Explanation

In human genetics, the sex chromosomes are X and Y.

Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

When a father passes on his chromosomes to his offspring, he can only contribute either an X or a Y chromosome.

If he passes on his Y chromosome, the child will be a boy (XY).

If he passes on his X chromosome, the child will be a girl (XX).

Therefore, the father can only transfer his Y chromosome to his son.