CBSE Class 10 Science 2022 · Paper 31/4/1 · Question 15

(a) The X chromosome is of normal size, while the Y chromosome is a short one, much smaller than X. The mismatched pair of sex chromosomes in humans is XY, found in males. (b) Human beings have one pair of sex chromosomes (the other 22 of their 23 pairs of chromosomes are autosomes). The perfect pair of sex chromosomes, XX, is present in the female parent (the mother). (c)(i) In some animals, for example many turtles and crocodiles, the temperature at which the fertilised eggs are kept decides whether the young developing in them will be male or female. In some other animals, for example snails, an individual can change its sex during its lifetime. In both cases the sex is decided by an environmental cue, or changes during life, instead of being fixed by the genes inherited, so the sex of an individual is not always determined genetically. OR (c)(ii) Flow chart (see the figure): father ($44+\text{XY}$) forms two kinds of sperm, $22+\text{X}$ and $22+\text{Y}$, in equal numbers; mother ($44+\text{XX}$) forms eggs that all carry $22+\text{X}$. Egg ($22+\text{X}$) + sperm ($22+\text{X}$) $\to$ zygote $44+\text{XX}$ $\to$ girl; egg ($22+\text{X}$) + sperm ($22+\text{Y}$) $\to$ zygote $44+\text{XY}$ $\to$ boy. The sex of the child therefore depends on which kind of sperm fertilises the egg, and girls and boys are expected in the ratio $1:1$.