# What is the probability that their first son will have hemophilia?

## 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.

Since the normal woman's father had hemophilia, she is a carrier. She inherited her father's $\text{X"^"h}$ chromosome and has the heterozygous genotype $\text{X}$$\text{X"^"h}$.