Question

One form of hemophilia is caused by a sex-linked recessive gene. Assume that a man with

hemophilia marries a phenotypically normal woman whose father had hemophilia. What is the
probability that their first son will have hemophilia?
A) 1/8
B) 1/16
C) 1/2
D) 1/4

Answer 1

The probability that the couple's first son will have hemophilia is 1/2. This is because the man cannot pass the gene for hemophilia, and the woman, being a carrier due to her father having hemophilia, has a 50% chance of passing the affected gene to a son.

Step-by-step explanation:

Considering hemophilia is a sex-linked recessive trait, we must look at the genetics of the parents. Since the female parent's father had hemophilia, she must have one hemophilia allele because the trait is X-linked, making her a carrier. The normal male (XY) can only pass on a Y chromosome to his sons, which does not carry the gene for hemophilia. Thus, the son will have a 50% chance of inheriting the X chromosome with the hemophilia allele from his mother. Therefore, the probability that their first son will have hemophilia is 1/2 (C).

Women need two mutated X chromosomes to be hemophilic; however, as carriers with one affected X chromosome, they do not exhibit symptoms but can pass the trait to their sons. Hemophilia A and B are linked to the X chromosome and lead to deficiencies in clotting factors VIII and IX, respectively. The condition is rarer in females because they would require two defective genes, one from each parent, to manifest the disease.