Hemophilia is caused by several genetic factors; one, a recessive allele of an x - linked gene, is the subject of this problem. assume that a man with hemophilia marries a normal woman whose father had hemophilia. what is the probability that their first son will have hemophilia?

50%.

First off, XhY means hemophilia for a male. The father has hemophilia, making him XhY. The mother doesn't have it, but must be a carrier because her father had it. This makes her XHXh.

Sons only receive a Y from their father, and either X from their mother. So theirs sons will either be XHY (normal) or XhY (hemophiliac).