CBSE Class 10 Science 2022 · Paper 31/1/2 · Question 15

(a) In a human male the two sex chromosomes are of different types, one X and one Y, and of different sizes, the X being of normal size and the Y short. Since the two members of the pair match neither in type nor in size, the male’s pair (XY) is called a mismatched pair; a female has a perfect pair, XX. (b) By fertilisation. Each gamete has half the number, $23$ chromosomes; when a male gamete fuses with a female gamete, the zygote gets $23+23=46$ chromosomes, the original number, and the body cells that develop from it carry $46$. (c)(i) Turtles and snails (crocodiles and some lizards are other examples). In many turtles the temperature at which the fertilised eggs are kept decides whether the young become males or females. In some snails an individual can change its sex during its life, which shows that its sex is not fixed by its genes. OR (c)(ii) Flowchart (see the figure): Father $(44+\text{XY})\to$ sperms $(22+\text{X})$ or $(22+\text{Y})$, half of each; Mother $(44+\text{XX})\to$ eggs, all $(22+\text{X})$. Sperm $(22+\text{X})$ + egg $(22+\text{X})\to$ zygote $(44+\text{XX})\to$ girl. Sperm $(22+\text{Y})$ + egg $(22+\text{X})\to$ zygote $(44+\text{XY})\to$ boy. So the type of sperm decides the sex, with an equal chance of a girl or a boy.