Explain : When and how the sex of the baby is determined ?

In human female the chromosome pattern is $\mathrm{XX}$ and that in the male is $\mathrm{XY}$.

Therefore, all the haploid gametes produced by the female ($ova$) have the chromosome $X.$

In the male gametes the sex chromosome could either be $\mathrm{X}$ or $\mathrm{Y}$ hence $50 $ percent of sperms carry $\mathrm{X}$ chromosome while the other $50$ percent carry the $Y.$

After fusion of the male and female gametes the zygote would carry either $XX$ or $XY$ depending on whether the sperm carrying $\mathrm{X}$ or $\mathrm{Y}$ fertilized the ovum.

The zygote carrying $XX$ would develop into a female baby and $\mathrm{XY}$ would form a male.

Thus the sex of the baby is determined by the father and not by the mother.

More Information:

Changes occuring during fertilization : Sperms of semen dropped in vaginas move towards fallopian tube through uterus. In this movement contraction of vagina and uterus wall become helpful.

The sticky secretion of the walls of oviducts also helps in this process. This process takes about $5$ to $6$ hours of time.

Secondary oocyte is surrounded by numerous sperms.

Secondary oocyte is surrounded by ovum layer and jelly layer.

Among many enzymes located in acrosome of sperm cell - one hyluronidase makes possible the entry of sperms into ovum.

The head of sperm and middle part enter into primary oocyte as soon as the nucleus located in the head of sperm cell is called male pronuclei.

The entry of sperm into secondary oocyte cause some immediate changes.

The ovum membrane layer separates little from plasma and now it is called fertilised membrane. This membrane prevents the entry of other sperm cells.

The entry of sperm causes mature division of ovum and female pronuclei.

$\because$ Thus one sperm and one ovum are associated in fertilisation.

$\Rightarrow$ The integration of male pronuclei and female pronuclei form diploid zygote nuclei.

Fertilised ovum is called zygote.