CBSE Class 10 Science 2019 · Paper 31/1/3/C · Question 16

(a) In asexual reproduction one parent's DNA is copied, so the offspring are almost identical; the only variations come from small errors in DNA copying. In sexual reproduction two parents form gametes, each with half of its DNA, and fertilisation combines DNA from two different individuals, so every offspring gets a new combination of the variations present in both parents. These variations have already been carried by living, reproducing parents, so they are not seriously harmful: they are viable. Hence sexual reproduction gives rise to more viable variations. Effect on evolution: in a population with many variations, some individuals are better suited to a changed environment. They survive and reproduce more (natural selection) and pass their useful variations to the next generation. Over many generations these variations accumulate, the population changes and new species may arise. So viable variations help sexually reproducing organisms to survive changes and to evolve. (b) No. The mother has the sex chromosomes XX, so every egg carries an X chromosome. The father has XY, so half of his sperms carry X and half carry Y. An X-carrying sperm gives an XX zygote (girl) and a Y-carrying sperm an XY zygote (boy). The sex of the newborn therefore depends on the sperm from the father, by chance, and not on the genetic make-up of the mother.