Final answer:
The mother must be a carrier with the genotype XhXh, making her an unaffected carrier of hemophilia, a sex-linked recessive disorder. This explains the son's hemophilia, as he would have inherited the hemophilic X chromosome from his mother.
Step-by-step explanation:
The genotype of the mother must be XhXh if a healthy couple has a son with hemophilia. Hemophilia is a sex-linked recessive disorder where a female must have two copies of the mutated gene (one on each X chromosome) to be affected, while a male needs only one. Males have only one X chromosome (XY) and will have the disorder if they inherit the mutated gene.
In this genetic scenario, the son's genotype is XhY, where 'Y' is the normal Y chromosome contributed by the father, and 'Xh' is the mutated X chromosome from the mother. Hence, the mother must carry one normal X chromosome (XH) and one mutated X chromosome (Xh) to be an unaffected carrier, or genotype XHXh.
As a result of being a recessive trait linked to the X chromosome, hemophilia primarily affects males more than females, contributing to the rarity of this disorder in females who need two mutated X chromosomes to be hemophilic (Option A).