Haemophilia is sex-linked and is caused by a recessive allele. A woman’s father has haemophilia, but her husband does not. The woman's mother does not have haemophilia and is not a carrier. What is the probability of the woman and her husband having a child who suffers from haemophilia?
The woman's father has haemophilia so his genotype must be XhY, so his daughter can only have the following genotype: XHXh. Her husband is not a haemophiliac so his genotype must be XHY. There is a 50% chance that the mother's gamete has the recessive allele for haemophilia. If she has a son that son will have a 50% chance of getting the Xh. This also applies to daughters, but they will get the normal allele XH from their father so the daughter will have a 0% chance of developing haemophilia.
