What determines the sex of a child?

          When we look at the human race we find that it is divided in two sexes – male and female. The question arises: Why don’t all people belong to one sex? What causes some babies to be girls and others, boys? In other words, what determines the sex of an individual?

          A woman becomes capable of bearing a child after she starts menstruation. Between the 10th and the 18th days from the onset of menstruation, the reproductive organs of a woman produce an egg called ‘ovum’. During this period if a woman mates with a man, a sperm present in the semen of the man fertilizes the egg. The combination of the ovum of the female and the sperm of the male results in the formation of the embryo in the womb.

          The ova of the female and the sperms of the male contain sex chromosomes. The ova have only one type of chromosome called X chromosomes. But the sperms have two types of sex chromosomes called X and Y chromosomes respectively. The X chromosomes are slightly bigger than the Y chromosomes in size.

          When female sex cells are to be formed, the two X chromosomes of the female germ cell come together. They then split lengthwise, forming four X chromosomes. In the formation of male sex cells, the single X chromosome pairs with its smaller partner, the Y chromosome. Each of these chromosomes split lengthwise. The cell then contains two X chromosomes and two Y chromosomes. When this cell gives rise to four new cells – the sex cells – half will contain X chromosomes, the other half, Y chromosomes. If these two kinds of sperm are equally efficient in reaching and fertilizing the egg cells, we should expect the two kinds of fertilized eggs to be equally numerous. Half of these would have two X chromosomes (XX) and would develop into girls. Half would carry one X and one Y chromosome (XY) and would give rise to boys. As a matter of fact, male births are slightly more frequent than female births. The reason is yet not known.