How do organisms, whether reproduced asexually or sexually maintain a constant chromosome number through several generations? Explain with the help of suitable example.

Explanation

(i) When organisms reproduce asexually, only mitotic divisions are involved and the chromosome number remains the same.
(ii) During asexual reproduction, the DNA (in the chromosomes) or the cells involved are copied and then equally divided among the two daughter cells. Thus, chromosome number remains unchanged.
(iii) In sexual reproduction, organisms produce gametes through a special type of division, meiosis- reductional division, in which the original number of chromosomes becomes half. These two gametes combine to form the zygote and the original number of chromosomes is restored.
(iv) In sexual reproduction specialized cells / germ cells with only half the number of chromosomes are formed. When these germ cells from two individuals combine to form a new individual, the original chromosome number is restored.
(v) Example : In humans, the parents father and mother each have 46 or 23 pairs of chromosomes. In the gametes-the sperm has half the number of chromosomes ie., 23 and the egg also has 23 chromosomes. When the spem and the egg fuse, the zygote has 46 or 23 pairs of chromosomes. Thus, the chromosome number remains constant.