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

(a) The statement means that the two sex chromosomes of a woman are alike - both are X chromosomes (XX), a perfect pair - whereas a man has two unlike sex chromosomes, one normal-sized X and one short Y (XY), a mismatched pair. (b) When gametes are formed, each egg or sperm receives only one chromosome of the pair. A woman (XX) can make only X-bearing eggs, so every child, girl or boy, receives an X from the mother. A man (XY) makes two kinds of sperm, half carrying X and half carrying Y. If a sperm carrying X fertilises the egg, the child is XX, a girl; if a sperm carrying Y fertilises it, the child is XY, a boy. Since the mother's contribution is always the same X, the sex of the child is decided by the chromosome in the father's sperm, so the mother is not responsible for the sex of the child.