NCERT Class 12 Biology: Chapter – 5 Principle of Inheritance and Variation Part 8 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 2:

a. In humans, males are heterogametic and females are homogametic. Explain. Are there any examples where males are homogametic and females heterogametic?

b. Also describe as to, who determines the sex of an unborn child? Mention whether temperature has a role in sex determination.


a. Humans have an XY type of sex determination were both male and females have same number of chromosomes. In males an X chromosome and Y chromosome is present. However females have a pair of X chromosome. Both male and females have same number of autosomes. Males have autosomes plus XY that is heterogametic and females have autosomes plus XX which is female homogametic. Since males produce two different types of gametes (some gametes with X chromosome and some with Y chromosome) ; such type of sex determining mechanisms are called as male heterogamety.

Birds are examples where males are homogametic and females heterogametic as birds have a different sex determining mechanism where two different types of gametes in terms of sex chromosomes are produced by females: female heterogamety. Thus female birds have one Z and one W chromosome (female heterogametic) whereas males have a pair of Z chromosomes (male homogametic) besides autosomes.

b. Father determines the sex of the unborn child. 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.

In some reptiles, they use incubation temperature to determine sex. In some species the pattern is; the eggs in extreme low or high temperature become male and eggs in medium temperature become female. Thus temperature has a role in sex determination.

Question 3:

A normal versioned woman, whose father is colour blind, marries a normal versioned man. What would be probability of her sons and daughters to be colour blind? Explain with the help of a pedigree chart.


Colour blindness is common hereditary condition which means it is passed down from parents. Red/green colour blindness is passed from mother to son on the 23rd chromosome, which is known as the sex chromosome. The faulty gene for colour blindness is found only on the X chromosome.

When the father is colour blind, he can only pass an X chromosome to his daughter. Thus she acquires the gene and become the carrier of the disease. When she has a child she will give one of her X chromosome to her child. If she gives the X chromosome with a faulty gene to her son he will be colour blind, but if the son gets good chromosome then he won՚t be colour blind. But there is no chance for her daughter to be colour blind; she can be a carrier if the faulty X chromosome is inherited by her from mother otherwise she is normal as the father is a normal visioned man.

X Chromosome to His Daughter