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

A human male has $46$ chromosomes ($23$ pairs) in his body cells: $44$ autosomes and the sex chromosomes X and Y, written $44+\text{XY}$. A human female also has $46$ chromosomes: $44$ autosomes and two X chromosomes, $44+\text{XX}$. In sexually reproducing organisms the gametes are formed by a special cell division (meiosis) in which the number of chromosomes is halved: each gamete receives only one chromosome of every pair (in humans $23$: an egg carries $22+\text{X}$, a sperm $22+\text{X}$ or $22+\text{Y}$). At fertilisation a male and a female gamete fuse, so the zygote gets one set from each parent and has $23+23=46$ chromosomes, the same number as the parents. Halving at gamete formation and fusion at fertilisation together keep the chromosome number the same from one generation to the next.