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LECTURE NOTES
Course: GBT 201; INTRODUCTORY GENETICS (3 Credits /Compulsory)
Course Duration : 15hrs Teaching and 45hrs Practical
Lecturer: BELLO, Omolaran Bashir
Ph.D, M.Sc. (Ilorin), B.Sc. (Ibadan), OND (Computer Studies)
Course: PST 204 Plant Morphology (2 Credits /Compulsory)
Course Duration: 15hrs Teaching and 45hrs Practical
E-mail : [email protected],
obbello [email protected]
[email protected]
Office Location: Department of Biological Sciences
Consultation Hours; 2.30-4.00 pm Monday-Thursdays.
Course Content
1. The Principles of Genetics
2. Mitosis and Meiosis
3. Mendelian Laws
4. Gene Interaction
5. Multiple Factors
6. Multiple alleles
7. Variation in gene expression
8. Linkage and chromosome mapping
9. Gene structure and function
10. Nucleic acid and coding
11. Mutation
12. Introduction to population genetics
 Hardy-Weinberg Law
 Evolutionary Processes and factors
1. Introductory Genetics
Genetics is the branch of biology which studies the origin of biological variations ,
organizations of variations and how these are transmitted from one generation to another or
from parents to offspring. The transmission of genetic materials from parents to offspring is
known as heredity.
Variation is the difference which may distinguish one from the other.
Types of Variation
1. Heritable Variation: exists in the parents and passed on to the offspring. In sexual
reproducing organisms, sperm, pollen grains , egg and ovum constitute the germ line in
containing attributes of parents that are passed on to the offspring e.g. diabetes milletus
(although can be controlled by the admission of insulin) is heritable. Others examples include
height, size, color, texture etc.
2. Non-heritable variation: are developmental errors which cannot be passed on to the
offspring from their parent e.g. post-natal accident such as blindness, or amputation of part of
the body. Partial heritable variations are heritable or non-heritable and are acquired from d
environment.
a. Morphological Variation .i.e. physical appearance induced by the environment .e.g. albinism
is induced in plants by raising them in the dark because it prevents chlorophyll formation.
The situation is reserved once such plants are allowed to receive rays of sun.
b. Muscle Development by weightlifters, wrestlers, boxers.
c. Differences between the yolk of poultry raised in battery cages and those raised in free range.
These raised in free range have more carotenoid. (Color pigment in plant organism).
d. Behavioral Tendencies which may be peculiar to group people e.g. cultural traits and
mannerism which are passed on to the offspring, also known as acquired characteristics.
3. Partly heritable and partly cultural includes body movement, Characteristics of the
female which differ from that of the male. This difference is partly due to the muscle
structure of the female and distribution of their sexual organs. However much of movement is
still learnt from others .i.e. learning from pairs.
Principles of Genetics
a..The site where genes work is the cell, each cells may have different functions
b.. Each cell function within an organism is determined by the genetic information encoded in
DNA.
c.. In Eukaryotes (Organisms whose cells contain a nucleus) DNA reside within membrane
bound structure in the cell. These structures include the nucleus, the energy producing
mitochondria and in plants, the chloroplasts (structure where photosynthesis takes place).
d.. In Prokaryotes(One cell Organism) and bacteria that lack internal membrane bound
structure, DNA floats freely within the cell body.
Importance of Genetics
The modern science of genetics influences many aspects of daily life from the food we eat to
how we identify criminals or treat diseases
1. In agriculture, genetic advances enable scientists to alter a plant or animal to make it more
useful. For instance, some food crops such as oranges, potatoes, Wheat and rice have been
genetically altered to withstand insect, pest and diseases resulting into higher crop yield.
Tomatoes and apples have being modified to resist discoloration and their way to the market.
Their genetic make-up of cattle (cows) have been modified to increase their milk production
and cattle raised for beef have been altered so that they grow faster.
2. Genetic technologies have also helped to convict criminals, DNA recovered from the semen
(sperm), blood, skin cells or hair found at a criminal scene can be analyzed in the laboratory
and compared with the DNA of the suspect. DNA match can also be used in courtroom as
evidence connecting a person to a crime or ownership of a child.
3. In medicine, scientists can generally alter bacteria so that they mass produce specific proteins
such as insulin used by diabetes patient or human growth hormone used by children who
suffer from growth disorder
4. In gene therapy, scientists try to cure disease by replacing malfunctioning genes with healthy
ones e.g. cancer, fibrosis, e.t.c Genetically engineered vaccines being tested for possible use
against the Human Immunodeficiency Virus (HIV). The virus that causes Acquired
Immunodeficiency Syndrome (AIDS).
5. Human Genome Project; Scientists have developed detailed maps that identified the
chromosomal locations of the estimated 20,000-25,000 human genes. The data bases help
scientists study previously unknown genes as well as many genes all at once to examine how
gene activity can cause disease. The scientists expected that their project would lead to the
development of new drugs targeted to specific disorders.
1. 1 Cell division
This the replication of cells for the growth and development the reproduction during
because of the cell of organism could not grow or function properly if the genetic information
encoded in the DNA was not passed from cell to cell. DNA is packed into structures called
chromosomes within a cell. Every chromosome in cell contains many genes, and each gene is
located at a particular site, or locus on the chromosomes. Chromosomes vary in size and
shape and usually occur in matched pairs called homologues. The number of homologues
chromosomes in cell depends upon the organism e.g. human body contains 23 pairs of
chromosomes, maize: 20, Fruitfly: 4, onion: 16 e.t.c.
Organisms use two types of cell division to ensure that DNA is passed down from cell to cell
during reprod uction. Asexual reproduction is by mitosis, a cell doubles its DNA before
dividing into cells and distributing the DNA evenly to each resulting cell. Organism that
reproduces sexually is by meiosis. The garment or egg and sperm (germline) The
chromosome in a gamete cell are reduced by half. During sexual reproduction, an egg and
sperm unite to form a zygote in which the full number of chromosome is restored.
1.2 Cell cycle
This is entire sequence of events happening from the end of one nuclear division to the
beginning of the next. It is divided into four stages namely G,S and G2 (Interphase) and M
(mitosis and cytokinesis) G1:After the N phase of previous cell cycle, the daughter cells
begins G1 of inter-phase of new cell cycle. It is the resting stage and involves in synthesis of
RNA and proteins.
S1: Period of DNA synthesis
G2: Period between the end of DNA synthesis and the beginning of the prophase
M:Is the period of chromosome division.
1.
2.
3.
4.
2.0 MITOSIS
This is conventionally divided into four stages
Prophase
Metaphase
Anaphase
Telophase
Overlapping with the latter stages of mitosis, cytokinesis completes the mitotic phase. Interphase which is a resting stage is the centre of intercellular activities such as energy
transformation, synthesis of protein, oil, starch e.t.c and also synthesis of genetic material and
duplication.
Mitosis can be defined as process by which the nucleus divides into two daughter nuclei each
with equivalent chromosome complement.
DIAGRAMS
1 Prophase
The chromatic fibres become more tightly coiled condensing into discrete chromosome
Observable with light microscope. The nucleoli disappear. Each duplicate chromosome
appears as two Identical sister chromatids joined together at their centromeres and all along
their arms by cohesion. The mitotic spindle begins to form. It is composed of the centrosome
and micro tubules that extend from them. The radial arrays of shorter microtubules that
extend from the centrosome are called asters (stars). The centrosome move away from each
other<apparently propelled by d leveling microtubules
between them.
The microtubules extending from each chromosome can now invade the nuclear eave
chromosome. The chromosome have become even more condensed, each of the two
chromatids of each chromosome now has a kinetochore,a special protein structure located at
the centromere. Some of the microtubule attached to the kinetochore becoming kinetochore
microtubules. This jack the chromosome back and front. Non kinetochore microtubule
interacts with those from the opposite pole of the spindle.
Early Prophase
Late Prophase
2 Metaphase
METAPHASE: This is the longest stage of mitosis< is the centromeres are now at the
opposite pole. The chromosome convenes on the metaphase plate and an imaginary plane that
is equidistance between the spindles two poles. The chromosome centromere lies on the
metaphase plate for each chromosome, the kinetochore of the sister chromatids are attached to
kinetochore microtubule coming from opposite pole.
3 Anaphase
It is the shortest stage of mitosis. it begins when the cohesion protein are leaved, these allow
the two sister chromatids of each pair to part suddenly. Each chromatids therefore becomes a
full fledged chromosome. The two liberated chromosome begins moving two opposite ends of
the cell as their kinetochore microtubule shortens. Because this microtubules are attached at
the centromere region, the chromosome makes centromere first. The cell elongate as the non
kinetochore microtubule lengthen. By the end of anaphase two ends of cell have equivalent
and complete collection of chromosome
Early Anaphase
Late Anaphase
4 Telophase
The two daughter nuclei formed in the cell nucleus envelops arrived from the fragment of the
parent cell. Sex nuclei envelop another portion of endo - membrane system. The chromosome
becomes less condensed. Mitosis, the division of one nucleus into two genetically identical
nuclei is now complete.
0 CYTOKINESIS
The division of the cytoplasm is usually well underway by late telophase so the two daughter
cell appear shortly after the end of mitosis. In animals cytokinesis involved the formation of
cleavage furrow which pitches the cell into two.
Two new "daughter cells" - the cycle is about to repeat itself.
1.
2.
3.
4.
5.
6.
7.
THE SIGNIFICANCE OF MITOSIS
It helps the cell to maintain proper size
It helps in maintaining an equilibrium in the amount of DNA and RNA in the cell
It provides the opportunity for the growth and development to organ and the body of the
organism
The old decaying and dead cell of the body are replaced
In certain organism, the mitosis is involved in asexual reproduction
The gonad and the sex cell depend on the mitosis for the increase in their number.
The cleavage of egg during embryogenesis involved mitosis.
2. 1 MEIOSIS
It can be defined as a process by which the chromosome in an organism is reduced into half
of its complement in the process of gamete formation. There are distinct phases namely:
reductional and equational division or meiosis1 and meiosis2 respectively. Meiosis1 can be
divided into stages with their first five i.e. (leptotene, zygotene, pachytene, diplotene,
diakinesis) Stages been referred to as prophase1. The other three stages are metaphase1,
anaphase1 and telophase1. Prophase1 is the longest stage of meiotic division.
LEPTOTENE (LEPTONEMA): The pro leptotene stage closely resembles with the early
mitotic prophase.in this stage the chromosome are extremely thin, long, uncoiled,
longitudinally single and slender threadlike structures. In the leptotene stage the
chromosomes at this stage becomes uncoiled and assumes a threadlike shape. The
chromosomes at this stage take out a` specific orientation inside the nucleus the end of the
chromosomes converge towards one side of the nucleus that side where the centromeres lies.
The centrioles duplicate each daughter centrioles migrate towards the opposite pole of the
cell. The centrioles duplicate and therefore each pole of the cell possess two centrioles of a
single diplosome.
ZYGOTENE(ZYGONEMA): the pairing of homologous chromosomes takes place. The
homologous chromosomes which come from the mother (by ova) and father(by sperm) are
attracted towards each other and there pairing takes place known as synapses which are exact
and specific. The paired homologous chromosomes are formed by synaptonema complex
(sc). This complex extends along the whole length of the paired chromosome and is usually
anchored at either end of the nuclei envelope. S.c helps to stabilize the pairing of homologous
chromosomes and to facilitate cytogenetical activity called bi combination or crossing over
(occurring during pachytene)
PACHYTENE: the pair of chromosomes becomes crested spirally around each other and can
not be distinguished separately. Pairing of homologous chromosome is completed and they
now form bivalent or petards. The number of bivalent is equal to the haploid number of
chromosomes. A change of parts between homologous chromosome also takes place during
pachytene(crossing over). The chromosomes nucleoli and nuclei organizers are very distinct
and the forces of paring which appears during zygotene start disappearing.
DIPLOTENE: in diplotene, unpairing or desynapses of homologous chromosomes is
started and chiasmata are first seen. The chromatids of each tetrad are usually clearly visible
but the synaptonema complex appeared to be dissolved, living participating chromatids of
they paired homologous chromosomes physically joined at one or more discrete point called
chiasmata. This point are here crossing over take place. Often there is some unfolding of the
chromatids at this stage, allowing for RNA synthesis and cellular growth.
DIAKINESIS: in diakinesis, the bivalent chromosomes become more condensed and evenly
distributed in the nucleus. The nucleus detaches from the organizer and ultimately
disappears. The nuclear envelop breaks down. the chiasma move from the centromere
towards the end of the chromosome to the intermediate chiasmata diminish. This type of
movement of chiasmata is known as Terminalization. The chromatids remain connected by
the terminal chiasmata and this exist up to the metaphase.
PROMETAPHASE:
in the prometaphase, the nuclear envelope disintegrates to the microtubules get arranged in
the form of spindle in the two centrioles which occupy the position of two opposite poles of
the cells. The chromosome become greatly coiled in the spiral manner and get arranged on
the equator of the spindle.
METAPHASE I:
It consists of spindle fibre attachment to chromosomes chromosomal alignment of the
equator. The microtubules are attached with the centromeres of homologous chromosome of
each tetrad. The centromere of each chromosome is directed towards the opposite poles. The
repulsive forces between the homologous chromosome increasingly greatly and the
chromosome become ready to separate.
ANAPHASE I
The homologous separate and move apart with the centromere leading the way to the poles.
This process completes the halving of the chromosome number since it’s the whole
chromosome(and not chromatids) each containing sister chromatids.
TELOPHASE I
The chromosome movement has stopped as they are now separated in different cells. As only
one partners of homologous pair goes to one pole, only half of the somatic number of
chromosome reaches one pole.
EQUATIONAL PHASE (MEIOSIS II):
This phase is essentially similar to mitosis in which the sister chromatids now separate and
enter into different cells.
PROPHASE II:
The chromosome becomes visible as tiny thread like structures in the cells. They continue to
condense and thickened. The phase is short lived.
METAPHASE II:
Each chromosome now appear doubled and is joined together only at the centromeres. The
spindle are organized on each side of the region as the chromosome align the muscles.
ANAPHASE II:
Each chromosome divides equationally producing 2 daughter chromosome each of which
now moves in opposite direction.
TELOPHASE II:
The separation of the chromatids occurred. Four groups of chromosome are organized.
3. MENDELIAN LAW
-
Principles of segregation: states that the two alleles of a character in the first filia generation
(F1) do not fuse together but exist independently of each other and pass on to the next
generation without been influenced by each other.
-
Principle of independent assortment states that separation of one gene pair occurs
independent of any other pair.
4. GENE INTERACTION
Gene interaction is a condition where a single character is governed by 2 or more genes and
every gene affects the expression of other genes involved.
Gene Interaction
Supplementary gene interaction
Epistasis
Duplicate Factor
Inhibitory Factor
Polymerism or Addictive Factor
F2 Ratio
9:3:4
12:3:1
15:1
13:1
9:6:1
Test Crossed Ratio
1:1:2
2:1:1
3:1
1:3
1:2:1
Intra-allelic: This is when one allele affects the expression of another allele in the same gene
locus. It’s a measure of how common an allele is in the proportion of all allele at one gene
locus.
Complete Dominance: is when one allele is completely dominant over the other allele, and
there are no intermediate phenotypes e.g. eye color
Co-Dominance: is a form of inheritance in which both alleles are equally shown e.g. A, B
blood is the co-dominant relationship between A protein & B protein both expressing
themselves completely AO ( type O allele means there is no protein), A is dominant and O is
recessive. B.O is also the same.
Co-dominance Working Example A B O Blood group
Human blood is classified into four groups based on the antigen on the surface of the blood
cell. An antigen is a protein that acts as the signal, enabling the body to recognize foreign
substances that enters the protein. When foreign substances enter the body, the antigen
responds by producing antibody i.e. antigen means antibody producing substance. Human
blood types are controlled by 3 allele ‘IA ,IB and I’ the allele IA and IB are co-dominant (both
are expressed together) and both are dominant to ‘I’ allele.
BLOOD TYPE
ANTIGEN
GENOTYPE
A
A
Ia ia or IAi
B
B
Ib iB or Ibi
Ab
A &b
IA ib
O
None
Ii
1. A woman with type O blood and a man who is type AB are expecting a child. What are the
possible blood types of the kid.
2. What are the possible blood types of a child whose parent are both heterozygous for B blood
type.
3. What are the chances of a woman with blood type AB & a man with type A having a child
type O.
4. Determine the possible genotype and phenotype with respect to blood group for a couple
whose blood group are homozygous A and heterozygous B.
5. Ajibise is blood type O, she has two elder brother with blood type A & B, what are the
genotype of her parents with respect to this traits.
6. A test was done to determine the biological father of a child. The child blood type is A and
the mother is B. Ridwan has a blood type O and alfa have a blood type AB. Who is the
biological father.
Note
A donate to A only, A accept from A & O
B donate to B and B accept from B & O
AB donate to AB, AB accepts from A,B,AB,O
O donate to A,B,AB,O, o accepts from O only.
Solution
1. Woman (O) - i i
Man (AB) - IAIB
2 IA i : 2IBi
1
:
50%
i i
1
50%
IA i
IB i
IA
IB
IA i
IB i
Incomplete Dominance: is a form of inheritance in which the heterozygous alleles are both
expressed, resulting in a combine phenotype e.g. ‘A’ red and a white allele gives pink.
Incomplete dominance is most commonly found in plants.
Factors Affecting Gene Interaction
i.
Penetrance: is how often a gene is expressed i.e. percentage of people who have the
gene and who develop the corresponding phenotype. A gene with incomplete (no penetrance)
may not be expressed even when the trait is dominant or when it is recessive and the gene
responsible for that trait is present on both chromosome. Penetrance of the same gene may
vary from person’s age even when the abnormal allele is not expressed (non penetrance) The
unaffected carrier of the abnormal allele can pass it to children who may have clinical
abnormalities.
ii.
Expressivity: is the extent to which a gene is expressed in one person. It can be graded
at a percentage e.g. when a gene have 50% expressivity, only half of the features are present
or the severity is only half of what can occur with full expression. Expressivity may be
influenced by the environment and by other genes so, people with the same gene may vary in
phenotype, expressivity can vary among the member of the same family.
iii.
Sex Limited Inheritance: A trait that appear in only one sex. It also refers to special
cases in which sex hormone and other physiological differences between male and female
alters the expressivity and penetrance of a gene e.g. premature baldness is an autosomal
dominant trait but such baldness is rarely expressed in female and then usually only after
menopause.
iv.
Genomic Imprinting: is the differential expression of genetic material depending on
whether it has been inherited from the father or mother. However, in less than 1% of allele,
expression is possible only from the paternal or maternal allele e.g. expression of gene for
insulin-like growth factor. It is normally expressed only from paternal allele.
v.
vi.
Co-dominance
Chromosomal inactivation: In females who have more than one X chromosome (except
in all but one of the X chromosome is inactivated; i.e. most of the allele on that chromosome
are not expressed. Inactivation occurs individually in each cell early in foetal life. Sometime
it is the X from the other that is inactivated or from the father. However, some of the allele
from inactive ‘X’ chromosome do express. Many of these alleles are on chromosomal region
corresponding to regions of the Y chromosome.
Multiple Factor or Multiple Gene
Multiple factor or multiple genes is two or more pair of allelic gene that acts as unit and
affect a part of a person’s characteristics. A gene that individually exacts a slight effect and
phenotype but along with few or many other gene that control quantitative trait is called
polygene or multiple factor.
a.
b.
c.
d.
e.
f.
Characteristics of Multiple Genes
Each contributing allele in series of multiple genes produce an equal effect.
Effect of each contributing allele are cumulative/additive
There is no dominant, rather there exist pairs of contributing alleles.
There are no epitasis (masking of the phenotype) of different loci.
The environmental condition have a considerable effect on the phenotypic expression of
polygene of the quantitative traits.
There is no linkage.
g.
The genotype determines the range at which an individual will occupy with regards to a
given quantitative character.
5. MULTIPLE ALLELES
Multiple alleles (Multiple factors) is a type of non-Mendelian inheritance pattern that
involves more than just the typical two alleles that usually code for a certain characteristic in
a species. With multiple alleles, that means there is more than two phenotypes available
depending on the dominant or recessive alleles that are available in the trait and the
dominance pattern the individual alleles follow when combined together.
Most of the time, when multiple alleles come into play for a trait, there is a mix of types of
dominance patterns that occur. Sometimes, one of the alleles is completely recessive to the
others and will be masked by any of those that are dominant to it. Other alleles may be codominant together and show their traits equally in the phenotype of the individual. There are
also some cases where some alleles exhibit incomplete dominance when put together in the
genotype. An individual with this type of inheritance connected to its multiple alleles will
show a blended phenotype that mixes both of the alleles' traits together.
Characteristics of multiple alleles
Multiple alleles are always at the same locus on the chromosome.
,,, They always affect the same characters.

wild type allele is nearly always dominant to all others at the same locus. It is also
possible that they may have intermediate phenotypic expressions when two different alleles
are brought together in the same genotype.

When any two multiple alleles are crossed, the phenotype is of a mutant character and
not of the wild type.

Multiple alleles control the same character. But each of them is characterized by
different manifestations.
Significance of multiple alleles:

The knowledge of the existence of allelic genes which act together modifying the
effects of dominant gene is of great value in artificial breeding for producing varied varieties.

The knowledge of alleles throws some light on the nature of genes.

Laws of segregation and recombination of characters are based upon different alleles.

Multiple alleles suggest that a gene can mutate in different ways.
6. VARIATION IN GENE EXPRESSION
GENE EXPRESSION
This is the process by which the information contained in the gene is converted into
molecules that determine the properties of cells and viruses. The transfer of genetic
information from DNA into protein constitutes the gene expression. The information transfer
is accomplished by a series of events in which the sequence of the bases in the DNA is first
copied into an RNA molecule and then, the RNA is used either directly or after some
chemical modification, to determine the amino acid sequence of a protein molecule. The
main ingredients necessary for complete translation are as follows;
1. Messenger RNA (mRNA): mRNA is needed to bring the ribosomal subunits together and to
provide the coding sequence of bases that the determines the amino acid sequence in the
resulting polypeptide chain.
2. Ribosomes: These components are particles in which protein synthesis takes place. They
move along an mRNA molecule and align successive transfer-DNA molecules. The amino
acids are attached one by one to the growing polypeptide chain by means of peptide bond.
Ribosome consist of two subunit particles- in E.coli, there sizes are 30S(the small subunit)
and 50S (the large subunit), the counterparts in eukaryotes are 40S and 60S.(The ‘S’ stands
for Svedberg unit which measures the rate of sedimentation of a particle in a centrifuge and
so it is an indicator of size). Together, the small and large particle forms functional
ribosome.
3. Transfer RNA(tRNA): The sequence of amino acid in polypeptide is determined by the
sequence in the mRNA by means of a set of adaptor molecules, the tRNA molecules, each of
which is attached to a particular amino acid. Each group of the three adjacent bases in the
mRNA forms a codon that binds to a particular group of three adjacent bases in the tRNA(an
anticodon), bringing the attached amino acid into lone for addition to the growing
polypeptide chain.
4. Amino acyl tRNA synthesis: this set of enzymes catalyzes the attachment of each amino
acid to its corresponding tRNA molecules or a changed tRNA.
5. Initiation, elongation and termination factors; polypeptides synthesis can be divided into
three stages-(i) initiation (ii) elongation (iii) termination, each of which requires specialized
proteins.
FACTORS AFFECTING GENE EXPRESSION
Over expression of genes involved in the control of rate of biosynthesis can increase oil
content. In some cases, introduction of additional copies of endogenous gene may interfere
with gene expression, thereby, rather than enhancing the gene product. This phenomenon is
called co- suppression, lowering gene expression by the use of anti-sense RNA which has
been successful in reduction of poly galactouronase levels in tomatoes.
7. LINKAGE CROSSOVER AND GENETIC MAPPING
Gene in the same chromosome will not assort independently and so mendel second law is not
universal but its limited to gene in different chromosome. Therefore, gene which reside in the
same chromosome are said to be limited. Such linkage may occur on one of the chromosome
or connected on the sex chromosome. For instance when two alleles AB come form the same
parent (AA/BB x aa/bb), they tend to enter different gamete and remain apart. The first is
known as repulsion.
Genetic mapping
WORKED EXAMPLE: In drosophila, round eye (ec+) bristle (sc+) and crossed veins(cv+) are
dominant over their respective allele, rough eye(ec), scute (sc) and cross veinless (cv), the 3
characters are sex linked. A drosophila of genotype ec+/sc+/cv+ is matedto another type of
genotype ec/sc/cv and the F1 female test crossed to a male recessive for the 3 character. The
following result were obtained.
Sc+/ec/cv+ = 810
Sc/ec+/cv+ =89
Sc/ec+/cv =828
Sc+/ec/cv =103
Sc/ec/cv+ =62
Sc+/ec+/cv+=0
Sc+/ec+/cv =88
Sc/ec/cv =0 .
Total = 1980
In other to construct d genetic map for the 3 genes, we need
1. Determine the linear order of the gene chromosome
2. Determine both single crossover (SCO) and double crossover(DCO) recombinant
3. use the information so obtain to calculate the relative distance between each gene
4. use the result to construct the genetic map
For this particular example, we need to note the most frequent are the parental. The
least frequent are the DCO recombinant. The other are the SCO recombinant
THE LINEAR ORDER OF THE GENE
In other to determine the linear order of the genes, it is necessary to look for the arrangement
of the genes in the parental (i.e. the 2 classes that are excess) that gives the DCO
recombinant(i.e. the least frequent classes). The example above the parental are 810 and 828,
sc+/ec/cv+ and Sc/ec+/cv, while the DCO are Sc+/ec+/cu+ and Sc/ec/cv therefore we look for
the parental that gives the 2 DCO recombinants.
Since this arrangement give the 2 least frequent DCO, we therefore accept this arrangement
as the correct order of the gene on the chromosome
DETERMINATION OF SCO CLASSES
In other to determine the SCO between each gene, we determine the single crossover classes
using the arrangement of the parental
Single crossover Sc+ and ec+
Sc+ ec cv+
Sc+/ec+/cv =88
X
=
+
Sc ec cv
Sc/ec/cv+ = 62
150
+
+
Distance between ec and cv
Sc+ ec cv+
Sc+/ec/cv =103
X
=
+
Sc ec cv
Sc/ec+/cv+ = 89
192
Calculating the distance between the gene
The distance between two genes is calculated by using the formular [(SCO+DCO) Total] X
100
[(88+62+0+0) 1980] X 100=7.6 or 7.6map unit
ec+ and cv+
[103+89+0+0) 1980] X 100
[192 1980] X 100= 9.7 oe 9.7 map unit
Construction Of Genetic Map
The map is then constructed by drawing straight line indicating the distance between each
gene. However, the map should be drawn to scale.
Example 2: in maize non color booster (b) legaless (lg) and viresent (v) are recessive to their
respective wild type (dominant). The F1 (heterozygous for all 3 characters) was test crossed
to the homozygous single parent. The resultant progeny wave in the following proportions.
What needs to be done is to move one of the outer gene into the middle
INTERFERENCE
If a cross over in one region of a chromosome does interfere with the crossover in the second
region, the expected frequencies of the crossover is equal to the product the single crossover.
Coefficient of coincidence measures the degree to which observe crossovers falls short od the
expected. In this maize example however, expected frequencies of DCO is equal to
0.18*0.28=0.0504=0.05.
Observe frequency of DCO is equal 0.18*0.22=0.04. Coefficient of coincidence (CI) is
calculated by the formula
If the crossover in one region does not interfere with another region, C.I
complete interference give C.I = 0, C.I =0.8= partial.
, Therefore
DEOXYRIBONUCLEIC ACID
DNA is the genetic material of all cellular organism and most of viruses. DNA carries the
info needed to direct protein synthesis and replication. Protein synthesis is the production of
the proteins needed by the cell or virus for its activities and development. Replication is the
process by which DNA copies itself for each descendant cell or viruses passing on the
information needed for protein synthesis. In most cellular organism, DNA is organized on
chromosome located in the nucleus of the cell.
DNA Structure
A molecule of DNA consist of two chain strands composed of a large number of chemical
compounds called Nucleotide, Linked together to form a chain. These chains are arranged
like ladder that has been twisted into the shape of a winding staircase called a double helix,
each nucleotide consist of 3 unit; a sugar molecule called deoxyribose, a phosphate group and
one of four different nitrogen contain compounds called bases. The bases are; Adenine (A),
Guanine (G), Thymine (T), Cytosine (C). deoxyribose molecule occupy the centre position in
the nucleotide flanked by a phosphate group on one side and a base on the other. The
phosphate group of each nucleotide is also linked to the deoxyribose of the adjacent
nucleotide in the chain. These linked deoxyribose PO4 subunit form the parallel side rail of
the ladder, the bases face inward towards each other, forming the rungs of the ladder.
The nucleotide in one DNA strand has a specific association with corresponding
nucleotide in the other DNA strand because of the chemical affinity of the bases, nucleotide
containing adenine are always paired with nucleotide containing thiamine and nucleotide
containing ‘C’ are always pairing with nucleotide G. the complementary bases are joined to
each other by weak chemical bond called Hydrogen bond.
PROTEIN SYNTHESIS
DNA carries the instruction for the production of protein. A protein is composed of smaller
compound called Ammo acid and the structure and function of protein is determined by the
sequence of its aa. The sequence of Aa is determined by the sequence of nucleotide bases in
the DNA. A sequence of 3 nucleotide bases called triplet, is a genetic code word or cordon
that specify a particular aa. For instance, the triplet G.A.C is the cordon for aa leucine and
triplet C.A.G is the codon for aa valine. A protein consisting of 100 aa is therefore encoded
by DNA segment consisting of 300 nucleotide. Of the two polynucleotide chains that form a
DNA molecule, only one strand contain the information needed for the production of a given
aa sequence. The other strands aids in replication. Protein synthesis begins with the
separation of DNA molecule into two strands in a process called transcription, a session of
one strand act as a template or pattern to produce a new strand called messenger RNA
(mRNA). The mRNA leaves the cell nucleus and attaches to the ribosome, specialize cellular
structures that are the sight of protein synthesis. Aa are carried to the ribosome by another
type of RNA called transfer RNA linked together in a particular sequence dictated by the
mRNA to form a protein.
A gene is equence of DNA nucleotides that occupy the order of aas in a protein viia an
intermediary mRNA molecule.
REPLICATION
In most cellular organism, replication of a DNA molecule takes place in the cell nucleus and
occurs just before the cell divide. Replication begins with the separation of the two
polynucleotide chains each of which then act as a template for the assembly of a new
complementary chain. As the old chain separate, each nucleotide in the two chain attract a
complimentary nucleotide that has been formed earlier by the cell. The nucleotides are
formed together by hydrogen bond to form the rings of a new DNA molecule. As the
complimentary nucleotide are filled into place, an enzyme called DNA molecule. This
process continues until a new poly nucleotide chain have been formed alongside the old one
forming a double helix sugar molecule.
Tools and Procedure
Specialize enzymes called restriction enzyme found in bacteria act like molecular scissors to
cut the PO4 backbone of DNA molecules at specific base sequences strand of DNA that have
been cut with restriction enzyme are left with single stranded tail that are called sticky- ends,
because they can easily realign with tails from advantage of restriction enzyme and the
sticky-end generated by this enzyme to carryout recombinant DNA technology or genetic
engineering. This technology involves removing a gene from one organism and inserting the
gene into another organism.
Application of DNA
Research into DNA has had a significant impact on medicine. Through recombinant DNA
technology, scientist can modify microorganism so that they become so called factories that
produce large quantities of medically useful drugs. This technology is used to produce insulin
which is used by some cancer patient study of human DNA are revealing gene that are
associated with specific diseases such as cystic fibroses. This information is helping
physician in diagnosing various diseases and it may lead to new treatment e.g. physician are
using a technology called thimeraplasty which involves a synthetic molecule containing both
DNA and RNA strand in an effort to develop a treatment for a form of hemophilia (inability
of the blood to clot)
Forensic science using techniques developed in DNA research to identify
individuals who have committed crime. DNA from semen, skin or blood taking from the
crime scene can be compared with the DNA of a suspect and the result can be used in count
as evidence. DNA has helped taxonomy to determine the evolutionary relationship among
plant, animal and other life forms. Closely related species have a similar DNA than those
with distant relationship. One surprising finding that emerge is that vultures of Americans are
more closely related to the clorks than to those of Europe, Asian, Agric.
Techniques of DNA manipulation in form of genetic engineering and biotechnologist strains
of crop plant to which gene have been transferred may produce higher yield and may be more
resistant to pest and diseases, cattle have been similarly treated to increase milk and beef
production as halves hogs to yield more milk with less fat.
9.0 NUCLEIC ACID AND CODING
A gene is a molecular unit of heredity of a living organism. It is widely accepted by the
scientific community as a name given to some stretches of deoxyribonucleic acids (DNA)
and ribonucleic acids (RNA) that code for a or for a polypeptide RNA chain that has a
function in the organism. A modern working definition of a gene is "a locatable
region of genomic sequence, corresponding to a unit of inheritance, which is associated with
regulatory regions, transcribed regions, and or other functional sequence regions”
The Functional structures of gene.
The vast majority of living organisms encode their genes in long strands of DNA
(deoxyribonucleic acid). DNA consists of a chain made from four types of nucleotide
subunits, each composed of: a five-carbon sugar (2'-deoxyribose), a phosphate group, and one
of the four bases adenine, cytosine, guanine, and thymine. The most common form of DNA
in a cell is in a double helix structure, in which two individual DNA strands twist around each
other in a right-handed spiral. In this structure, the base pairing rules specify that guanine
pairs with cytosine and adenine pairs with thymine. The base pairing between guanine and
cytosine forms three hydrogen bonds, whereas the base pairing between adenine and thymine
forms two hydrogen bonds. The two strands in a double helix must therefore be
complementary, that is, their bases must align such that the adenines of one strand are paired
with the thymine of the other strand, and so on.
Chromosomes
The total complement of genes in an organism or cell is known as its genome, which may be
stored on one or more chromosomes; the region of the chromosome at which a particular
gene is located is called its locus. A chromosome consists of a single, very long DNA helix
on which thousands of genes are encoded. Prokaryotes—bacteria and archaea—typically
store their genomes on a single large, circular chromosome, sometimes supplemented by
additional small circles of DNA called plasmids, which usually encode only a few genes and
are easily transferable between individuals. For example, the genes for antibiotic
resistance are usually encoded on bacterial plasmids and can be passed between individual
cells, even those of different species, via horizontal gene transfer. h
Transcription
The process of genetic transcription produces a single-stranded RNA molecule known
as messenger RNA, whose nucleotide sequence is complementary to the DNA from which it
was transcribed. The DNA strand whose sequence matches that of the RNA is known as
the coding strand and the strand from which the RNA was synthesized is the template strand.
Transcription is performed by an enzyme called an RNA polymerase, which reads the
template strand in the 3' to5' direction and synthesizes the RNA from 5' to 3'. To initiate
transcription, the polymerase first recognizes and binds a promoter region of the gene. Thus a
major mechanism of gene regulation is the blocking or sequestering of the promoter region,
either by tight binding by repressor molecules that physically block the polymerase, or by
organizing the DNA so that the promoter region is not accessible.
Translation
Translation is the process by which a mature mRNA molecule is used as a template for
synthesizing a new protein. Translation is carried out by ribosome, large complexes of RNA
and protein responsible for carrying out the chemical reactions to add new amino acids to a
growing polypeptide chain by the formation of peptide bonds. The genetic code is read three
nucleotides at a time, in units called codons, via interactions with specialized RNA molecules
called transfer RNA (tRNA). Each tRNA has three unpaired bases known as
the anticodon that are complementary to the codon it reads; the tRNA is
also covalently attached to the amino acid specified by the complementary codon. When the
tRNA binds to its complementary codon in an mRNA strand, the ribosome ligates its amino
acid cargo to the new polypeptide chain, which is synthesized from amino
terminus to carboxyl terminus. During and after its synthesis, the new protein must fold to its
active three-dimensional structure before it can carry out its cellular function.
Nucleic acid
A comparison of the two principal nucleic acids: RNA (left) and DNA (right), showing the
helices and nucleobases each employs.
Nucleic acids are polymeric macromolecules, or large biological molecules, essential for
all known forms of life. Nucleic acids, which include DNA (deoxyribonucleic acid)
and RNA (ribonucleic acid), are made from monomers known as nucleotides. Each
nucleotide has three components: a 5-carbon sugar, a phosphate group, and a nitrogenous
base. If the sugar is deoxyribose, the polymer is DNA. If the sugar is ribose, the polymer is
RNA.
Together with proteins, nucleic acids are the most important biological
macromolecules; each is found in abundance in all living things, where they function in
encoding, transmitting and expressing genetic information—in other words, information is
conveyed through the nucleic acid sequence, or the order of nucleotides within a DNA or
RNA molecule. Strings of nucleotides strung together in a specific sequence are the
mechanism for storing and transmitting hereditary or genetic, information via protein
synthesis.
TYPES OF NUCLEIC ACIDS
Deoxyribonucleic acid
Deoxyribonucleic acid (DNA) is a nucleic acid containing the genetic instructions used in the
development and functioning of all known living organisms (with the exception of RNA
viruses). The DNA segments carrying this genetic information are called genes. Likewise,
other DNA sequences have structural purposes, or are involved in regulating the use of this
genetic information. Along with RNA and proteins, DNA is one of the three major
macromolecules that are essential for all known forms of life. 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
nucleobases (informally, bases). It is the sequence of these four nucleobases 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.
Ribonucleic acid
Ribonucleic acid (RNA) functions in converting genetic information from genes into the
amino acid sequences of proteins. The three universal types of RNA include transfer RNA
(tRNA), messenger RNA (mRNA), and ribosomal RNA (rRNA). Messenger RNA acts to
carry genetic sequence information between DNA and ribosome, directing protein
synthesis. Ribosomal RNA is a major component of the ribosome, and catalyzes peptide bond
formation. Transfer RNA serves as the carrier molecule for amino acids to be used in protein
synthesis, and is responsible for decoding the mRNA. In addition, many other classes of
RNA are now known.
Genetic coding.
A Schematic diagram of a single-stranded RNA molecule illustrating the position of threebase codons.
10. MUTATION
It is a sudden genetic change which is heritable. It involves alteration in the coding sequence
of the gene. The change is usually in the sequence of the organic basis in the DNA
constituting the particular gene. The original gene ‘A’ is dominant while the mutant gene ‘a’
is recessive. The mutation is a reversible process and the recessive allele ‘a’ will mutate back
to its former form ‘A’ although not necessarily at the same rate that ‘A’ mutate to ‘a
Effect of mutation on gene activity.
Normally, the activity of a gene is lost due to mutation. This is the reason while the original
or wild type genes are dominant and the mutant genes are recessive. In a diploid (2n)
organism, if a mutation occurs in one of the genes, it leads to heterozygosis since other alleles
remain unaffected and because of the presence of the wild type, a recessive mutation is not
expressed. In a haploid (n) organism which have only one copy of each gene, each and every
mutation is expressed.
Fate of undesirable mutation
Undesirable mutation are selected against by the environment therefore, the probability of
such mutant are lost forever or eliminated from a population are greater in haploid than in
diploid.
Mutation in Somatic cells
To get transmitted to the progeny through the gamete use the result that they are lost with the
death of the organism. In vegetatively propagated or asexually reproducing organism.
Somatic mutation are transferred to the progeny and therefore perpetuated.
Detection of Mutation
Mutation are usually detected since they bring about phenopitic change in the organism.
TYPES OF MUTATION
a. Dominant Vs Recessive Mutation: Mutation which is expressed in the first filial
generation (f1) of all organism which arises from the mutant germplasm of a diploid organism
is reffered to as dominant mutation. It is usually expressed in the heterozygous condition and
is therefore easy to detect. On the other hand, mutation which is expressed in the second filial
generation (F2) and in only small fraction of its progeny is known as recessive mutation.
Recessive are not lethal mutation and the most difficult to detect and at the same time most
frequent. However, recessive mutation are of great evolutionary significance because they
can be perpetuated and maintain in heterozygous condition.
b. Germinal Mutation: occurs in the germ plasm (germline) of an organism. It may be
expressed in its lifecycle or expressed in the offspring.
c. Recurrent Vs Non-recurrent mutation: In recurrent mutation, each mutational event
occur regularly with characteristics frequency. It is an agent of change in the gene frequency
in a PP. non-recurrent mutation on the other hand is a mutational event that give rise to first
one representative of mutated gene or chromosome in the PP. it is a unique event and the
product have very little chance of survival. It is therefore of little importance in changing the
gene frequency in a PP.
d. Neutral Mutation: is a unique mutation that is neutral with respect tofitness of the
individual.
e. Lethal Mutation: is a mutational event which leads to the death of the individual in which
it occurs. As the case with the dominant mutation, lethal mutation is easy to detect since it
leads to the death of the individual carrying it.
f. Deleterious Mutation: is a mutational event which is detrimental to the gene of the
individual. It may be lethal or decrease the physiological function of the individual
concerned.
g. Reverse Mutation: sometimes, Mutants reverse back to the wild type due to reverse
mutation e.g. A forward mutation in a red flower plant may result in white flowered plant and
a reverse mutation in the white flower plant may result in the red flower plant. Reverse
mutation are very useful in genetic studies because they can be selected much more easily.
h.
Suppressor Mutation: Sometime, a forward mutation at one locus may hide the effect
of another forward mutation and a combination of the two appears as if a reverse
mutation has taken place. Such mutation which suppress the phenotype of other mutation
are known as suppressor mutation.
i. Mutable and Mutator gene: genes which are capable of undergoing mutation are known
as mutable genes and the gene which control the mutation frequency of other gene are known
as mutator gene . Sometimes, a single mutator gene affects mutator frequency of a number of
loci. However, there are examples of mutator gene that affect only one mutable gene. In
Males for example, the mutation frequency of the gene controlling Anthocyanide production
is under the control of a single mutator locus.
Induction of Mutation
Apart from mutation that arises spontaneously, mutation can be induced. A detectable
mutation occur if the change cordon stands for a different aas sequence of the protein, and
therefore it is not expressed.
a.
Ionizing radiation :such as Xray causes breakage in the DNA synthesis result in
mutation.
b.
Ultraviolent radiation: ultraviolent rays cause hydration and deammation which
interferes with DNA synthesis, resulting in mis-replication and mutation
c.
Chemical mutagen: These include nitrous acid, carboxylic compound and nitrogen
musterds. The overall effect is that they cause abnormal base, DNA, distortion of normal
DNA double helix, addition or division of single base pairs. This result in different tyoe of
architectural change such as transition, transaction and frame shift as well as nonsense
mutation and mix sense mutation.
Frame shift may occur due to division or addition of bases which causes gross change in the
aa sequence of protein since it affect the reading frame of codon beginning from the site of
. transistion and translation of the gene is therefore affected because this process start from a
fixed point.
Transition: Transition involves changes from one purine to another purine or pyrimidine to
another pyrimidine.
Transaction: This involve a change from purine to pyrimidine or vice versa
A=T
C=G
A=T
T=A
Mix sense Vs Nonsense mutation: mix sense in mutation occur if the change codon stands for
a different aa which grossly affects the catalytic properties of the protein. Nonsense mutation
occurs when a codon which stand for an aa is change to a chain-terminating one
11. POPULATION GENETICS
It is the study of genetic variation in a population for many years.
Factors that Affect Influence Genetic Change In a Population
a. Mutation: They are the change that occur to the gene of an individual during a lifetime and
are transmitted directly to the offspring. It occurs at the level of molecular genetic material,
DNA. Mutation are replacement of one nucleotides base by another and may allow an
individual to adapt to the changing environmental occurrences e.g. Climate
b. Natural Selection: is the principal way that organism adapt to their environment. It is very
basis of evolution of all living organism. In natural selection, a trait that provides individual
with greater evolutionary fitness( reproductive success) will increase in frequency over
generations among that make individual less fit may decrease and initially rear gene result
from a single mutation will become common in a PP for it to provide an effect that enable
individuals to adapt to their environment. Those with the beneficial gene will survive longer
and produce more offspring than those without such gene. Offspring who inherited such
favorable gene will also have more offspring and individual with genes will soon outnumber
those without the gene.
c. Random genetic drift: beside mutation and genetic gene, pure chance factor may choose
the frequencies of the genes present in a PP. most gene occurs in two or more forms of
alleles. For each gene an individual inherit one allele from the mother and from the father
even though a parent may have two different allele of the gene, only one will pass down to a
child. The allele to the passed down is determined entirely by chance at the time of
conception. Random genetic drift therefore refers to the change in a pp, gene frequency
resulting from the change factor due to genetic drift certain allele will therefore appear from a
pp even if they confer evolutionary fitness simply because they occur in a very low
frequencies at the same time. Other alleles will become wide spread because they occur in
higher frequency. Random genetic drift does not usually have a major effect on a pp gene
pool if the size of the pp is larger. If the pp is small, it can dramatically influence gene
frequency. Epidemic, disease, wars and destructive natural events such as volcanic eruption,
earthquake and flood can create situation known as bottle neck. The calamity leads a reduce a
pp in which the genes of only a few survivors remain.
Gene Flow and Migration
Gene flow occurs directly, when individual from one pp meet with others from another
thereby introducing their genes into the pp. increase gene flows between pp generally make
them more alive than they have been previously. Gene flow also occur indirectly for example
if a pp A interbreed with pp B and B interbreed with C, some gene from pp A will pass to pp
C in this ways flow occurs across vast geographic regions and connect distant pp.
11.0 HARDY-WEINBERG
A set of algebraic formulae that describe how the proportion of different genes. The
hereditary unit that determine a particular allele should occur in pp. the rule also reveals how
often particular genotype, the actual combination of gene and individual carries and may
pass on to the offspring should appear in the same proportion by studying this alleles and
genetic frequency, scientist can identify pp that are changing genetically oe evolving. They
can also predict the occurrence of genetic effect in a pp.
Each individual in a pp has two allele of every gene. This allele may be the same and one
allele maybe dominant over the other for example: in a sample group of 100 individual from
a particular pp, the gene from a certain individual have allele Aa in which A is dominant over
a, each individual of the group carries each of this allele in one of the pp or genotype AA or
Aa. In a sample of group of 100 people, 33 people has AA genotype 54 has Aa genotype and
13 have aa genotype. The actual frequency of the two alleles in the group i.e. the number of
each allele type by the total number of all alleles (2 each from the 33 individual with the AA
genotype and one each from the 54 individual with the Aa genotype) by 200, the total number
of all allele, 2 each from 100 individuals. The hardy-weinberg rule uses the actual allele
frequency of a pp to predict the pp expected genotypic frequency. i.e. the number of genotype
that should be in a pp. Assuming that a gene pp has 2 pp Aa whose frequencies are
represented mathematically as ‘p & q’ that can form the 3 type AA, Aa, aa.
The following formular can be used to predict genotypic frequency
Frequency
AA: p x p= p2
Aa: p x q= 2pq
aa : q x q= q2
p2+2pq+q2 is used to find genotypic frequency
i.
For example: If the frequency of the A allele in a pp is =0.60 then the expected genotypic
frequency with Aa genotype is 0.36 i.e. (0.60 x 0.60 which is 0.36) expected genotype
frequency. Scientist compare a pp expected genotypic frequency to its actual genotypic
frequency (determined by dividing the total no of each genotype in the group by the total
number individual number in the group). To determine whether the pp is maintaining the
same ratio or equilibrium of the genotype over time according to the rule each equilibrium
remain the same in as long as 4 conditions are meant;
i. Individual must select or mate randomly without regards or phenotypic trait
ii. No genotype can be favoured in such a way that it will increase the frequency in pp
ovetime.
iii. No new allele can be deduced to the pp or by alleles that have changed or mutated from
one form to another.
The number of individual and genotype in pp remain high.
The pp that meet this condition maintain the same proportions of different genes overtime i.e.
genetic make of pp never changes.
33 have AA ->
33+33=66
54 have Aa
13 have aa
A= 33+ 33+54=120
a= 54+ 13 + 13=80
A+a=200(120+80)
Actual allele frequency A=
= 0.6
Expected genotypic frequency of AA= 0.6 x0.6 = 0.36
Actual genotypic frequency =
= 0.33