CBSE Class 10 Science 2022 · Paper 31/4/3 · Question 15

(a) The probability of a male child is $\frac{1}{2}$ (50%) and of a female child is $\frac{1}{2}$ (50%): every egg carries X, while half the sperms carry X and half carry Y, so each child is equally likely to be a boy or a girl (and the two chances add up to 1). (b) Human beings have one pair of sex chromosomes. The perfect pair (XX) is present in the female (mother); the male has the mismatched pair XY. (c)(i) (1) In some reptiles, such as many kinds of turtle, the temperature at which the fertilised eggs are kept decides whether the young are male or female. (2) In some animals, such as snails, individuals can change their sex during their lifetime. In neither case is sex fixed by the chromosomes, so it is not always determined genetically. OR (c)(ii) Flow chart. Parents: father ($44+\text{XY}$) $\times$ mother ($44+\text{XX}$). Gametes: the father makes sperms carrying $22+\text{X}$ or $22+\text{Y}$; every egg carries $22+\text{X}$. Fertilisation: an X-carrying sperm and an egg give the zygote $44+\text{XX}$, a girl; a Y-carrying sperm and an egg give the zygote $44+\text{XY}$, a boy. So $\text{girl}:\text{boy}=1:1$.