CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 12

Basic requirement: male and female gametes (germ cells) must be formed and must fuse, in fertilisation, to form a zygote. The gametes usually come from two different parents, a male and a female; each gamete carries half the number of chromosomes, so the zygote gets the full number. Importance in nature: (i) The offspring combine DNA from two parents, so sexual reproduction produces more variation in a population than asexual reproduction does. (ii) Variation helps a species survive changes in its surroundings: some individuals may have variations that let them survive where others would die. (iii) Variations accumulated over many generations are the basis of evolution and of the origin of new species.