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GENETICS
CHAPTER
1
Mendel’s Experiments and Principles of Inheritance
The contribution of Mendel to Genetics is called Mendelism. Gregor Johann Mendel from 1822–1884,
the father of genetics who was an Austrian monk made crosses in garden pea (Pisum sativum) and
conducted experiments during 1856-1863. He presented his results of the experiments before the Natural History of Society at Brunn in 1865. His paper was published in 1866 in the Annual Proceedings of
the Society and distributed to libraries in Europe and America but no one appreciated and it was neglected until 1900. In 1900 the principles of genetics worked by Mendel were rediscovered by three
botanists, namely Correns (Germany), De Vries (Holland) and Tshermark (Austria). Bateson confirmed
Mendel’s work by a series of hybridization experiments.
MENDEL’S EXPERIMENTS
Mendel crossed varieties of edible peas (Pisum sativum). For example, he crossed a red-flowered variety with a white flowered variety. He did this by dusting the pollen of one variety (the red) on the pistils
of other (the white). Of course, he prevented the white plant from pollinating itself. This he did by
removing the stamens of the white flowers before the flowers had opened and shed their pollen. After
pollinating the emasculated white flowers with the red pollen, he enclosed them in bags in order to
prevent insects from getting to them with pollen from unknown sources. Thus he crossed the red variety and the white. The offspring of the cross was red. Mendel then self-fertilized the off-spring and he
found that they produced off spring of their own in the ratio of 3 reds : 1 white.
The pea plant contains a number of contrasting characters. Out of these contrasting characters,
Mendel selected only seven characters. Mendel in his first experiment crossed two plants differing in
one character (height) only. A plant having a tall stem was crossed with another plant having dwarf
stem. Tall and dwarf are the two varieties of a single character, height. Such crosses, where parents
differ in one pair of alternative characters are known as monohybrid crosses. The resulting hybrids are
known as monohybrids. When the behaviour of each single character was established, Mendel crossed
two plants differing in two characters, such as flower position and height of the stem. A plant having
axial flower and a tall stem was crossed with a plant having terminal flower and a short stem. Such
crosses, where parents differ in two pairs of alternative characters are known as dihybrid crosses. The
resulting hybrids are known dihybrids. The plants involved in the above crosses are called parent
plants. They form the parental generation which is marked by P. The first hybrid generation resulting
from a cross between parental plants is called the first filial generation and is marked as F1. The second
generation of hybrids arising from the self or cross fertilization of F1 hybrid generation is called the
second filial generation and is marked as F2.
3
4
GENETICS AND PLANT BREEDING
1. Law of Dominance
Each organism is formed of many characters and each character is controlled by a pair of factors
or genes (T or t). Mendel’s law of dominance states that one factor in a pair may mask or prevent the
expression of the other. He called the character appeared in the F1 generation or his monohybrid cross
as dominant and those which did not appear in the F1 generation as recessive. A recessive factor freely
expresses itself in the absence of its dominant allele. This law is formulated based on the monohybrid
experiment.
2. Law of Segregation (Segregation of Genes)
From his experiments Mendel concluded that each parent contributed one factor for a character
to the F1 hybrid. In this way the F1 hybrid has two factors for each character. When the F1 hybrid forms
gametes the two factors separate from each other. There is no mixing up of factors thus emphasizing
the purity of gametes. The phenomenon of separation became Mendel’s second law of principle and
was later termed as the Law of Segregation. This is explained diagrammatically as follows:
P
Tall plant
(true breeding)
Factors
Gametes
F1 hybrid
F1 gametes
F2 segregation
X
TT
(T)
Dwarf plant
(true breeding)
tt
(t)
Tall Tt (Selfing)
i.e. Tall Tt
(T) (t) X (T) (t)
3 Tall : 1 Dwarf
TT, Tt, Tt, tt
3. Law of Independent Assortment
This law is based on dihybrid experiment. According to this law, the genes for each pair of
characters separate independently from those of other characters during gamete formation.
Example :
P
Round Yellow
RRYY
Gametes :
F1
(RY)
F1 gametes
X
Round Yellow
RrYy
(RY) (Ry) (rY) (ry)
1:1:1:1
Wrinkled Green
rryy
(ry)
X Self (Round Yellow)
RrYy
(RY) (Ry) (rY) (ry)
5
MENDEL’S EXPERIMENTS AND PRINCIPLES
F2 Checker board
RY
Ry
rY
ry
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
9:3:3:1 ratio
Mendel applied the principles of a monohybrid cross in the dihybrid cross, the true breeding
round yellow parent must be homozygous RRYY, and the wrinkled green parent rryy. Since each character is determined by two factors, in a dihybrid cross there must be four factors present in each parent.
Likewise the F1 hybrid must be RrYy. Mendel found that the pair of factors for roundness will be
behaving independently of the pair of factors for yellow colour of seeds. In other words, one factor for
a character must be passing independently of a factor for another character. Thus in the F1 hybrids, R
and r pass into different gametes. Now the probability of an R gamete formed is one-half, and of r
gamete also one-half. Similar probabilities exist for Y and y gametes. It follows that the probability that
R and Y will go to the same gamete is one fourth, as also of R and y, r and Y, and r and y. Therefore,
gametes containing factors RY, Ry, rY and ry will form in equal proportions (1:1:1:1).
The F1 hybrid producing the four types of gametes mentioned above was selfed. The results
expected in the F2 progeny can be predicted by making a checker board or a Punnett Square. Gametes
produced by one parent are plotted on top of the checker board, and the gametes of the other parent on
the vertical side. The 16 square of the checker board are filled up by making various possible combinations of male and female gametes during fertilization. The phenotypes read out from the checker board
indicate a 9:3:3:1 ratio exactly as observed by Mendel.
Back Cross and Test Cross
“Back cross is the cross of F1 hybrid with any one of its parents”. “Test cross is the cross of F1
hybrid with recessive parent”. Mendel verified his results by performing the test cross. He crossed the
F1 hybrid heterozygous for both characters with a double recessive parent (rryy) which will produce
only one type of gamete ry. The uniformity in the gametes of the recessive parent determines the differences in the types of gametes produced by the heterozygous parent. Now the hybrid RrYy produces
gametes carrying RY, Ry, rY and ry with equal frequency (1:1:1:1). It follows that during fertilization if
all these four types of gametes unite with ry gamete of the recessive parent, the resulting progeny will
show all the four combinations of characters also in equal proportions. Indeed, Mendel observed the
test cross progeny to consist of Round yellow, Round green, Wrinkled yellow and Wrinkled green
plants in the ratio of 1:1:1:1.
F1 :
Gametes :
F2
ry
Round yellow
RrYy
(RY) (Ry) (rY) (ry)
1:1:1:1
RY Ry rY ry
RrYy Rryy rrYy rryy
X
1:1:1:1
Wrinkled green
rryy
(ry)
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GENETICS AND PLANT BREEDING
From the results of his dihybrid crosses, Mendel realized the following facts. At the time of
gamete formation the segregation of alleles R and r into separate gametes occurs independently of the
segregation of alleles Y and y. That is why the resulting gametes contain all possible combinations of
these alleles. i.e., RY, Ry, rY, ry. In this way Mendel proved that when two characters are considered in
a cross, there is independent assortment of genes for each character, and this became the Law of Independent Assortment.
The Chromosomal Theory of Heredity
The Mendelian laws of inheritance, formulated in 1865, still form the basis of our understanding
of the transmission of heritable variation, and the inheritance test is still the basic technique for following
phenotypic characters through the several generations. It is the genius of Mendel that a few, but
quantitatively predictable, results enabled him to postulate that an abstract entity—the element or factor,
as he called it, the gene as we now designate it—was responsible for the phenotypic character he was
following in his breeding experiments, that this entity was singly represented in gametes and doubly so
in zygotes and somatic tissues, and that factors, when together in pairs, could exhibit dominance or
recessiveness in expression.
The egg and sperm of animals, despite obvious differences in size and shape, had earlier been
shown to be cells; the physical continuity between generations was, therefore, a slender cellular bridge.
Fertilization was shown to be the union of gametes, with the fusion of parental nuclei in the cytoplasm
of the egg being the crucial event, and with each nucleus providing equal, or nearly equal, numbers of
chromosomes to the newly formed zygote and hence to the developing embryo. As the embryo grew by
adding to its cell numbers, the critical acts of cell division were the longitudinal replication of each
chromosome, and the segregation of these longitudinal halves to the two daughter cells, thus providing
both a physical basis for the qualitative and quantitative chromosomal equality of each daughter nucleus
and a functional basis for the conservation of the genotype. Meiosis, a kind of cell division leading to
the formation of eggs or sperm in animals and spores in most plant species, was revealed as a mechanism
for halving the chromosome number; thus providing a means for counteracting the chromosome doubling
that took place earlier in the fusion of parental nuclei in fertilization and making clear the patterns of
haploidy and diploidy in the life cycle. Chromosomes had been demonstrated to have a physical continuity
from one cell to another, and from one generation to another, and to be qualitatively different from each
other in so far as they affected developmental process. By 1902, Garrod, through his study of the
metabolic diseases of humans—“in born errors of metabolism”, as he called them—would forge the
first link that would couple the biochemistry of disease syndromes to patterns of inheritance. With the
further observation that species were characterized by a constant number of chromosomes, although
the two sexes of a species may differ slightly from each other, it was becoming more and more obvious
that the patterns of heredity or inheritance were somehow linked to, or mirrored in, the behaviour of
chromosomes.
In retrospect, therefore, the discoveries made during the last quarter century of 1900s and the
first few years of the 20th century made it increasingly clear that the chromosomes were key elements
whose behaviour in division and fertilization revealed a regularity that could, on the one hand, account
for the transmission of heredity factors from one generation to another, whether the cells or of individual organisms, and on the other for the conservation of species reproduction, i.e., the preservation of
type, generation after generation. The science of cytogenetics was thus launched with a brilliant correlation of factor (gene) and chromosome behaviour.
MENDEL’S EXPERIMENTS AND PRINCIPLES
7
The basis of the theory, as stated by Sutton, is as follows:
1. In somatic cells, arising from a fertilized egg, or zygote, the chromosomes consist of similar
groups, one of maternal origin inherited through the egg, the other of paternal origin and
inherited through the sperm. Each somatic nucleus, therefore, contains pairs of like
chromosomes, or homologues, the number of pairs being the same as the haploid set of
chromosomes in a gamete. Hence, the chromosomes, like Mendel’s factors, are doubly
represented in the somatic cells of an organism, and singly, or pure, in the gametes.
2. The chromosomes retain their structural individuality and their continuity throughout the life
cycle of an organism. Again, the factors of Mendel retain their individuality and continuity
even though the character they determine might not be expressed. The basis of genetic
homogeneity and heterogeneity, dominance and recessiveness, was suggested by chromosomal
behaviour.
3. In meiosis, synapsis brings together pairs of homologous chromosomes and leads to their
subsequent segregation into different cells, establishing thereby a quantitative basis for both
the segregation of factors and the independent assortment found when two pairs of contrasting
factors are considered together.
4. Each chromosome, or chromosome pair, plays a definite role in the development of the
individual. This conclusion derived not from any specific facet of Mendel’s investigations
but rather from studies of Boveri of abnormal larvae of Sea Urchins which lacked certain
normal chromosomes, as well as Sutton’s own studies of the size differences among the
chromosomes of the insect Brachystola.
Sutton, consequently, visualized the chromosomes as the physical carriers of Mendelian factors
and the segregation of a pair of homologous chromosomes and the independent assortment of nonhomologous chromosomes as the physical basis for the qualitative and quantitative aspects of Mendelian segregation. Sutton, together with Roux and Boveri, also anticipated the phenomenon of linkage
when he stated that all the factors in any one chromosome must be inherited together. As the theory of
gene and the concept of Mendelian factors, the heredity constitution of an organism, and eventually of
a species, the notions would eventually lead to a further refinement, namely, the molecular basis of
hereditary phenomena.
Gene Interactions and Modified Dihybrid Ratios
According to Mendel each character is controlled by a pair of factors or genes. But later discoveries proved that in many cases the expression of a single character is controlled by the interaction of
more than one pair of genes. This is called “Interaction of genes” or “Factor hypothesis”. This was
proposed by Bateson and Punnet. This hypothesis states that certain characters are controlled by the
interaction of two or more genes. The genic interaction may occur in between genes located in the same
chromosome or different chromosomes. This type of genic interaction is known as non-allelic genes
interactions. The genic interaction may also occur between the two alleles of a single type of gene. This
type of genic interaction is known as allelic gene interaction. However, Mendelian laws will hold good
in all the type of segregation and interactions. As a matter of interaction the phenotypic ratio will alone
be modified and not the genotypic ratio.
Complementary Genes (9:7)
Complementary genes may be defined as “two or more dominant genes occurring in different
loci of the same chromosome or different chromosomes interact with one another to produce a character
8
GENETICS AND PLANT BREEDING
but neither of them produces that character in the absence of the other”. The action of these independent
genes are complementary.
Bateson and Punnet studied the inheritance of flower colour in sweet pea, Lathyrus odoratus.
There are two varieties of pea plants, one producing red flower and the other white flower.
P:
White flower
CCaa
(Ca)
Red flower
CcAa
(Selfing)
Cc Aa
(CA) (Ca) (cA) (ca)
Gametes :
F1
Gametes :
CA
Ca
cA
ca
X
White flower
ccAA
(cA)
X
CcAa
(CA) (Ca) (cA) (ca)
CA
Ca
cA
ca
CCAA
Red
CCAa
Red
CcAA
Red
CcAa
Red
CCAa
Red
CCaa
White
CcAa
Red
Ccaa
White
CcAA
Red
CcAa
Red
ccAA
White
ccAa
White
CcAa
Red
Ccaa
White
ccAa
White
ccaa
White
9(Red) : 7 (White)
The cross between two white varieties can be explained by assuming two genes for red colour
which must be present together, i.e., must act in a complementary way to each other. Thus each gene
independently contributes something different but essential for synthesis of red pigment. If one of the
two genes for red colour is absent, the result is a white flower.
The inheritance of the colour of aleurone layer in corn also demonstrates interaction of
complementary genes. The outermost layers of endosperm in the maturing corn kernels become modified
into a specialized aleurone tissue, so named because the cells have rich deposits of aleurone grains. In
corn the aleurone layer is coloured due to anthocyanin pigments in the cells, and is controlled by
complementary effect of two genes.
Supplementary Genes (9:3:4)
Supplementary genes may be defined as two independent pairs of dominant genes, which interact in such a way that one dominant gene produces its effect, when the second dominant gene is added
to the first, a new character is expressed.
Coat Colour in Mice
Inheritance of coat colour in mice was studied by Castle. There are three different varieties of
mice, they are agouti (grey), black and albino (white). Agouti colour is dominant to both black and
9
MENDEL’S EXPERIMENTS AND PRINCIPLES
albino. Black is dominant to albino but recessive to agouti. Albino is recessive to both agouti and black.
Agouti is produced by dominant gene A in the presence of another dominant gene B. Dominant gene B
alone produces black colour. Dominant gene A produces albino. The recessive condition of these genes
causes albino.
Castle crossed a homozygous black mice (BBaa) with a homozygous albino (bbAA). The F1
individuals are Agouti. When the F1 Agouties are inbred their progeny consists of 9 agouti, 3 black and
4 albino.
Parents :
Black male
BBaa
(Ba)
X
Albino female
bbAA
(bA)
BbAa Agouti
(two F1 individuals
crossed)
Bb Aa
(BA) (Ba) (bA) (ba)
X
Bb Aa
(BA) (Ba) (bA) (ba)
Gametes :
F1 generation:
F1 Gametes :
BA
Ba
bA
ba
BA
BBAA
Agouti
BBAa
Agouti
BbAA
Agouti
BbAa
Agouti
Ba
BBAa
Agouti
BBaa
Black
BbAa
Agouti
Bbaa
Black
bA
BbAA
Agouti
BbAa
Agouti
bbAA
Albino
bbAa
Albino
ba
BbAa
Agouti
Bbaa
Black
bbAa
Albino
bbaa
Albino
9:3:4
Agouti : Black : albino
Duplicate Gene (15:1)
When two or more genes have the same effect on a given trait, they are referred to as duplicate
genes. In maize the gene for yellow endosperm is dominant over white endosperm. A pure breeding
yellow endosperm plant when crossed to a white endosperm plant yields yellow endosperm in F1. On
self pollination of F1 hybrids in F2 generation of 15 yellow and 1 white endosperm is obtained. The
yellow endosperm results from two independent dominant genes Y1 and Y2. When anyone of these two
dominant genes or both together are present, yellow endosperm is produced. When only recessive
alleles are present in the homozygous condition (y1y1y2y2) it forms white endosperm. Thus the dominant
genes Y1 and Y2 have an identical effect on endosperm colour and are consequently termed duplicate
genes or isogenes.
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GENETICS AND PLANT BREEDING
Parent :
Yellow endosperm
Y1Y1y2y2
(Y1y2)
Yellow endosperm
Y1y1Y2y2
(Y1Y2) (Y1y2)
(y1Y2) (y1y2)
Gametes :
F1
F1 gametes
X
Yellow endosperm
y1y1Y2Y2
(y1Y2)
(Selfing)
Y1Y2
Y1y2
y1Y2
y1y2
Y1Y2
Y1Y1Y2Y2
Yellow
Y1Y1Y2y2
Yellow
Y1y1Y2Y2
Yellow
Y1y1Y2y2
Yellow
Y1y2
Y1Y1Y2y2
Yellow
Y1Y1y2y2
Yellow
Y1y1Y2y2
Yellow
Y1y1y2y2
Yellow
y1Y2
Y1y1Y2Y2
Yellow
Y1y1Y2y2
Yellow
y1y1Y2Y2
Yellow
y1y1Y2y2
Yellow
y1y2
Y1y1Y2y2
Yellow
Y1y1y2y2
Yellow
y1y1Y2y2
Yellow
y1y1y2y2
White
15 (Yellow) : 1 (White)
George H. Shull (1914) reported a case of duplicate gene in the common weeds, shepherd purse
Bursa bursapastoris. There are two varieties of seeds. Normal variety produces a triangular seed case
and the other mutant variety produces oval seed case. When these two varieties are crossed, the resulting
F1 plants produce triangular seeds (which is thus dominant). When the F1 hybrids are selfed, in F2
plants with triangular seeds and oval seeds are produced in the ratio 15:1 instead of the normal Mendelian
ratio.
Epistatic Genes (13:3)
Epistasis is a Greek word which means stopping or suppression. In some animals a gene at one
locus on a chromosome suppresses or masks the expression of a gene at another locus. Such genes are
known as inhibiting genes, since they inhibit the expression of other genes. A gene that inhibits or
masks the expression of another gene (non-allelic gene) is said to be epistatic. The gene that is masked
is said to be hypostatic.
In poultry white birds belong to two different varieties namely white leghorns or white wyandottes.
Experiments reveal that the gene for white plumage of white leghorns is dominant over the gene for
coloured plumage of coloured varieties. But the gene for white plumage of white wyandottes is recessive
to the gene for coloured plumage of coloured varieties. Therefore, the gene which produces white
plumage in white leghorns is different from the gene for white plumage in white wyanodottes.
A cross between a white leghorn and a white wyanodotte gives an F1 of white birds. When such
birds are inbred, the F2 progeny segregates in the ratio of 13 white to 3 coloured birds. The experiment
is explained below by postulating two gene C and I for the white leghorns.
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MENDEL’S EXPERIMENTS AND PRINCIPLES
Parents :
White Leghorn
CCII
White CcIi
F1
F1 gametes :
X
White Wyanodotte
ccii
X Inbred
(CI) (Ci) (cI) (ci)
(CI) (Ci) (cI) (ci)
F2
CI
Ci
cI
ci
CI
CCII
White
CCIi
White
CcII
White
CcIi
White
Ci
CCIi
White
CCii
Colour
CcIi
White
Ccii
Colour
cI
CcII
White
CcIi
White
ccII
White
ccIi
White
ci
CcIi
White
Ccii
Colour
ccIi
White
ccii
White
F2 ratio :
13 White : 3 colour
The F2 ratio indicates that only three out of 16 genotypes, that is CCii, Ccii, Ccii produce coloured
birds. The white leghorn obviously contain a gene I, which in the dominant state inhibits or suppresses
the expression of the dominant colour gene C, resulting in white plumage. The recessive alleles of the
inhibitor gene (ii) produce coloured birds due to expression of gene C. In other words gene I is epistatic
to gene C. This is a case of dominant epistasis because even one dominant allele of gene I is able to
express itself.
Lethal genes
A lethal gene produces an effect which differs from the normal condition that its possessor is
unable to survive. There is a mutant recessive gene which causes internal adhesions of the lungs. A
child homozygous for the lethal gene might survive upto embryonic development. But at birth, when it
suddenly becomes dependent upon its lungs for its oxygen supply, it would die because its lungs could
not expand properly. Being recessive, it could be carried by normal parents in heterozygous condition
without any ill effects. In a mice cross heterozygous yellow (Yy) XYy produces offspring 1YY dies
(homozygous yellow) : 2Yy (heterozygous yellow) : 1yy homozygous non-yellow. Hence a 2 : 1 ratio
is realised. The lethal gene dominant in mouse, is thus lethal, causing death in homozygous condition
YY.