Download 1 SMOLENSK STATE MEDICAL ACADEMY NINA E

SMOLENSK
STATE
MEDICAL
ACADEMY
NINA E. SCHEBNIKOVA
COLLECTION ON GENETICS
FOR THE OVERSEAS STUDENTS
Reviewed by professor ALEXANDR S. SOLOVJOV, D.M.
FIRST EDITION
Smolensk
2005
2
УДК 576.3/.4
Учебное пособие по генетике для иностранных студентов,
Н.Е.Щебникова /Под редакцией профессора А.С.Соловьева. Смоленск:
Изд. СГМА, 2005 – с.
Учебное пособие составлено в соответствии с программой обучения в
медицинских ВУЗах Российской Федерации. Пособие включает материал по
основным разделам генетики, включая классические типы наследования
признаков у растений, животных и человека. Кроме того, в пособии отражены
классические представления и достижения последних лет в области
мутационного процесса и наследственных болезней человека.
Пособие разработано для студентов факультета иностранных учащихся СГМА.
Рецензенты: кандидат биологических наук, доцент кафедры ботаники СГПУ
Е.М.Елагина, старший преподаватель кафедры иностранных языков СГМА
В.Я. Ионова.
Смоленская государственная медицинская академия 2005
3
CONTENTS:
1. Mendel’s laws of inheritance.
2. Gene linkage. Sex determination. Sex-linked genes.
3. Variability. Gene (point) mutations. Genetic disorders.
4. Chromosomal mutations. Chromosomal disorders.
5. Genetic tasks.
6. Pre-exam questions.
4
1. MENDEL’S LAWS OF INHERITANCE.
The Problem of Inheritance
A characteristic common to all organisms is the capacity to reproduce offspring, to create a new
generation of similar organisms. People have known for centuries several important facts about
reproduction. Within a population of organisms variability (or "varieties") usually exists for the
characteristics of the organism. For example, all human beings have a blood type, but not all
have the same blood type. People have also known that among sexually reproducing
organisms, both parents contribute in some way to the production of offspring. For centuries it
has been known that offspring usually resemble their parents and each other. Also, many traits
in the offspring are not exactly like those of either parent. Children usually resemble their parents and each other but display varying combinations of parental traits.
With these and other observations of nature, humans have been able to manipulate both
animal and plant breeding to develop particular characteristics. Over 4000 years ago in Egypt,
farmers realized they could improve their crops and animals by selective breeding. For example,
artificial pollination of date palms was common by 2000 B.C.
By 1727 the French government began a seed breeding program to improve food and wine
production. They were able to develop the sugar beet, used to produce table sugar. This had
great commercial value because sugar usually had to be imported. By 1760 the British
government began a 30-year program to improve livestock herds. Many of the sheep and dairy
and beef cattle common today were first developed then.
Although the results were impressive, selective breeding was based as much on trial and
error as it was on luck because no one knew exactly what determined the pattern of
inheritance. Many plant and animal breeders had noticed that some characteristics passed from
one generation to the next unchanged,
some
appeared to be a mixture
of parental
characteristics, and others appeared and disappeared from generation to generation.
Gregor Mendel
Gregor Mendel began to unravel this confusing knot of heredity. He was born in 1822 to peasant
parents in a small Austrian farming village with a long tradition of supplying skilled gardeners to
the great landowning families. Mendel's father was himself a skilled fruit grower. Mendel
attended grammar school and was an extremely gifted mathematics student. After grammar
school Mendel entered a monastery at Brunn, Czechoslovakia, as a monk to continue his
5
studies. Five years later he was sent to study mathematics and science at the University of
Vienna. After graduation he planned to become a science teacher at the monastery. Largely
because of his agricultural background Mendel knew much about botany and basic biology.
However, he failed his examination in botany because he disagreed with the examiners over
several questions about plant breeding and consequently did not obtain his teaching certificate.
Having given up on being a teacher, Mendel returned to Brunn, where he did administrative
work in the monastery. However, still convinced his answers on the examination were correct,
he quietly carried out some plant breeding experiments in his spare time. Mendel began his
experiments in 1856 behind monastery walls, in a modest garden plot measuring 120 feet by 20
feet. These experiments occupied him for the next 8 years and resulted in solving the riddle of
inheritance.
Mendel chose the common garden pea plant for his breeding experiments for several important
reasons.
First, pea plants are small, easy to grow, and they produce large numbers of offspring. A generation is completed in a single growing season.
Second, pea plant pollination is easily controlled because plants contain both male and
female reproductive structures in the flower. Normally the plants undergo selfpollination. The female reproductive cells in each flower are pollinated by the male
reproductive cells from the pollen of the same flower. Offspring are produced from a
single pea plant.
However, different plants can be mated manually. In cross-pollination the flowers are
carefully picked apart. Pollen is collected from one plant and transferred to a flower of
another plant. In this way a plant breeder could mate two different plants and see what
varieties of offspring resulted. The offspring produced from cross-pollination are called
hybrids.
Third, Mendel chose the pea plant because many varieties of pea plants were
readily available. He was able to obtain 34 varieties of pea plants from several commercial
6
growers. These differed in such characteristics as flower color, seed shape, and height.
Each variety of pea plant had been cultivated for many generations and was considered
true breeding. True breeding means that all offspring produced through self-pollination
are identical to the parent. For example, if a particular pea plant produced purple flowers,
all its offspring, generation after generation, also produced only purple flowers.
Mendel chose seven different characteristics to observe and manipulate. He then
selected two contrasting varieties for each of these characteristics. For example, for
flower color, he used purple or white, for seed shape he chose smooth or wrinkled, and
for plant height he selected either tall or short (Figure 1). These contrasting varieties were
distinctly different and therefore would be easily identified from generation to generation.
Mendel thus initially made only monohybrid crosses.
A monohybrid cross is breeding for only one characteristic at a time.
Figure 1. Seven characteristics of pea plants with their contrasting varieties
7
Note. Mendel selected seven different characteristics of the pea plant, each with two contrasting
varieties, for his breeding experiments
.----------------------------------------------------------------------------------------------------------------------------- ------
Mendel's Experiments and Results
Mendel conducted his monohybrid breeding experiments in two steps. First he crosspollinated true-breeding plants having alternative varieties for a particular characteristic.
He removed the male parts from flowers of the pea plants to prevent self-pollination. Next
he manually removed the pollen from one plant and carefully pollinated the female parts
of another plant. For example, pollen from a white flower was placed on the female parts
of a purple flower. He also made a reciprocal cross by taking the pollen from a purpleflowered pea plant and introducing it to a white-flowered plant. The second step was to
allow the hybrid offspring produced by cross-pollination to self-pollinate and produce
offspring. To keep track of his experiments
Mendel referred to the first cross as the parental generation, or P generation. The first
offspring generation was called the first filial or F1 generation. The next generation he
designated as the second filial or F2 generation.
When Mendel crossed two contrasting varieties, such as purple-flowered pea plants with
white-flowered plants (P generation), the hybrid offspring (F 1 generation) produced
always resembled one of the parental varieties. The flowers of the F1 generation hybrids
were as purple as the flowers of the purple parent, and there were no white-flowered
offspring (Figure 2-A).
Figure 2. Monohybrid cross.
Note. In his initial breeding experiments Mendel cross-pollinated two true-breeding pea
plants, each with a different variety of the same characteristic. For example, he took
8
pollen from a purple-flowered plant and introduced it into a white-flowered plant. He also
made a reciprocal cross in which he took pollen from a white-flowered plant and
introduced it into a purple-flowered plant. The flowers of all of the F 1 pea plants were
purple (A). Mendel allowed F 1 hybrids to self-pollinate. Both purple and white flowers
were produced in the F 2 generation. He also discovered that for every three purple
flowers produced there was 1 white flower produced (B).
-------------------------------------------------------------------------------------------------------------------
The variety expressed in the F1 plants, purple, was called dominant (The law of
dominance). The alternative variety, white, not expressed in the F1 plants, was called
recessive. For the contrasting varieties of each of the seven characteristics Mendel
tested, one variety was dominant and the other was recessive. But was that an accurate
conclusion, and if so, what happened to the recessive variety?
Mendel began the second step of his experiment to find this out. After the individual
F 1 plants matured and were allowed to self-pollinate, Mendel collected and planted the
seeds from each plant to observe what the F 2 would look like. He found that F1 plants
produced white flowers as well as purple flowers in the F 2 generation. The recessive
variety was not lost in the F 1 generation but remained "hidden" and then expressed in the
F2 plants.
Mendel counted the numbers of each variety among the F2 offspring. For example, of 929
F2 individuals produced, 705 had purple flowers and 224 had white flowers. Almost one
quarter of the F2 offspring had white flowers, the recessive variety (Figure 2-B).
Mendel then examined each of the seven characteristics by counting the varieties present among the offspring produced from the F, generation. He discovered that for every
three plants displaying the dominant variety, one plant displayed the recessive variety.
The ratio was always the same.
Mendel called the unknown agent responsible for each characteristic a hereditary
factor. In modern terminology Mendel’s factors are called genes. However, it was still
not clear how many factors were involved. He reasoned that the purple flower F 1
hybrid must contain at least one hereditary factor for purple because it is purple, but it
must also contain at least one hereditary factor for white because when self -fertilized, it
produced some white flowered plants in the F 2 generation. To find out how many factors
were present for each characteristic, he allowed the F 2 to self-pollinate. The white-flowered F2 plants produced only white-flowered offspring. However, only one third of the
purple-flowered dominant F 2 plants produced only purple-flowered offspring. The
remaining two thirds were hybrids and produced both purple- and white-flowered
offspring in a ratio of 3:1. The F2 generation consisted of one-fourth true-breeding
dominant purple plants, one-half hybrid purple plants that produced both purple- and
white-flowered offspring, and one-fourth recessive true-breeding white-flowered plants
(Figure 3).
Figure 3. Mendel’s experiment to determine the number of hereditary factors
9
controlling a characteristic.
Note. To determine the number of hereditary factors responsible for a characteristic,
Mendel let the F2 generation plants self-pollinate. He found that only one third of the
purple-flowered plants produced only white-flowered offspring.
----------------------------------------------------------------------------------------------------------------------
Mendel's background in mathematics helped him at this point to solve the problem of
inheritance and explain what these ratios meant.
Mendel saw that the F 1 hybrid purple-flowered plants producing both purple- and whiteflowered offspring in the F 2 generation must contain hereditary factors for both varieties.
The simplest conclusion was that a hybrid plant for' flower color contains two hereditary
factors, which he labeled P and p. The capital letter P represents the dominant
hereditary factor for purple. The lowercase letter p represents the recessive hereditary
factor for white. The true-breeding purple-flowered and white-flowered plants of the
parental generation must also contain PP and pp, respectively. If this is correct, then
clearly PP and pp plants breed true because when they self-pollinate, only true breeds
could be produced with no other factors present. The Pp hybrids, however, do not
necessarily breed true because they are carrying both P and p factors. Mendel
designed a final set of experiments to determine if his reasoning was correct.
10
DIAGRAM OF
P (Parents)
Phenotype
PP
MONOHYBRID
x
pp
purple white
G (Gametes)
(P)
(p)
F1 generation – genotype -
Pp
Phenotype
P (Parents F1)
CROSS.
purple (the law of dominance)
Pp
x
Pp
Phenotype
purple
purple
G (Gametes)
(P) (p)
(P) (p)
F2 generation – genotype Phenotype
-
PP
Pp
purple
Pp
pp (genotypic ratio is 1: 2: 1)
purple purple white (phenotypic ratio is 3 : 1).
THE LAW OF SEGREGATION.
Mendel proposed the hypothesis that two hereditary factors determine a particular
characteristic and they must separate from each other when reproductive cells are
formed. For example, the hybrid plant must produce two kinds of pollen and two kinds
of eggs, either P-bearing or p-bearing. If this were true, then the three possible pollenand-egg combinations were PP, Pp, and pp. However, observation alone could not
confirm this hypothesis because of the dominant and recessive relationship between
the two varieties. Purple flowers could thus be the PP or Pp combination.
To test this hypothesis, Mendel devised a simple breeding technique called a
testcross.
In the testcross, the F1 generation is crossed with a recessive parent.
To learn which hereditary factors a self-pollinated purple-flowered plant from the F,
generation possessed, Mendel crossed it with a white-flowered pea plant. Two
alternatives from a testcross were possible. If the test plant from the F1 generation
contained identical factors, PP for example, and was crossed with a white-flowered
recessive plant, pp, all of the offspring would have purple flowers. The other possibility
was that if the test plant contained two different factors, Pp, one half of the offspring would
have white flowers and the other half would have purple flowers. The dominant and
recessive varieties would then appear in a 1:1 ratio (Figure 4). The results were that for
each pair of contrasting varieties Mendel investigated, each testcross produced a 1:1
ratio, just as his hypothesis predicted.
Figure 4. Testcross.
11
Note. To test the hypothesis that two hereditary factors were responsible for a
characteristic, Mendel devised a testcross in which a hybrid plant is cross-pollinated with
a true-breeding recessive plant. If the hybrid contained two identical factors, all the
offspring would have purple flowers (A). If the hybrid contained two different factors ,
however, one half of the offspring would have white flowers and the other half would ha ve
purple flowers (B). For each pair of contrasting varieties Mendel investigated, each
testcross produced a 1:1 ratio, confirming his hypothesis that two factors determined a
characteristic.
---------------------------------------------------------------------------------------------------------------------
Using conclusions from his experiments, Mendel proposed the law of segregation.
The law of segregation states that pairs of hereditary factors (genes) segregate or
separate from one another during the formation of reproductive cells and each reproductive cell receives only one of the pair of hereditary factors (genes).
This law of segregation is also called the ‘law of purity of gametes’, which shows that the
organism may be hybrid (Pp) but its gametes are always pure having only (P) or (p), but never
both. Thus, the gametes are always pure even when its producer is hybrid organism.
DIAGRAM OF TEST CROSS.
P (Parents)
Pp(F1)
Phenotype
purple
G (Gametes)
(P)
pp
white
(p)
F2 generation – genotype Phenotype
x
(p)
Pp
purple
pp (genotypic ratio is 1:1)
white (phenotypic ratio is 1:1)
Significance of test cross.
1. The test cross method is very useful to breeders and geneticists in
12
determining the genotype constitution of any plant or animal.
2. It helps to determine whether the dominant character of F1
offspring is due to homozygous (PP) or heterozygous (Pp)
condition. In other words it helps to determine unknown genotype
of individual.
3. The appearance of recessive character in F2 generation
immediately shows that the organism in question is heterozygous.
Thus, a breeder will discard unwanted progeny from the breeding
programme.
The Law of Independent Assortment
Mendel then went on to ask whether the factors for different characteristics also
segregated independently of one another. Would, for example, the particular factor that
determined seed shape influence which factors a plant inherited for seed or flower color?
Mendel conducted breeding experiments similar to his original monohybrid crosses but
this time made dihybrid crosses.
A dihybrid cross involves breeding for two characteristics at a time.
He again started with true-breeding lines of peas that differed from one another in two of
his seven chosen characteristics. Then he cross-pollinated contrasting pairs of the truebreeding lines. In a cross involving different seed-shape varieties, smooth (S) and wrinkled (s), and different seed-color varieties, yellow (Y) and green (y), all of the F 1 offspring
were identical (the law of dominance). Each plant has smooth and yellow seeds. From his
monohybrid experiments Mendel knew each plant contained different hereditary factors
for both seed shape, Ss, and seed color, Yy. The F 1 plants were then allowed to selfpollinate. Mendel examined 556 plants produced from a dihybrid cross that had been
allowed to self-pollinate and obtained the following F2 generation results: 9/16 smooth and
yellow, 3/16 smooth and green, 3/16 wrinkled and yellow, 1/16 wrinkled and green.
These results express a 9:3:3:1 ratio. From this, Mendel concluded that the chances for
a seed to be smooth or wrinkled are independent of its chances to be yellow or green.
The characteristics assort independently of one another. How did Mendel reach this
conclusion?
If a seed has a three-fourths chance of being smooth and a three-fourths chance of
being yellow, and if the characteristics assort independently of one another, its
chances of being both smooth and yellow at the same time would be 3/4 x3/4 -9/16
(Figure 5).
Figure 5. Dihybrid cross.
13
Note. Mendel wanted to know whether the factors for different characteristics assorted
independently. After he allowed the F 1 hybrids to self-pollinate, the different
combinations of varieties expressed a 9:3:3:1 ratio. This ratio could occur if the
hereditary factors for different characteristics assorted independently in reproductive
cells.
-----------------------------------------------------------------------------------------------------------------This is the ratio Mendel observed. If the characteristics were assorting independently of
one another, he should have been able to predict the expected ratio of a particular
combination in the F 2 dihybrid cross by multiplying the chances of occurrence of the
individual varieties of the two characteristics together. For example, what are the
chances of a seed being both smooth and green?
There is a three-fourths chance of a seed being smooth. There is a one-quarter
chance of a seed being green. So the chance of being both smooth and green would
be ¾ x ¼ - 3/16. Each of his predicted ratios matched the observed ratio in the F 2 generation. From these dihybrid crosses Mendel reached a second conclusion about the pattern
of inheritance, called the law of independent assortment.
The law of independent assortment states that t h e distribution of hereditary
factors (genes) for one characteristic into the reproductive cells does not affect the
distribution of hereditary factors (genes) for other characteristics.
A modern restatement of Mendel’s law of independent assortment would be that genes that
are located on different chromosomes assort independently during meiosis.
DIAGRAM OF DIHYBRID CROSS.
P (Parents)
Phenotype
YYRR
yellow, round
G (Gametes)
(YR)
F1 generation
- genotype
Phenotype
x
yyrr
green, wrinkled
(yr)
YyRr
(the law of dominance)
yellow, round
14
P (Parents F1)
YyRr
Phenotype
G (Gametes)
x
yellow, green
(YR) (Yr) (yR) (yr)
YyRr
yellow, green
(YR) (Yr) (yR) (yr)
F2 generation
gametes
YR
Yr
yR
Yr
YR
YYRR
YYRr
YyRR
YyRr
yellow, smooth yellow, smooth yellow, smooth yellow, smooth
Yr
YYRr
YYrr
YyRr
Yyrr
yellow, smooth yellow, wrinkled yellow, smooth yellow, wrinkled
yR
yr
YyRR
YyRr
yyRR
yellow, smooth
yellow, smooth green, smooth
green, smooth
YyRr
Yyrr
yyrr
yyRr
yellow, smooth yellow, wrinkled green, smooth
yyRr
green, wrinkled
15
Mendelian traits in human beings.
There are more than 8000 Mendelian traits in human population. These traits may be normal
and abnormal such as genetic disorders.
Figure 6. Some dominant and recessive human traits.
Questions for the discussion.
Zygotegiving
(2N) example.
1. State and explain Mendel’s law of dominance,
2. What is monohybrid ratio?
3. State and explain Mendel’s law of segregation, giving example.
4. Explain the term “test cross”. Give the significance of test cross.
5. State and explain Mendel’s law of independent assortment, giving example.
6. List at least five Mendelian human traits.
16
2. GENE LINKAGE. SEX DETERMINATION. SEX - LINKED GENES.
The numerous genes present on the chromosomes are responsible for the immense diversity
in characters among living organisms. During the forming of sex cells (gametes) there is a
separation of the chromosomes along with which the genes located on the chromosomes
are also separated. Based on this phenomenon, Mendel proposed the law of independent
assortment of these factors (genes). According to this law, gene alleles of different characters are
assorted independent of other gene pairs. However, this law holds true only when the genes
considered are located on separate, non-homologous
chromosomes, and they can be
separated independently. However, researchers after Mendel obtained different results.
According to their findings, they reported that certain gene groups do not s eparate during the
production of gametes.
Morgan described such genes as linked genes and termed the
phenomenon as Linkage. The characters occurring due to this phenomenon are termed as linked
characters. Hence, the phenomenon in which two or more than two genes occurring on one
chromosome form a linked group and are inherited from one generation to the next generation,
known as linkage.
Morgan, through his experiments on Drosophila gave the following conclusions:
Genes exhibiting the phenomenon of linkage remain together on one chromosome, in a regular order
or sequence. The strength of linkage will depend upon the closeness of linked genes on the same
chromosome. Linked genes remain together in their original parents combination during
inheritance.
Linkage in Drosophila melanogaster: In one variety of Drosophila (fruit fly), the gene responsible
for body colour is linked to the gene for wing length. In Drosophila, the gene b is responsible for the
black body colour and the gene B - for grey colour. The gene B is dominant. Similarly the gene V is
responsible for the development of normal, long wings, while the gene v causes development of short
and vestigial wings. The gene V is dominant.
When a Drosophila with grey body and normal long wings (BBVV) is crossed with a fruit fly having
black body and short vestigial wing (bbvv) at the F1 generation, all offspring have grey bodies and
long wings ( Mendel’s law of dominance).
P (Parents)
female BBVV
x male bbvv
G (Gametes)
(BV)
(bv)
F1 generation
BbVv (the law of dominance)
If the male of this F1 generation is test crossed with homozygous recessive female the offspring
in F2 generation are of two varieties in a 1:1 ratio. This differs F2 from the expected Mendelian ratio
1:1:1:1. These offspring show characters typical of the parental (P) generation, because the genes B
and V remain linked and the genes b and v remain linked together.
Complete gene-linkage in Drosophila and its inheritance (as absence of crossing-over in
male drosophila)
Diagram of test cross:
P (Parents)
male BbVv
x female bbvv
G (Gametes)
(BV) (bv)
F2 generation
BbVv (50%);
(bv)
bbvv (50%)
Crossing-Over
In the living cell homologous chromosomes normally occur in many pairs. It is known that one
chromosome is inherited from father and the other is inherited from the mother. During
development of the gametes these chromosomal pairs undergo reduction division. At the time of
17
division, during Prophase 1 of meiosis, i.e., at the Zygotene stage, these homologous pairs come near
to each other. In substage, pachytene the non-sister chromatids of each homologous chromosome in a
pair are closely associated and exchange the segments of DNA with each other. This exchange of
genes has been termed as crossing-over.
Morgan showed the complete linkage rarely occurs because linked genes do not always remain
together. Linked genes are often separated due to exchange of chromosomal segments in crossingover during meiosis and new gene combinations are formed. Linked genes which are widely separated
on chromosomes have more chance of separation by crossing-over and hence can also be called
incompletely linked genes.
When Drosophila with grey body and long wings (BBVV) is crossed with fruit fly having black
coloured bodies and short, vestigial wings (bbvv), the offspring of F1 generation obtained show grey
bodies and long wings (the law of dominance). When the females of this F1 generation were crossed with
the homozygous recessive males (test cross), four different phenotypic types are observed in the
offspring. Of these four phenotypic types, two types resemble the parents of which 41.5 per cent of
the offspring have black bodies and short, vestigial wings. Another 41.5 per cent offspring have
grey bodies and long wings. Hence a total of 83% offspring resemble the parents. The
remaining 17% offspring show two different characters, of which 8.5% fruit flies have grey
bodies and short, vestigial wings and another 8.5% fruit flies have black bodies and long wings.
From this it is evident, that in 17% of the offspring during production of gametes, the crossingover and an exchange of DNA segments possibly occurred between non-sister chromatids. Hence,
these offspring differ from their parents in genotype and phenotype.
Incomplete linkage of genes in Drosophila and its inheritance (crossing-over in female
drosophila)
Diagram of test cross:
P (Parents) female BbVv
x male bbvv
G (Gametes) (BV) (bv) (Bv) (bV)
(bv)
F1 generation BbVv – 41,5%; bbvv – 41,5%; Bbvv- 8,5%; bbVv- 8,5%.
SEX DETERMINATION.
In the most higher organisms, male and female reproductive organs are present on separate
individuals. These are sexually dimorphic forms. In such forms, precise mechanisms and factors
are involved in determining whether individuals will develop into males or females.
Determination of sex is influenced by genetic, hormonal and in some cases, environmental
factors.
Sex Determination : Henking found that in insects, the male had an extra chromosome
which, unlike other chromosomes, was not paired. This extra chromosome in the male was
termed as the X-body. As this chromosome was involved in sex determination, it was termed as
Sex chromosome. The second sex chromosome differs in the male and female. If it is similar
to the first sex chromosome, it is called X-chromosome and if it differs then it is called a Ychromosome. The remaining chromosomes are termed autosomes.
In the somatic cells of several organisms, besides the several pairs of autosomes, the sex
chromosomes may be homozygous (XX), in certain cases, they may be different, heterozygous
(XY) or there may be only a single sex chromosome (XO).
During meiosis, in diploid (2n) living organisms, by reduction division, the 2n number of
chromosomes are reduced to half (haploid-n) in both the egg and sperm cells. At the time of
fertilization, the chromosomal constitution of the male and female gametes (sperm and egg)
determines the genetic make up of the zygote. This establishes, the genetic sex of the
developing embryo, whether male or female. At this stage, during embryonic development the
process of development of male or female phenotypic and genotypic characters and sex
determination are thought to be completed.
1. Sex Determination in Man : In man, the female has chromosomal constitution of 2A + XX
and therefore produces similar egg cells with I A + X chromosomal constitution. The female
is homogametic. The male has 2A + XY chromosomes and produces sperms of 2 types, half
with A + X and I half with A + Y chromosomes. The male is heterogametic. Sex determination
in man depends upon the type of sperm that fertilizes the egg cell. Therefore, in human
18
beings sex is determined at the time of fertilization.
P (Parents)
female XX
x
G (Gametes)
(X)
F1 generation
XX(female)
male
XY
(X) (Y)
XY(male) (genotypic and phenotypic ratio is 1:1)
2.Homogametic male and heterogametic female.
It is not necessary that in all organisms, the female is homogametic and male is heterogametic. For
example, in birds, certain moths, butterflies, and bony fishes, the males are homogametic (XX)
and produces gametes of a similar chromosomal constitution. The females are heterogametic
(XY) and produces two types of eggs. Thus, the egg determines the sex of the offspring. To avoid
confusion, these forms are usually expressed as ZZ (male) and ZW (female).
P (Parents) male ZZ
x
G (Gametes))
(Z)
F1 generation
ZZ (male);
female ZW
(Z) (W)
ZW (female) (phenotypic and genotypic ratio is 1:1).
3. XO-sex determination.
In certain cases(bed-bugs, grasshoppers), the females are homogametic
and males are heterogametic because they have a single sex chromosome.
P (Parents) female XX x male XO
G (Gametes)
(X)
(X) (O)-these gametes contain only autosomes.
F1 generation
XX(female)
XO(male) (phenotypic and genotypic
ratio is 1:1).
SEX-LINKED GENES.
The X and Y chromosomes are considered as sex chromosomes. In the female, the sex
chromosomes are of XX type. In certain cases, it has been observed that genes responsible for
dominant traits are associated with the X-chromosomes. In certain other cases, genes responsible
for recessive traits has been observed associated with the X-chromosome. Genes for certain traits
are also associated with the Y-chromosome.
The genes associated with sex chromosomes are known as sex-linked genes.
Inheritance of X-linked recessive genes.
Colour blindness and haemophilia in man are X-linked recessive genes. A man with colour
blindness cannot identify certain colour specially red and green. When a person with haemophilia
bleeds the blood fails to clot in this case. He bleeds continuously. Such a defect 'is more common in
males than in femaies.
Females have 2X-chromosomes (XX). If one of the X-chromosome carries the defective gene
then the 2 nd X-chromosome has the homologous dominant gene and hence in the female the
expression of the recessive defective genes does not occur. Therefore females with such a
heterozygous gene constitution are carriers of such diseases, while in the male, there is only one
X-chromosome (XY). If such X-chromosome carries a defective gene, due to the absence of a second
X-chromosome with a dominant gene, the recessive trait will be expressed in the male.
If both the X-chromosomes carry recessive genes (homologous) then the defect is expressed in the
female. The trait of both the X-chromosomes having defective genes is rare compared, to one of
chromosomes carrying such a gene. Due to this, such hereditary diseases are expressed more
commonly in males than in females.
Inheritance of X-linked dominant genes :
The pattern of inheritance of X-linked dominant genes differs from that of the recessive genes. If the
dominant genes are inherited then they affect both female and male and females are affected more
than that of males.
19
The X-chromosome of the cells carries the dominant gene for abnormal tooth enamel, so that
even one of X-chromosomes out of 2X chromorome in female carries this dominant defective gene, tooth
enamel abnormality occurs in female. Hence the carrier of this defect is not possible in female.
Inheritance of Y-linked genes : Certain genes are located only on the Y-chromosome, of which
many are involved in the mechanisms of sex determination and differentiation.
In man, the growth of hair on the ears rims and beard and development of testis, are controlled by
genes located on the Y-chromosomes and hence they are expressed only in the male. Such genes are
termed as holandric genes. Such characters are transmitted from the father to the son.
Questions for the discussion.
1. What is complete gene - linkage? Explain with suitable example.
2. Explain incomplete gene- linkage, giving example.
3. What is sex determination? Explain how genes control this phenomenon.
4. Explain the phenomenon of sex determination in man, birds and bed-bugs.
5. Describe the inheritance of X-linked recessive genes in human beings.
6. Describe the inheritance of X-linked dominant genes in human beings.
7. Describe the inheritance of Y-linked genes in human beings.
3.VARIABILITY. THE SOURCE OF GENETIC VARIABILITY. MUTATIONS.
GENE OR POINT MUTATIONS. GENETIC DISORDERS.
The source of genetic variability.
There are three methods of changing gene combinations and maintaining population variation.
The first method is through sexual reproduction. Each parent contributes different alleles to the
offspring via its contribution of one half of each pair of the offspring’s chromosomes. The
offspring represents a unique combinations of parental genes.
The second method is crossing-over in which new combinations of alleles are produced during
the exchange of chromosome segments.
The third method is segregation and independent assortment of homologous chromosomes
during anaphase 1 of meiosis 1.
Despite the accuracy of chromosomal replication, random mistakes occur, altering
nucleotide sequences of DNA. Such changes in the genetic message are rare. For example, in a
human male the probability that a particular gene is altered is about one out of every million
gametes (sperm) he produced. Limited as this appears to be, this steady trickle of change is
ultimate cause of genetic variation and the basis for biological evolution.
20
Mutations.
The term mutation was first coined in 1901 by Hugo de Vries to explain the variations he
observed in the plant evening primrose, Oenothera amarckiana.
Mutation is a sudden, discrete change in the genetic material which is heritable.
When a mutation occurs within a gamete that fuses with another gamete to produce a zygote, the
mutational change is passed along as part of the hereditary information to future generations. This
introduces new variability. In contrast, when a mutation occurs in a somatic cell, it is not passed along
to later generations because the mutation does not directly affect the gametes.
CAUSES OF MUTATION.
Aside from their spontaneous occurrence, mutations can also be produced by external factors, such as ionizing
radiation, ultraviolet radiation or chemicals. Any external factor or agent capable of damaging DNA is called
mutagen. When mutations are caused by a mutagen, the mutation is said to be induced, rather than spontaneous.
Gene Mutations (point mutations).
The most common type of mutation occuring in DNA replication is a gene (point) mutation.
A point mutation is the change in one nucleotide base pair in a gene.
It may be a substitution, deletion or addition of a single nucleotide in a gene.
Four different effects may result from a single base substitution in a nucleotide sequence.
-The first possibility is that a mutation may not change the amino acid sequence of the encoded protein.
The genetic code is redundant in that one amino acid may be encoded by several different codons. If a
mutation changes the codon CTC to CTT, the new sequence still codes for glutamic acid and the
protein synthesized from the mutated gene does not change.
DNA triplet changed from
CTC ----------------------------------► CTT
The mRNA would transcribe
GAG----------------------------------►GAA
Note. Originally GAG coded for glutamic acid. Although the
mutation has changed the sequence, it still specifies the
amino acid glutamic acid.
-The_second possibility is that the mutation may code for an amino acid functionally equivalent to the
original amino acid. Many proteins contain sequences of amino acids that can vary. For example, in
hemoglobin the amino acids that face outward in the three-dimensional protein must be hydrophilic to
keep the protein dissolved in the red blood cell's cytoplasm. Which hydrophilic amino acids are on the
outside is not significant. For example, a point mutation causing a change of the codon CTC to СТА
replaces glutamic acid, which is hydrophilic, with aspartic acid, which is also hydrophilic. This
substitution does not change the solubility of hemoglobin.
Mutations that do not change the function of an encoded protein are called neutral mutations.
DNA changed from
21
CTC ----------------------------------- ► СТА
The mRNA would transcribe
GAG----------------------------------- ► GAU
Note. Originally GAG coded for glutamic acid. The mutation has
changed the sequence so that it now specifies the amino
acid aspartic acid.
-The third possible outcome is that the mutation may encode for a functionally different amino acid. A
mutation from the codon CTC to CAC replaces glutamic acid, which is hydrophilic, with valine, a
hydrophobic amino acid. In human hemoglobin this substitution results in the genetic defect of sicklecell anemia. This hemoglobin variant sticks to itself, causing the red blood cells to become distorted and
malfunction. This mutation is called “missense”- mutation.
DNA triplet changed from
CTC ---------------------------------- ► CAC
The mRNA would transcribe
GAG ---------------------------------- ► GUG
Note. Originally GAG coded for glutamic acid. The mutation has changed the sequence so that it
now specifies the amino acid valine.
- The fourth possibility is that a new triplet is one of three termination codons or stop-codons (UAG,
UAA or UGA). These do not code for amino acid and, therefore, the translation stops. This mutation is
called “nonsense”-mutation.
DNA triplet changed from
ATG---------------------------►ATT
The mRNA would transcribe
UAC----------------------------►UAA
Note. Originally UAC coded for tyrosine. The mutation has changed the sequence so that termination
codon appears. And protein synthesis is stopped.
Mutations can also involve adding or deleting single nucleotide base pair. In these frameshift
mutations one nucleotide base pair in a gene is added or deleted. Because the DNA code is read as
triplets of nucleotides, frameshift mutations cause all triplets after the deletion or addition to be read
differently.
22
Dots represent
other nucleotides
Original
DNA triplets read
CTT/AAA/. . .
The mRNA transcribes
GAA/UUU/. . .
UUU codes for phenylalanine
Insertion
Deletion
If С is added
if C is dropped
DNA triplets now read
DNA triplets now read
CCT/TAA/A . .
TTA/AA./
The mRNA transcribes
The mRNA transcribes
GGA/AUU/U . .
AAU/UU./
GGA codes for glycine
AAU codes for
AUU codes for isoleucine
phenylalanine
Genetic disorders.
The cause of these disorders is gene mutations. There are more than 1000 genetic disorders
in human beings. Genetic disorders are caused by alleles that encode abnormal proteins; the
effects of these proteins lead to serious health problems. Many of the most important genetic
disorders are associated with recessive alleles, the functioning of which may lead to the
production of defective versions of enzymes that normally perform critical functions. Because
such traits are determined by recessive alleles and therefore expressed only in homozygotes,
the alleles are not eliminated from the human population, even though their effects in
homozygotes may be lethal. Dominant alleles that lead to severe genetic disorders are less
common; in some of the more frequent ones, the expression of the alleles does not occur until
after the individuals that possess them have reached reproductive age.
23
Table 2. Some important genetic disorders.
One type of genetic disorders is metabolic disorders.
Metabolic Disorders
Many proteins function as enzymes in diverse metabolic pathways, making our lives possible. A block in
any single reaction results in a shutdown of the metabolic pathway beyond that point and of substrate
accumulation before that point. Usually these metabolic pathway disorders are caused by recessive
autosomal genes. They were first identified by Archibald Garrod and are called inborn errors of
metabolism.
Have you ever noticed warning labels on cans of diet soda that read "Phenylketonurics: Contains
phenylalanine" and wondered what it means? An inborn error of metabolism known as
phenylketonuria , or PKU, is the reason.
People normally have an allele for the production of an enzyme – phenylalanine hydroxylase- that
converts the amino acid phenylalanine to tyrosine. If this allele is functioning properly, phenylalanine
will be converted to tyrosine, which will be available to be converted into thyroxine and melanin by
other enzymes. If the enzyme that normally converts phenylalanine to tyrosine is absent or not
functioning properly, toxic materials can accumulate and result in a loss of nerve cells, causing mental
retardation. Because less tyrosine is produced, there is also less of the growth hormone thyroxine,
resulting in abnormal body growth. Because tyrosine is necessary to form the pigment melanin, people
who have this condition have lighter skin color. These effects be limited by reducing the amount of
phenylalanine in the diet, a difficult thing to do since this amino acid is very common. The one
abnormal allele produces three different phenotypic effects: mental retardation, abnormal growth, and
light skin.
24
The pathway of the conversion of phenylalanine in the human body
Proteins
from food
Phenylalanine
Enzyme
phenylalanine
hydroxylase
Gene
Other sources
of tyrosine
1.
Phenylpyruvic acid
Toxic to nerve cells
Absence of enzyme encourages phenylpyruvic
acid production and prevents tyrosine
production
Tyrosine
Thyroxine (results in normal growth)
2. Less thyroxine produced
(may result in abnormal growth)
Less melanin produced
3. Light skin pigment
Melanin
(skin pigment)
Note. This diagram shows how the normal pathways work (these are in black). If the enzyme
phenylalanine hydroxylase is not produced because of an abnormal gene, there are three major
results: 1) mental retardation, because phenylpyruvic acid kills nerve cells, 2) abnormal body
growth, because less of the growth hormone thyroxine is produced, and 3) pale skin
pigmentation, because less melanin is produced.
Questions for the discussion.
1. List the mechanisms responsible for generating genetic variability in a population.
2. Explain and give examples of neutral mutations.
3. Explain and give example of “missense”-mutation.
4. Explain and give example of “nonsense”-mutation.
5. Explain and give examples of frameshift mutations.
25
6. Describe metabolic disorders in human beings, giving example.
4. CHROMOSOMAL MUTATIONS. CHROMOSOMAL DISORDERS.
Besides gene mutations, alterations in the chromosomes can also occur. Since these changes
may bring about visible changes in the phenotype, they have been referred to as chromosomal
mutations. These mutations involve a change in the chromosome number or a change in the structure of
chromosomes.
Changes in the chromosome number: euploidy and aneuploidy.
EUPLOIDY: living organisms have 2n number of chromosomes. In several cases, when reduction division is
taking place or at the time of mitosis there can occur abnormalities in the chromosome number. The changes in the
number of chromosomes in the living cells could occur in multiplies of odd number and such changes are termed
as euploidy. Instead of the normal number of chromosomes in living organisms, the number could occur as a single
set (Monoploidy) or in many multiples of the basic set of chromosomes (Polyploidy).
Monoploidy : if, among diploid organisms, due to certain reasons irregularities occurring in chromosomal
number result in individual having only a single basic set of chromosomes (haploid), the condition is termed as
Monoploidy.
Polyploidy (Multiple sets of chromosomes): Euploidic organisms having multiple sets of chromosomes 3n
(triploid), 4n (tetraploid), or 5n (pentaploid) or more are termed as Polyploid and the phenomenon is termed
as Polyploidy. This phenomenon is commonly observed among plants, whereas it occurs very rarely among
animale. Experimentally, polyploidy has been induced in tomatoes, apples, chickoos, grapes, strawbery plants, all
of which have been made polyploid. Besides these, crops such as wheat, jowar have also been made polyploid.
Polyploidy is rare among human beings. Only cancer cells have been observed to be polyploid.
ANEUPLOIDY. Organisms with changes that involve individual chromosomes are referred to as Aneuploidy.
At the time of reduction division, in the different stages sometimes, one or more of a pair of
homologous chromosomes may enter the same gamete while the other gamete does not receive
any of the chromosomes of that pair. In this way, where the homologous pair of chromosomea
fail to separate, it is known as nondisjunction. During mitosis, the centromeres of two
chromatids of any chromosome pair fail to divide on time during cell division. These chromatids
fail to separate from each other and remain in the same daughter cell. Nondisjunction may take
place in either the autosomes or the sex chromosomes.
The condition in which organisms with lack only one chromosome of a homologous pair (2n - 1) is
known as Monosomy and when two chromosomes of a homologous pair are missing (2n - 2), this event is called
Nullisomy. In organisms having (2n + 1) an extra chromosome in any one of the homologous pair of any group
of chromosomes, the condition is termed Trisomy and an increase in more than one chromosome in a pair is
termed as polysomy.
If the above-mentioned chromosomal irregularities occur in autosomes, the condition is termed autosomal aneuploidy,
and if it occurs in the sex chromosome, it is known as sex chromosomal aneuploidy.
CHROMOSOMAL DISORDERS.
The cause of these disorders is chromosomal mutations.
Autosomal Aneuploidy: These types of disorders in human beings are associated mostly with trisomy of
chromosome Numbers 3, 13, 15, 18, 21 and 22.
Individuals born with trisomy of the 3rd chromosome, die after birth, in such children, anomalies are observed
in the eyes, lips, palate and ears.
In the D group chromosomes, i.e. 13-15 chromosome pairs has trisomy, then such children have cleft palate and
deformed lips, as well as they have hearing defects and impaired brain development. Trisomy of chromosome
No.13 is called Patau syndrome.
Trisomy of chromosome No. 18 is called Edward syndrome. This trisomy leads to serious defects like brain and
ear defects, flattening of the skull, receeding lower jaw, short neck and kidney abnormalities. Such children do not
survive more than a year.
Trisomy of chromosomes in В and С group, i.e. No. 4 -> 12, result in abortion of the embryos, which do not
complete the embryonic development. Usually such embryos are aborted in the early stages of pregnancy.
The occurrence of trisomy in chromosome number 21 is comparatively more frequent. This defect is known as
Down syndrome. Such individuals manifest certain characteristics : short stature, large head, short neck,
26
flat and small nose, protruding or drooping lower lip, puffed eyelids, widely spaced eyes and upward slant of eye,
open mouth, thick protruding tongue, short stubby fingers with flat palm, poorly developed genitalia, mental
retardation, etc. There is an increased frequency of occurrence of this disorder among older women who bear
children.
Sex chromosomal Aneuploidy :
In females, the 23 rd pair of chromosomes is XX and XY in males. Any increase or decrease in this number results
in disorders such as :
Turner Syndrome : This genetic disorder is observed among females. The karyotype of such a female has the 23rd pair
as Monosomy (XO), This disorder among females has been termed as Turner Syndrome. The reproductive organs
are poorly developed. In several cases the ovaries may be absent or poorly developed. In such females characters
such as shield-shaped chest, poorly developed breasts, small uterus, short stature and short neck, are observed.
Super female: Such females have sex chromosomes as XXX, XXXX or XXXXX. A female with such a karyotype
does not have well-developed female secondary sex characters, but she may have physical disabilities, mental
retardation and sterility.
Klinefelter syndrome : This disorder is observed among males. S u c h m a l e s h a v e p o l y s o m y i n t h e i r sex
chromosomes, where two to three X-chro mo so mes are with a single Y-chromosome, i.e., XXY, XXXY.
Male with such disorders have shorter upper body segment than the lower i.e. length of the legs is greater. The
testis are poorly developed,
breasts are well developed, lack of hair on body, absence of facial hair, mental retardation and sterility also
occur.
In certain men, XYY chromosomal complements are found. The secondary sex characters and phenotypic
features of such individuals are similar to normal, but they are taller than normal, mentally retarded, highly
aggressive and have criminal tendencies and therefore they can be a nuisance to society. Sometimes their
karyotype can also be XXYY.
Changes in chromosome structure (chromosome aberrations).
Every living cell possesses a specific set of chromosomes which bear a specific size
and structure, and occur in pairs in a diploid organism. Any alteration in the chromosome
architecture can be detected either by genetic tests or cytologically by simply viewing the
chromosomes, preferably in paired condition under the microscope. Four different types
of structural alterations (deletion, duplication, inversion, translocation) have been
identified all of which involve chromosome breakage and then rearrangements in some
cases. Many of these aberrations change the order of genes on the affected
chromosome.
1. Sometimes a part of the chromosome is lost. This is known as deletion or deficiency.
This loss can be from one end or from chromosome parts between the ends.
Accordingly, the former is known as terminal and the latter interstitial deletion.
For example, ABCDE -----------► ADC. A part of chromosome (DE) is lost and broken
by enzymes.
ABCDE ---------► ADE. A part of chromosome (BC) is lost and broken by enzymes.
2.A duplication occurs when a portion of a chromosome is repeated. The duplication
can either be in tandem sequence or in reverse order. Acquisition of additional genetic
material can lead to new functions or serious deleterious effects.
For example, ABCDE ---------► ABCBCDE. A portion of chromosome (BC) is repeated
twice.
ABCDE ---------► ABCCBDE. A portion of chromosome (BC) is repeated twice in
reverse order.
3.When a segment of chromosome breaks but later rejoins after rotating by 180° it results in inversion.
If the centromere is included in the inverted segment, it is known as pericentric but if inversion occurs
only in one arm and the centromere is not involved, it is referred as paracentric inversion.
For example, ABCDE ---------►ACBDE. A portion of chromosome (BC) is rotated by 180 0.
4.Translocations occur when a segment of one chromosome is moved to another nonhomologous chromosome.
Reciprocal translocations occur when two nonhomologous chromosomes exchange their segments.
For example, ABCD and
KLMN
---------►
ABMN and
KLCD.
Nonreciprocal translocations or transpositions occur when a segment of one chromosome is moved to another
27
place of the same chromosome or to another chromosome.
ABCD and
KLMN
---------►
AB and
KLMNCD
One type of transposons is mobile DNA segments or jumping genes that move from place to place within or
between chromosomes.
Chromosome aberrations are generally harmful to the individual. These mutations are the cause of chromosomal
disorders.
Recently many physical and mental symptoms known as Prader-Willi syndrome have been associated with a
deletion of part of chromosome number 15. As infants these individuals do not feed well because of a poor sucking
reflex. However, by the age of five, they develop an uncontrollable urge to eat that results in obesity and related
health problems, such as diabetes.
A deletion of a small segment of the short arm of chromosome number 5 in human beings is known as the Cri- duchat ( «cat’s cry») syndrome. As a result, such individuals have small head circumference, with poor mental
development. Such infants have abnormal cry like the cry of a cat in their childhood. Such a child rarely survives
beyond childhood.
Philadelphia syndrome: A child with this syndrome may have loss or deletion of a small segment of the long arm
of chromosome No.22. Such a child may have leukemia (blood cancer).
Chromosome aberrations produce unbalanced meiotic products thus leading to
sterility. Some of them cause new location for a gene and so may often lead to changed
expression.
Questions for the discussion.
1. Describe polyploidy.
2. Explain origin of nondisjunction of chromosomes.
3. Explain and give examples of autosomal aneuploidy in human beings.
4. Explain and give examples of sex chromosomal aneuploidy in human beings.
5. Describe deletions, duplications, translocations, transpositions and inversions, giving
examples.
6. Indicate the cause of chromosomal disorders and list chromosomal disorders in human
beings.
5. GENETIC
TASKS.
REMEMBER!
Solving a heredity problem consists of five basic steps:
Step 1: Assign a symbol for each allele.
Step 2: Determine the genotype of each parent and indicate a mating.
Step 3: Determine all the possible kinds of gametes each parent can produce.
Step 4: Determine all the gene combinations that can result when these gametes unite.
Step 5: Determine the phenotype of each possible gene combination.
Task №1.
In certain pea plants, the allele for purple flowers is dominant over the allele for white
flowers.
A) If the homozygous-recessive plants and heterozygous plants are crossed, what
will the phenotype and genotype of the offspring be?
28
B) If both individuals are heterozygous, what will the phenotypic and genotypic ratios
of offspring be?
Task №2
In humans, the allele for free earlobes is dominant and the allele for attached earlobes
is recessive. If both parents are heterozygous, what is the probability that they can have
the child with free earlobes? With attached earlobes?
Task №3.
Some people are unable to convert the amino acid phenylalanine into the amino acid
tyrosine. The buildup of phenylalanine in the body prevents the normal development of
the nervous system. Such individuals suffer from phenylketonuria (PKU) and may
become mentally retarded. The normal condition is to convert phenylalanine to tyrosine.
It is dominant over the condition for PKU. If one parent is heterozygous and the other
parent is homozygous for PKU, what is the probability that they will have the child who
is normal? A child with PKU?
Task №4
In humans, the allele for fair- haired is recessive and the allele for dark- haired is
dominant. A fair-haired man marries a dark-haired woman. What will the phenotype
and genotype of the offspring be?
Task №5
In humans, the allele for short fingers is dominant over the allele for normal length of
fingers. The allele for blue-eyed is recessive and the allele for brown-eyed is dominant.
It’s known, that two genes are located on different chromosomes. The blue-eyed
woman with short fingers married the brown-eyed man with normal length of fingers.
They have got two babies: the brown-eyed girl with normal length of fingers and the
blue-eyed boy with short fingers. What is the genotype of each parent? What is the
probability that the next baby will have brown eyes and normal length of fingers?
Indicate the genotype and phenotype of offspring.
Task №6
In humans, the alleles for albinism and left-handed use are recessive. Both parents
have normal skin pigmentation and use a right hand. Their baby is an albino using left
hand. What is the genotype of each parent? What is the probability that the next baby
will have normal skin pigmentation and right-handed use? It’s known, that two genes
are located on different chromosomes.
Task №7
In humans, the allele for free earlobes is dominant over the allele for attached earlobes.
The allele for dark –haired dominates the allele for fair- haired. If both parents are
heterozygous for earlobes shape and hair color, what types of offspring can they
produce, and what is the probability for each type? It is known, that two genes are
located on different chromosomes.
Task №8.
In humans, the allele for brown eyes is dominant over the allele for blue eyes. The
allele for right-handed use dominates the allele for left-handed use. The blue -eyed
man using a right hand married the brown -eyed woman using a right hand. They have
got two babies: a brown - eyed baby using left hand and a blue eyed baby using right
29
hand. What is the genotype of each parent? What is the probability that the next baby
will have blue eyes and left-hand use? It is known, that two pairs of alleles are located
on separate chromosomes.
Task №9.
In humans, the allele for brown eyes is dominant over the allele for blue eyes. The
allele for right-handed use dominates the allele for left-handed use. Both parents are
heterozygous. What will the phenotype and genotype of the offspring be? What is the
probability for the blue-eyed babies using left hand? It’s known, that two genes are
located on different chromosomes.
Task №10.
In humans, the allele for short fingers is dominant over the allele for normal length of
fingers. The allele for curly- haired dominates the allele for straight -haired. It’s known,
that two genes are located on different chromosomes.
The husband has normal length of fingers and straight hair. His wife has short fingers
and curly hair. They have got two babies: a girl with normal length of fingers and curly
hair, and a boy with short fingers and straight hair. What is the genotype of each
parent? What is the probability that the next baby will have short fingers and curly
hair?
Task №11.
In certain pea plants, the allele for height is dominant over the allele for shortness. The
allele for round pods is dominant over the allele for constricted pods. It’s known, that
these traits are inherited independently. If both individuals are heterozygous, what will
the phenotypic and genotypic ratios of the offspring be?
Task №12.
In guinea-pigs, the allele for dark – coloured coat dominates the allele for whitecoloured coat. And the allele for smooth-haired is recessive and the allele for irregularhaired is dominant. It’s known, that two genes are located on the different
chromosomes. Heterozygous irregular-haired, white-coloured females are crossed with
heterozygous smooth-haired, dark-coloured males. What is the genotype of each
parent? What will the phenotypic and the genotypic ratios of the offspring?
Task №13.
In humans, the allele for tongue-rolling is dominant over the lack of this ability. Freckles
appear to be dominant over no freckles. It’s known, that two genes are located on the
same chromosome. Both parents are heterozygous. What is the genotype and
phenotype of each parent? What will be the phenotype and genotype of the offspring?
Task №14.
In humans, fair- haired is a recessive trait, and dark- haired is a dominant trait. Having
extra fingers is dominant over a normal number of digits. The husband has fair hair and
a normal number of fingers. His wife has dark hair and extra fingers. They have got four
babies: a fair-haired girl with a normal number of fingers, a dark-haired boy having extra
fingers, a fair-haired boy having extra fingers and a dark-haired girl with a normal
number of fingers. What is the genotype of each parent and the offspring? It’s known,
that two genes are located on the same chromosome.
Task №15.
30
In rats, the allele for dark – coloured coat is dominant over the allele for the white –
coloured coat. The allele for pink eyes is dominant over the allele for red eyes. Two
genes are located on the same chromosome. Pink-eyed rats with dark – coloured coat
are mated with red-eyed rats having white –coloured coat. There are two kinds of
offspring: pink-eyed rats with dark- coloured coat and red-eyed rats with white –
coloured coat. What are the genotypes of both parents and offspring?
Task №16.
In certain corn plants the allele for round seeds is dominant over the allele for wrinkled
seeds. The allele for coloured seeds is dominant over the allele for colourless seeds.
Two genes are located on the same chromosome. If both individuals are heterozygous,
what will the phenotypic and genotypic ratios for the offspring be?
Task № 17.
In certain tomato plants the allele for shortness is recessive and the allele for height is
dominant. The allele for round-shaped fruit dominates the allele for pear-shaped fruit.
It’s known, that two genes are located on the same chromosome. High plants with
round-shaped fruit were crossed with short plants having pear-shaped fruit. There were
100 high plants with round-shaped fruit and also 100 short plants with pear-shaped
fruit. What is the genotype for each parent and offspring?
Task №18.
In humans, the absence of sweat glands is a recessive gene. It’s located on the Xchromosome. The husband doesn’t suffer from this disorder. His wife has sweat glands,
but her father doesn’t have sweat glands and her mother is healthy. What are the
genotypes of the parents and grandparents? What is the probability that this couple will
have babies suffering from this disorder?
Task №19.
In humans, night blindness is a recessive gene. It’s located on the X-chromosome. The
husband has a normal vision, but his father suffered from this disorder. The wife has a
normal vision too, but his father had night blindness. What vision will the babies have?
Task №20.
In humans, hemophilia A is a disease that prevents blood from clotting normally. It’s
caused by a recessive allele located on the X-chromosome. A boy has the disease. But
both parents don’t have the disease. What are the genotypes of his parents? What is
the probability that the next boy will be healthy? What are the genotypes and
phenotypes of this couple’s offspring?
Task №21.
In humans, the gene of color blindness is recessive. It’s located on the X-chromosome.
The wife has a normal vision, but her father suffered from this disorder. The husband
has a normal vision too, but his father suffered from color blindness. What vision will
their babies have? What are the genotypes of parents and grandparents?
Task №22.
In humans, enamel hypoplasia of teeth is caused by a dominant gene located on the X-
31
chromosome. Both parents suffer from this disorder. They have got a boy with normal
teeth. What teeth will the next boy have?
Task №23.
In fruit flies, the allele for red eyes dominates the allele for white eyes. These genes are
located on the X- chromosomes. Red-eyed females were crossed with white-eyed
males. There were the offspring: red-eyed and white-eyed males and red-eyed
females. What are the genotypes of the parents and offspring?
Task №24.
In humans, hemophilia A is caused by a recessive allele located on the X-chromosome.
The man suffering from hemophilia married a normal woman, but her father suffered
from this disorder. What is the genotype of each parent? What is the probability that
their babies will be healthy?
6.Pre-exam guestions.
1. The main points of a cell theory.
2. What are the differences between the
prokaryotic and eukaryotic cells?
3. The structure of the cell membrane.
4. The functions of the cell membrane.
5. Cytoplasm, its composites: hyaloplasm,
organelles and inclusions.
6. Hyaloplasm, its chemical composition
and functions.
7. The endoplasmatic reticulum, its
structure and functions.
8. The Golgi complex, its structure and
functions.
9. The lysosomes, their structure and
functions.
10.The mitochondria, their structure and
functions.
11.The ribosomes, their structure and
functions.
12.The centrioles, their structure and
functions.
13.Cytoskeleton,
its
structure
and
functions.
14.Cilia and flagella, their structure and
functions.
15.Inclusions and their role in the cells.
16.The structure and functions of the
nuclear envelope, nucleoplasm and
nucleoli.
17.The structure and importance of DNA.
32
18.Genome of the cell. Genome of the
eukaryotic cell.
19.What are the peculiarities of genome of
the prokaryotic cell?
20.Chromosomes,
their
chemical
composition and structure.
21.Types of metaphase chromosomes.
22.Heterochromatin and euchromatin.
23.What is a karyotype? Human karyotype
and the classification of human
chromosomes.
24.What is the cell cycle? The events in the
cell during interphase.
25.The DNA replication.
26.The stages of mitosis.
27.The importance of mitosis.
28.The structure of female sex cells (eggs).
29.The structure of male sex cells (sperm
cells).
30.The stages of gametogenesis.
31.The stages of meioses 1 and meiosis 2.
32.The importance of meiosis.
33.Mendel`s law of dominance.
34.Mendel`s law of segregation. Test cross.
35.Mendel`s
law
of
independent
assortment.
36.Complete gene-linkage.
37.Incomplete gene-linkage.
38.Sex determination.
39.Sex-linked genes.
40. Gene mutations.
41.Chromosomal mutations.
REFERENCES:
1. Биология. Под редакцией проф. В.Н.Ярыгина, М., «Высшая школа»,
Т.1,2003, с.432.
2. Молекулярная биология клетки. М., «Мир»,Т.1-3, 1998.
33
3. Н.П.Бочков. Клиническая генетика.2002.
4. В.А.Шевченко. Генетика человека. 2002.
5.Chakraborti D.P. et al. Biology. Textbook for class X1, 2002.
6. Chares V.Mann, R.C.G.Russel, Norman S. Williams. Bailey and Love’s. Short
Practise of Surgery. 22nd edition, Chapman and Hall Medical, 1995, 1041p.
7. Enger Ross. Concepts in biology, 8th edition, WCB, 1997, 458p.
8. General Medicine, Syllabi, Moscow, 1967, 308p.
9. George W. Thorn et al. Harrison’s Principles of Internal Medicine, 8th edition,
1977, 1063p.
10. Joseph Mannino. Human Biology, Mosby,- Year Book, Inc., 1995, 456p.
11. Raven, Jonson. Biology, third edition, Mosby,-Year Book, Inc., 1992, 1217p.
12. Biology Digest. Std.11, Navneet Publications, India, Ltd., Dantali, Gujarat,
2000, 334p.
13. Shri R.C. Raval. Biology, Std.12, Gujarat State Board of School Textbook,
Gandhinagar, 2000, 316p.