Question

What are chromosomes? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained.

Answer 1

Hint: Chromosomes are the hereditary unit through which the genetic information is passed on from one generation to another. They are nothing but the packed DNA. We look like our parents and many of their features and bodily characteristics are present in our body too. This is because of the passing on of chromosomes.

Complete answer:
To know the correct answer to the question, we must be well aware of the chromosomes and its transfer through generations.
We all know that the male gamete (the sperm) and the female gamete (the ovum) fuse to form a zygote which grows up to become an embryo. These gametes are formed through special processes known as gametogenesis. The sperm is formed through spermatogenesis and the egg or the ova is formed through oogenesis. The egg and the sperm both contain half the number of chromosomes that are present in the germ cells of both the parents. These male and female germ cells undergo meiosis which is a reduction division and give half of their chromosomes (i.e 23 chromosomes) to the spermatids and ovum respectively. When these fuse the number of chromosomes in the zygote is restored (i.e. 23 chromosomes of ovum and 23 chromosomes of the sperm fuse to make a zygote have 46 chromosomes).

Note: Here, it must be noted that if the reduction division or meiosis had not taken place in the germ cells, then the number of chromosomes in the gametes would have been 46. When these would have fused the zygote would have 92 chromosomes in them which would have created an abnormality.