Download Lecture 3. - Government Degree College Pulwama

An Introduction to Genetics
Dr Rouf Ahmad Mir, Department of Botany
Govt. Degree College Pulwama j&k
Email: [email protected]
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 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.
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.
Mendel’s Laws of Inheritance
Dr Rouf Ahmad Mir, Department of Botany
Govt. Degree College Pulwama j&k
Email: [email protected]
1. 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
Dwarf plant
(true breeding)
Factors
TT (T)
Gametes
Gametes (T) (t)
X
(true breeding)
Tall Tt (Selfing)
tt
F1 Hybrid
i.e. Tall Tt
(t)
F1 gametes
(T) (t) X (T) (t)
F2 segregation
3 Tall : 1 Dwarf
TT, Tt, Tt, tt
2. 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
P
Round Yellow
X
RRYY
Gametes :
F1
RY
Wrinkled Green
rryy
Round Yellow
(ry)
RrYy
X Self (Round
RY) (Ry) (rY) (ry)
Yellow)
1:1:1:1
RrYy
(RY) (Ry) (rY) (ry)
F2 Checker board
Ry rY ry
Ry rY ry
Ry rY ry
Ry rY ry
RY RRYY
RY RRYY
RY RRYY
RY RRYY
ratio
RRYy RrYY
RRYy RrYY
RRYy RrYY
RRYy RrYY
9:3:3:1
RrYy
RrYy
RrYy
RrYy
Ry RRYy RRyy
Ry RRYy RRyy
Ry RRYy RRyy
Ry RRYy RRyy
RrYy Rryy
RrYy Rryy
RrYy Rryy
RrYy Rryy
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
Round yellow
Wrinkled green
RrYy
rryy
(RY) (Ry) (rY) (ry)
(ry)
1:1:1:1
X
RY Ry rY ry
F2
ry RrYy Rryy rrYy
rryy 1:1:1:1
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