CBSE Class 10 Science 2018 · Paper 31/1/C · Question 19

(a) Variation is the occurrence of differences in the characters of the individuals of a species; offspring resemble their parents but are not identical to them or to one another. It is created in two ways: the copying of DNA during reproduction is not absolutely accurate, so small copying errors arise; and sexual reproduction combines the DNA of two parents, giving new combinations of variations. The variations accumulate over generations. If the surroundings change drastically, a population without variation could be wiped out, but individuals whose variations suit the new conditions survive and reproduce, so the species survives. For example, when the water they live in becomes much warmer, heat-resistant variants of bacteria survive while the others die. (b) The gametes (sperms and eggs) are formed by a special cell division that halves the chromosome number, so each gamete carries only one chromosome of each pair (23 in human beings instead of 46). At fertilisation a male and a female gamete fuse, so the zygote receives half from each parent and the original number is restored ($23+23=46$). Hence the offspring has the same number of chromosomes as its parents in every generation.