A phenotypically normal woman whose father was haemophilic marries a normal non-haemophilic man. What proportion of their sons is expected to be haemophilic?

The woman must be a carrier (X^H X^h) since she inherited her father's X^h. The man is X^H Y. Sons receive Y from the father and either X^H or X^h from the mother with equal probability, so one half of the sons are haemophilic.