Here's why:

* Females have two X chromosomes (XX).

* Males have one X and one Y chromosome (XY).

During fertilization, the mother always contributes an X chromosome. The father can contribute either an X or a Y chromosome.

* If the father contributes an X chromosome, the offspring will be female (XX).

* If the father contributes a Y chromosome, the offspring will be male (XY).

Therefore, the father's sperm determines the sex of the child.