CBSE Class 10 Science 2020 · Paper 31/2/1 · Question 21

Chromosomes are thread-like structures present in the nucleus of a cell. They are made of DNA and proteins and carry the genes, the hereditary information passed from parents to offspring. In sexually reproducing organisms, body cells have two sets of chromosomes (a pair of each kind; $46$ in human beings). During the formation of germ cells (gametes), the number of chromosomes is reduced to half, so each gamete receives only one chromosome of each pair ($23$ in human beings). At fertilisation a male and a female gamete fuse, and the zygote receives one set from each parent, restoring the original number ($23+23=46$). In this way the number of chromosomes in the progeny is maintained, generation after generation.