CBSE Class 10 Science 2019 · Paper 31/5/3 · Question 17

(a) In sexual reproduction the progeny develops from a zygote formed by the fusion of two gametes, a sperm from the father and an egg from the mother. Gametes are formed by a special cell division that halves the chromosome number, so each carries only one chromosome of every pair: a human body cell has $46$ chromosomes ($23$ pairs), while a sperm and an egg have $23$ each. At fertilisation $23+23=46$ restores the normal number, and every pair in the zygote has one chromosome from each parent. So each parent contributes exactly half of the chromosomes, and of the DNA, of the progeny. (b) No. In the light of evolution, the diversity of animals is the result of divergence from common ancestors, not evidence of diverse ancestry. Variations arise and accumulate, natural selection and genetic drift act on them, and when a population is split, for example by geographical isolation, the separated groups become new species. Repeated over millions of years, this branching gave today's variety. Homologous organs, such as the forelimbs of a frog, a lizard, a bird and a human, and fossils such as Archaeopteryx show that very different animals share common ancestors.