The rather miraculous process of human reproduction is surrounded in mystery and myth. Up to now, there is a belief that a mother is responsible for the sex of her baby.
In many ignorant Indian families, a girl is no less than a boy. Science has put it clearly that the sex of a child depends on what takes place at fertilization.
If a sperm carrying X chromosome fertilises an ovum (or egg) which carries X chromosome, then the child born will be a girl (or female). This is because the child will have XX combination of sex chromosomes. And if a sperm carrying Y chromosome fertilises an ovum (or egg) which carries X chromosome, then the child born will be a boy (or male). This is because the child will have XY combination of sex chromosomes.
It is the sperm which determines the sex of the child, and half of the sperms have X chromosomes while the other half have Y chromosomes. Thus, there is a 50 per cent chance of a boy and a 50 per cent chance of a girl being born to the parents. This is why the human population is roughly half males and half females.
In light of the above discussion we can reliably conclude that if the father contributes X sex chromosome at fertilisation through his sperm, the baby born will be a girl. On the other hand, if he contributes a Y sex chromosome at fertilisation through his sperm, then the baby born will be a boy. Therefore the type of chromosome contributed by the father determines the sex of the child.
It is important to know that in some animals, sex determination is also controlled by the environmental factors. For example, in some reptiles, the temperature at which the fertilized egg is incubated before hatching plays a role in determining the sex of the offspring. It has been found that in a turtle, high incubation temperature leads to the development of female offsprings. In the case of a lizard, however, high incubation temperature results in male off springs.
As humans, our bodies have no mechanism to segregate the X sperm from the Y sperm. If you have a strong preference for a boy or a girl, your options are limited. You could plan to have enough babies so that both genders are eventually represented. Or it may be better to rethink your position and hope for a healthy baby of either sex.