A phenotypically normal woman, whose father suffered from haemophilia, marries a normal man. What percentage of their total progeny is expected to be affected by haemophilia?

Step-by-step solution

Haemophilia is an X-linked recessive disorder. Let the normal allele be X^ H and the haemophilia allele be X^ h . The genotype of the woman's father is X^ h Y . Since the woman is phenotypically normal but her father was haemophilic, she must have inherited the X^ h chromosome from him. Thus, she is a carrier and her genotype is X^ H X^ h . The woman marries a normal man, whose genotype is X^ H Y . The cross is X^ H X^ h X^ H Y . The possible genotypes of the progeny are: X^ H X^ H (Normal female) - 25 % X^ H X^ h