Hemophilia is a sex-linked recessive trait in humans. If a father and a son are both hemophiliacs, but the mother is normal, her genotype must be. (Note: the H's are superscripts.) * XhXh XHXh XHXH XhY

XHXh (Note: the H's are superscripts.)

Explanation:

Hemophilia is a sex-linked recessive trait which means that the males having one altered copy of the gene is enough to cause the medical condition( as they have only one X chromosomes and females have two)

In this case the genotype of father and son is such that they are affected by hemophilia.The son has mother's X chromosome and father's Y chromosome.

XhY is the genotype of father and son. As mother would be heterozygous because she is not affected, her genotype will be XHXh.