How is the sex of a child determined in humans?

The sex of a child in humans is determined by the combination of sex chromosomes inherited from the mother and father during fertilization. Humans have two types of sex chromosomes: X and Y chromosomes.

A female has two X chromosomes (XX), while a male has one X and one Y chromosome (XY). The mother's egg always contributes an X chromosome, whereas the father's sperm can contribute either an X chromosome or a Y chromosome.

If an X-carrying sperm fertilizes the egg, the combination becomes XX, and the child develops as female. If a Y-carrying sperm fertilizes the egg, the combination becomes XY, and the child develops as male.

Therefore, the chromosome contributed by the father's sperm determines the biological sex of the child. This concept helps students understand genetics, inheritance, and human reproduction.