CBSE Class 10 Science 2022 · Paper 31/6/1 · Question 3

In sexually reproducing organisms such as human beings, the body cells carry two sets of chromosomes, one chromosome of each pair from each parent; the nucleus of a human body cell normally has 46 chromosomes in 23 pairs. The germ cells are formed through a special reduction division, meiosis, in which each germ cell receives only one chromosome of every pair - that is, half the number of chromosomes (23 in a human sperm or egg). At fertilisation a male germ cell fuses with a female germ cell, and the zygote receives one set from each, so the normal number is re-established ($23+23=46$). The zygote then divides repeatedly by mitosis, which copies every chromosome, so the body cells of the progeny carry the same number as the parents. Halving the number in the germ cells and restoring it at fertilisation keeps the chromosome number, and the DNA content, of the species constant from one generation to the next.