Final answer:
Kevin and Kate have a 25% chance of having a son with hemophilia since Kate is a carrier of the hemophilia gene and Kevin has a normal X chromosome.
Step-by-step explanation:
Hemophilia is an X-linked recessive trait. Since both fathers of the couple had hemophilia, Kate must be a carrier of the gene, as females need two mutated X chromosomes to have hemophilia, and she does not have it. Therefore, Kate has one normal X chromosome and one X chromosome with the hemophilia gene. Assuming Kevin does not carry the hemophilia gene (as indicated), any son they have will have a 50% chance of having hemophilia, as he will receive his Y chromosome from Kevin and has an equal chance of receiving either the normal X or the X with the hemophilia gene from Kate.When dealing with recessive X-linked traits, a female needs two copies of the mutated gene to express the trait, while a male needs only one since males have just one X chromosome. Since Kate's father had the disease, she must have inherited the hemophilia gene from him, making her a carrier. If they have a daughter, she must inherit the normal X chromosome from Kevin (as males provide an X chromosome to daughters). The daughter would have a 50% chance of being a carrier, like her mother, depending on whether the X chromosome she inherits from her mother carries the hemophilia gene. But because Kate must be a carrier and Kevin has a normal X chromosome, the probability that Kevin and Kate will have a son with hemophilia is 25%, because there is a 50% chance the son will inherit the mutant X chromosome and a 50% chance of having a son in the first place.