(a) 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.

(b) 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.

Please note that it is the sperm which determines the sex of the child. This is because half of the sperms have X chromosomes and 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.

From the above discussion we conclude that if the father (man or husband) contributes X sex chromosome at fertilisation through his sperm, the baby born will be a girl. On the other hand, if the father (man or husband) contributes a Y sex chromosome at fertilisation through his sperm, then the baby born will be a boy.

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This means that it is the sex chromosome contributed by father (man or husband) which decides the sex of the baby which the mother (woman or wife) will give birth to. Thus, father (man or husband) is responsible for the sex of the baby (boy or girl) which is born.

The belief that mother (woman or wife) is responsible for the sex of her baby is absolutely wrong. In many ignorant Indian families, the mother (woman or wife) is held responsible for the birth of a girl child and unnecessarily harassed by her in-laws (sasural). Such people should understand that it is the husband who is responsible for the birth of a girl child (and not his wife). Moreover, a girl is no less than a boy.

In some of the animals, sex determination is also controlled by the environmental factors. For example, in some reptiles, the temperature at which the fertilised egg is incubated before hatching plays a role in determining the sex of the offspring. It has been found that in a turtle (Chrysema picta), high incubation temperature leads to the development of female offsprings (or female progeny).

On the other hand, in the case of a lizard (Agama agama), high incubation temperature results in male offsprings (or male progeny). In some animals, such as snails, individuals can change sex, indicating that sex is not determined genetically in such animals.