CBSE Class 10 Science 2017 · Paper 31(B)/C · Question 21

(a) A human male and a human female each have $46$ chromosomes ($23$ pairs) in their body cells: $22$ pairs of autosomes and $1$ pair of sex chromosomes. The female has two sex chromosomes of the same type, XX; the male has two sex chromosomes of different types, one X and one Y (XY), the Y being short. (b) Acquired traits versus inherited traits: 1. Acquired: developed during the individual's own lifetime, by experience, practice, use or injury. Inherited: received from the parents through their genes. 2. Acquired: changes in body (non-reproductive) cells, with no change in the DNA of the germ cells. Inherited: controlled by the DNA of the germ cells. 3. Acquired: not passed on to the progeny, e.g. weight lost through starvation, a skill such as swimming. Inherited: passed on from generation to generation, e.g. eye colour, free or attached earlobes. We cannot pass on our acquired experiences because they change only our non-reproductive tissues (such as the brain and muscles), not the DNA of our germ cells. The progeny develops from a zygote that receives DNA only from the egg and the sperm, so a change that has not reached the germ cells cannot be inherited.