CBSE Class 10 Science 2022 · Paper 31/6/1 · Question 5(a)

A woman has two X chromosomes (XX), so every egg she produces carries an X chromosome. A man has one X and one Y chromosome (XY), so half of his sperms carry an X chromosome and half carry a Y chromosome. At fertilisation the egg is equally likely to be fertilised by either kind of sperm. If an X-carrying sperm fertilises the egg, the zygote is XX and develops into a girl; if a Y-carrying sperm fertilises it, the zygote is XY and develops into a boy. Crossing XX with XY therefore gives XX : XY = $1:1$, so in each pregnancy the probability of a girl and of a boy is $\frac{1}{2}$ each, i.e. $50\%$. The sex of the child is decided by the sperm, that is, by what the child inherits from the father.