How is the sex of the child fixed during the fertilisation step in human beings ? Explain.

In human beings, the sex of a child is genetically determined at the moment of fertilisation. Humans have 23 pairs of chromosomes, of which one pair is the sex chromosomes. Females have a perfect pair of $X$ chromosomes ($XX$), while males have a mismatched pair consisting of one normal $X$ and one short $Y$ chromosome ($XY$). During gamete formation, all eggs produced by the mother carry an $X$ chromosome. However, the father produces two types of sperm: 50% carry the $X$ chromosome and 50% carry the $Y$ chromosome.

1. If a sperm carrying the $X$ chromosome fertilises the egg, the resulting zygote will be $XX$, developing into a girl.
2. If a sperm carrying the $Y$ chromosome fertilises the egg, the resulting zygote will be $XY$, developing into a boy.

Thus, the sex of the child is fixed by the paternal chromosome inherited during fertilisation.