CBSE Class 10 Science 2017 · Paper 31/1/2 · Question 11

A human female has $46$ chromosomes ($23$ pairs): $22$ pairs of autosomes and one pair of sex chromosomes, XX, that is $44+\text{XX}$. A human male also has $46$: $22$ pairs of autosomes and the sex chromosomes XY, that is $44+\text{XY}$. In sexually reproducing organisms the germ cells (gametes) are formed by a special cell division, meiosis, in which each gamete receives only one chromosome of each pair, that is half the number: in humans an egg has $22+\text{X}=23$ chromosomes and a sperm has $22+\text{X}$ or $22+\text{Y}=23$. At fertilisation a male gamete fuses with a female gamete, and the zygote gets one set from each parent, so the original number is restored ($23+23=46$). The progeny therefore have the same number of chromosomes as the parents.