiments, the logical counterpart of knockouts. These are
sometimes performed in conjunction with knockout experiments to more finely establish
the function of the desired gene. The process is much the same as that in knockout
engineering, except that the construct is designed to increase the function of the gene,
usually by providing extra copies of the gene or inducing synthesis of the protein more
frequently.
Tracking experiments, which seek to gain information about the localization and
interaction of the desired protein. One way to do this is to replace the wild-type gene with
a 'fusion' gene, which is a juxtaposition of the wild-type gene with a reporting element
such as Green Fluorescent Protein (GFP) that will allow easy visualization of the
products of the genetic modification. While this is a useful technique, the manipulation
can destroy the function of the gene, creating secondary effects and possibly calling into
question the results of the experiment. More sophisticated techniques are now in
development that can track protein products without mitigating their function, such as the
addition of small sequences which will serve as binding motifs to monoclonal antibodies.
Expression studies aim to discover where and when specific proteins are produced. In
these experiments the DNA sequence before the DNA that codes for a protein, known as
a gene's promoter is reintroduced into an organism with the protein coding region
93
replaced by a reporter gene such as GFP or an enzyme that catalyzes the production of a
dye. Thus the time and place where a particular protein is produced can be observed.
Expression studies can be taken a step further by altering the promoter to find which
pieces are crucial for the proper expression of the gene and are actually bound by
transcription factor proteins; this process is known as promoter bashing.
15.3 Human genetic engineering
Human genetic engineering can be used to treat genetic disease, but there is a difference between
treating the disease in an individual and in changing the genome that gets passed down to that
person's descendants (germ-line genetic engineering).
Human genetic engineering is already being used on a small scale to allow infertile women with
genetic defects in their mitochondria to have children.[2] Healthy human eggs from a second
mother are used. The child produced this way has genetic information from two mothers and one
father. The changes made are germ line changes and will likely be passed down from generation
to generation, thus are a permanent change to the human genome
Human genetic engineering has the potential to change human beings' appearance, adaptability,
intelligence, character and behaviour. It may potentially be used in creating more dramatic
changes in humans. There are many unresolved ethical issues and concerns surrounding this
technology, and it remains a controversial topic.
Techniques of Genetic Engineering
Much before the advent of modern biotechnology, there were several practices that
were commonly followed. Breeding (hybridisation) of plants and animals to change the genotype
and phenotype of the hybrids is probably the oldest and the most widely used technique of
genetic engineering.
New technique
In the recent years, many modern techniques have been developed which have enabled
scientists to manipulate the genetic material. These include recombinant DNA technology,
transformation and transduction.
. 1 GENETIC ENGINEERING
Genetic engineering enables scientists to produce clones of cells or organisms that contain the
same genes. 1. Scientists use restriction enzymes to isolate a segment of deoxyribonucleic acid
(DNA) that contains a gene of interest—for example, the gene regulating insulin production. 2.
A plasmid removed from a bacterium and treated with the same restriction enzyme binds with
94
the DNA fragment to form a hybrid plasma. 3. The hybrid plasmid is re-inserted back into the
bacterium, where it replicates as part of the cell’s DNA. 4. A large number of identical daughter
cells (clones) can be cultured and their gene products extracted for human use.
Genetic engineering, alteration of an organism's genetic, or hereditary, material to eliminate
undesirable characteristics or to produce desirable new ones. Genetic engineering is used to
increase plant and animal food production; to help dispose of industrial wastes; and to diagnose
disease, improve medical treatment, and produce vaccines and other useful drugs. Included in
genetic engineering techniques are the selective breeding of plants and animals, hybridization
(reproduction between different strains or species), and recombinant deoxyribonucleic acid
(DNA).
In 1977 scientists successfully manipulated bacteria to produce a human protein. That same year
American molecular biologist Walter Gilbert found a way to accelerate dramatically the
laborious task of sequencing the chemicals that make up a strand of genetic material. He shared
the 1980 Nobel Prize in chemistry for this achievement. In a 1980 Scientific American article,
Gilbert and American molecular biologist Lydia Villa-Komaroff describe basic biotechnology
techniques and their laboratory’s success in producing rat insulin with genetically engineered
bacteria. Human insulin was first produced in the lab using recombinant (genetically engineered)
bacteria in 1978, and five years later recombinant human insulin, used to treat diabetes mellitus,
became the first biopharmaceutical on the market.
2 SELECTIVE BREEDING
The first-known genetic engineering technique, still used today, was the selective breeding of
plants and animals, usually for increased food production. In selective breeding, only those
plants or animals with desirable characteristics are chosen for further breeding. Corn has been
selectively bred for increased kernel size and number and for nutritional content for about 7,000
years. More recently, selective breeding of wheat and rice to produce higher yields has helped
supply the world's ever-increasing need for food.
Cattle and pigs were first domesticated about 8,500 to 9,000 years ago and through selective
breeding have become main sources of animal food for humans. Dogs and horses have been
selectively bred for thousands of years for work and recreational purposes, resulting in more than
150 dog breeds and 100 horse breeds.
3
HYBRIDIZATION
Hybridization (crossbreeding) may involve combining different strains of a species (that is,
members of the same species with different characteristics) or members of different species
in an effort to combine the most desirable characteristics of both. For at least 3,000 years,
female horses have been bred with male donkeys to produce mules, and male horses have
been bred with female donkeys to produce hinnies, for use as work animals.
4. Modern Gene manipulation techniques
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•
•
Recombinant DNA technology is a set of techniques for recombinant genes from
different sources and transferring the recombinant DNA into other cells, where it may be
expressed.
Such gene manipulation has sparked an explosion of discoveries in molecular biology
and an industrial revolution in biotechnology
Basic Strategies of Gene Manipulation
Before the advent in 1975 of recombinant DNA technology molecular genetics were
largely limited to studying laboriously obtained mutant organisms. Only phage and
prokaryotic genes were readily transferable from one cell to another, by use of the natural
processes of transformation, conjugation, and transduction.
• Recombinant DNA technology has now made it possible to study virtually any gene in
detail by using one or more of a number of biochemical tools and methods.
• A variety of bacterial restriction enzymes recognize short, specific nucleotide sequences
in DNA and cut the sequences at specific points on both strands to yield a set of double
•
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stranded DNA fragments with single "stick-ends", which other
•
Recombinant DNA DNA molecules can be introduced into host cells by means of
bacteriophage or plasmid vectors. in either case, gene cloning results when the
Foreign genes replicate inside the host bacterium
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bacterium.
•
The two major sources of desired genes for insertion into vectors are
o (a) genomic libraries contain that all the plasmid or phage-carried DNA segments
isolated directly from an organism.
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o
o
(b) complementary DNA synthesized from mRNA templates. The latter source is
especially useful for manipulating and expressing intron-rich eukaryotic DNA.
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•
A useful tool in Genetic engineering is gel electrophoresis. This techinique separates
particles based on size.
•
•
•
The presence of a particular gene can be determined directly, using radioactivity labeled
nucleic acid segments of complementary sequence called probes. Detection can also be
done indirectly by identifying the protein product of the gene.
Machines are now used to synthesize short genes of known nucleotide sequences long
stretches of DNA, using restriction enzymes and gel electrophoresis as key tools. DNA
sequences collected in computer banks allow rapid analysis and automatic translation into
amino acid sequences.
Bacteria and yeast have been successfully used to make the gene products of most
recombinant DNA. However, antibodies, requires the use of cultured animal and plant
cells that posses the requisite complex biosynthetic machinery.
15.4 Applications of Recombinant DNA technology
•
•
Recombinant DNA technology has been a boon for biological research, allowing
investigators to answer questions about molecular evolution, probe details of gene
organization and control, produce and catalog proteins of interest, and map eukaryotic
genes.
The human genome project involves linkage mapping, physical mapping, and sequencing
of the human genome as well as similar analysis of the genome of some sample species.
• Restriction fragment length polymorphisms are differences in DNA sequence on
homologous chromosomes that result in different patterns of restriction fragment lengths,
which are visualized as bands upon gel electrophoresis after treating the DNA fragments
with restriction enzymes. RFLPs can be used as markers for genetic mapping and for
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fingerprinting DNA for medical or forensic purposes.
•
Polymerase chain reaction (PCR) is a technique for quickly amplifying DNA in vitro.
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•
•
•
•
Exciting medical applications of recombinant DNA technology include the large-scale
production of previously scarce pharmaceutical products; the design of safer, more
effective vaccines; the development of diagnostic tests for detecting mutations that cause
genetic disease; and the ultimate prospect of curing and preventing genetic disorders
caused by single defective genes. The last application faces a host of formidable
technological and ethical challenges that have yet to be resolved.
Potential agricultural benefits of genetic engineering include the design of more
productive and disease-resistant plants and animals and improvement in food quality.
Plant cells and amenable to gene manipulation, using the Ti plasmid from Agrobacterium
as the principle vector.
Preliminary results with engineering of single gene traits in plants have created optimism
for increasing crop yields by biotechnology. Increasing biological nitrogen fixation by
gene splicing promises to have tremendous potential for increasing the worlds food
supply.
Safety and Policy Matters
•
When scientists realized that recombinant DNA technology might have some potentially
dangerous consequences, they outlined a set of safety measures that have been
implemented in laboratories across the country.
Several U.S. government agencies are involved in regulating the use of recombinant DNA
technology
15.5 Applications of Biotechnology in Development
Today biotechnology is applied in various fields. In waste management, for example,
biotechnology is used to create new biodegradable materials. One such material is made from the
lactic acid produced during the bacterial fermentation of discarded corn stalks. When individual
lactic acid molecules are joined chemically, they form a material that has the properties of
plastics but is biodegradable. Widespread production of plastic from this material is expected to
become more economically viable in the future
As the number of people donating blood has declined and the number of people needing blood
transfusions has increased, biotechnology researchers have been hard at work seeking substances
that can safely reproduce some of the functions of blood. The most advanced research focuses on
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the oxygen-carrying properties of blood. In this February 1998 Scientific American article,
biotechnologists Mary L. Nucci and Abraham Abuchowski discuss the strategies being pursued
and the challenges that researchers still face in the search for blood substitutes.
Biotechnology also has applications in the mining industry. In its natural state, copper is found
combined with other elements in the mineral chalcopyrite. The bacterium Thiobacillus
ferrooxidans can use the molecules of copper found in chalcopyrite to form the compound
copper sulfate (CuSO4), which, in turn, can be treated chemically to obtain pure copper. This
microbiological mining process is used only with low-grade ores and currently accounts for
about 10 percent of copper production in the United States. The percentage will rise, however, as
conventionally mined high-grade deposits are exhausted. Procedures have also been developed
for the use of bacteria in the mining of zinc, lead, and other metals.
The field of medicine employs some of the most dramatic applications in biotechnology. One
advance came in 1986 with the first significant laboratory production of factor VIII, a bloodclotting protein that is not produced, or has greatly reduced activity, in people who have
hemophilia. As a result of this condition, hemophiliacs are at risk of bleeding to death after
suffering minor cuts or bruises. In this biotechnological procedure, the human gene that codes for
the blood-clotting protein is transferred to hamster cells grown in tissue culture, which then
produce factor VIII for use by hemophiliacs. Factor VIII was approved for commercial
production in 1992.
Study Questiosns:
1) List three traditional methods of breeding
2) Enumerate five steps involved in modern genetic manipulation.
3) List the applications of Biotechnology in development..
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