Sex Determination Based on Chromosomes

The sex of a child is determined by the specific pair of sex chromosomes inherited from the parents.

Humans have 23 pairs of chromosomes, with one pair being the sex chromosomes.

• Females typically have two X chromosomes ($XX$).
• Males typically have one X chromosome and one Y chromosome ($XY$).

Chromosome Inheritance for Girl Child

During reproduction:

• The mother always contributes an $X$ chromosome from her egg cell.
• The father contributes either an $X$ or a $Y$ chromosome through his sperm.

The combination of chromosomes determines the child's sex:

• If the sperm carries an $X$ chromosome, it fertilizes the egg to form an $XX$ pair. This results in the birth of a girl child.
• If the sperm carries a $Y$ chromosome, it fertilizes the egg to form an $XY$ pair. This results in the birth of a boy child.

Therefore, the $XX$ set of chromosomes leads to the birth of a girl child.