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.
