Chapter No.8 : Variation And Genetics - Botany - For HSC Part 2 (XII) - Science Group

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Chapter No.8
Variation And Genetics

Botany

For HSC Part 2 (XII) Science Group

Q. No.6: Define "Law of Independent assortment" and explain with checkerboard.
Ans: Statement Of Mendel's Second Principle (Law of Independent Assortment):
Gregor John Mendel's proposed the Law of Independent Assortment, which state that:

"The memberof one pair of genes segregate independently of the other pairs."

OR

"Members of one pair of factor segregate (assort) independently of members of another pair of factor. Therefore all possible combinations of factors can occur in gametes."

OR

"When a pair or contrasting characters are followed in one cross, their genes assort independently into gametes, gave of one pair inherit independently of the other pair, therefore all possible combinations of genes can occur in gametes."

Mendel's Procedure:
After working on several monobybrid crosses, Mendel took two traits for cross between two individuals. Such cross is called dihybrid cross, and the ratio obtained in F₂ generation is called dihybrid ratio. The result achieved as a consequence of dihybrid-cross is spoken of as Inheritance of two traits.
Mendel experiments on garden peas. He took 4 units of characters such as;

1. Yellow Flower (YY) with round seeds (RR)
2. Green Flower (yy) with wrinkled seeds (rr).

The cross was made between true breeder parents.
F₁ generation produced all flower yellow with round seed as both the traits were dominant.
But when seeds of F₁ generation were collected and sown, the segregation of characters took all possible combinations and produced 4 kinds or offspring in F₂ in the following ratio:-

9 Yellow round.

3 Yellow wrinkled.

3 Green round.

1 Green wrinkled.

i.e. 9 : 3 ; 3 : 1.

It has also been noticed that some of them were homozygous and some were heterozygous.
Among the 4 kinds of offspring, two showed parental combination (Yellow round and green wrinkled) while two were new i.e. Green round and Yellow wrinkled. This means that trait of round seed was not tied with yellow colour and it was inherited independently. This exchange of genes (trait) takes place during crossing over which provides an opportunity for new varieties.

Result:- Thus we obtained a ratio 9 : 3 : 3 : 1

Q.No7: What do you mean by sex determination? Discuss linked inheritance with reference to colour blindness.

Ans: SEX CHROMOSOME:
The biologist tried to determine, whether an offspring will be male or female in an egg. The first information on sex came from Cytological studies by C.E. Mc Clung, E.B Wilson and N Steven between 1901-1905.
It was found that in plants and animals most of the chromosomes are pairs of homologous chromosomes, the members of one pair are not homologous, i.e. the two differ from each other morphologically in both male and female.

"This odd chromosome, which determines the sex of the individual is termed as "sex chromosomes".

The sex chromosomes are non-homologous and are mostly represented by 'X' and 'Y' chromosomes and they can not form pair.
For example: In human being, the difference in males and females is not of one whole chromosome, but of the shape of one chromosome in one sex, which is unlike its mate and unlike any chromosome in the opposite sex. In man each sex cell has 46 chromosomes, of which 44 i.e. 22 pairs are similar in both sexes, these are called autosomes. The 23^rd pair in female consists of two similar chromosomes, but in the male of the 23^rd pair differ in shape, one of the two is like those of the female, but the second is much smaller, these are called sex chromosomes. The two chromosomes of the female and one of the male which are alike known as X chromosomes, while the unlike smaller one only in male is called Y chromosome. The human female possesses a genotype of 44 + XX chromosomes, whereas the genotype of male is 44 + XY chromosomes.

Linked Inheritance:
A form of inheritance in which the transmission of the genetic material is coreeelated with the sex of the parents is called sex linked inheritance.

OR

Any genetic trait which is transmitted through sex chromosome is called sex linked inheritance.

Linked Inheritance With Reference To Colour Blindness:
It is a sex-linked inheritance found in human beings. Persons suffering from colour blindness have difficulty in distinguishing red from green. It is a rather common trait and more common in males. It is because Y chromosome is inherit for this trait. Only one gene of this trait will render a colour blind man. While on the other hand a woman must have two genes for this trait to become colour blind. This trait of colour blindness can easily be detected by using special charts made up of a number of coloured dots so arranged that colour blind persons see a different pattern than other persons do.
The possible results of various crosses are shown in the following charts.

Gene on chromosome
N = Normal eyed male
n = Colour blind female

(i) CROSS BETWEEN COLOUR BLIND MALE AND NORMAL FEMALE:

Result: When a colour blind male marries a normal female, all children will be normal but daughters will be colour blind carrier.