NCERT Class 12 Biology: Chapter – 5 Principle of Inheritance and Variation Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 8:

How alleles of particular gene are differ from each other? Explain its significance.


When the allelic pairs of genes are identical then they are called homozygous and when the alleles of gene are different it is heterozygous. For example in case of the trait: height; we have TT and tt to show homozygous and Tt to show heterozygous.

Now in a pair of dissimilar factors one dominates the other and hence called dominant factor while the other called as recessive factor. The dominant one is denoted by upper case and recessive one with lower case. For example T is used for tall and t for dwarf of the same character- height.

Question 9:

In a monohybrid cross of plants with red and white flowered plants, Mendel got only red flowered plants. On self-pollinating these plants got both red and white flowered plants in ratio. Explain the basis of using RR and symbols to represent the genotype of plants of parental generation.


Symbols RR and rr are used for used for the sake of convenience and using a particular alphabet has no scientific basis. Usually, the first letter of a particular trait is used to describe that character and the contrasting character is shown by the same letter but in a different case. As per convention, dominant trait is shown by capital letter and recessive trait is shown by lower case letter.

Question 10:

For the expression of traits genes provide only the potentiality and the environment provides the opportunity. Comment on the veracity of the statement.


The phenotype of individuals are a result of the combined effect of environment and genotype.

Question 11:

A, B, D are three independently assorting genes with their recessive alleles a, b, d, respectively. A cross was made between individuals of genotype with . Find out the type of genotypes of the offspring produced.


The given cross is Aa bb DD X aa bb dd. Hence the offspring՚s would be:

Genotypes are Aa bb Dd and aa bb Dd.

Question 12:

In our society a woman is often blamed for not bearing male child. Do you think it is right? Justify.


I don՚t think it is right to blame women for not bearing male child.

As the sex determining mechanism in case of humans is of XY type. Of the 23 pairs of chromosomes, 22 pairs are autosomes and are same in both male and females. The remaining one pair is different that is a pair of X chromosomes in females and an X and Y chromosome in males. During spermatogenesis half of the sperms produced by males carry X chromosomes and the other half of sperms carry Y chromosomes besides autosomes. However females produce ovum with X chromosome. There are equal chances of fertilisation of ovum with sperm carrying X or Y chromosome. An ovum fusing with a sperm carrying X chromosome results in zygote developing into a female (XX) and an ovum fusing with a sperm carrying Y chromosome results in a zygote developing into male (XY) offspring.

Thus it is clear that genetic makeup of sperm determines the sex of the child. Also for each pregnancy there is always 50 % probability of either a male or female child.

Question 13:

Discuss the genetic basis of wrinkled phenotype of pea seed.


Starch synthesis in pea seeds is controlled by one gene. It has two alleles (B and b) . Starch is synthesized effectively by BB homozygotes and therefore, large starch grains are produced. In contrast bb homozygotes have lesser efficiency in starch synthesis and produce smaller starch grains. After maturation of seeds, BB seeds are round and the bb seeds are wrinkled. This is the genetic basis of wrinkled phenotype of pea seed.

Question 14:

Even if a character shows multiple allelism, an individual will only have two alleles for that character. Why?


Humans are diploid organisms and each person possess only two alleles. Hence even if a character is controlled by a gene of three alleles like:- the ABO blood group controlled by gene I with three alleles , i in humans; an individual will only have two alleles for that gene to control blood sgroup, which could be a combination of these three alleles like , and ii.

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