CBSE Class 10 Science 2017 · Paper 531/1 · Question 22

Human beings have 23 pairs of chromosomes (46 chromosomes) in each body cell. Out of these, one pair is the pair of sex chromosomes; the other 22 pairs are autosomes. There are two types of sex chromosomes, X and Y: a woman has XX and a man has XY. Justification (flow chart): Mother ($44+\text{XX}$) forms eggs that all carry $22+\text{X}$. Father ($44+\text{XY}$) forms two kinds of sperm in equal numbers, $22+\text{X}$ and $22+\text{Y}$. Egg ($22+\text{X}$) + sperm ($22+\text{X}$) gives a zygote $44+\text{XX}$, a girl; egg ($22+\text{X}$) + sperm ($22+\text{Y}$) gives a zygote $44+\text{XY}$, a boy. Which kind of sperm fertilises the egg is a matter of chance, so a girl and a boy are equally likely (50% each). The mother contributes the same X chromosome every time, and the father cannot control which of his two kinds of sperm fertilises the egg, so neither parent can be held responsible for the sex of the child.