Question

A male hemophiliac, marries a normal woman, and they have four normal children, two boys (Mark and Mike) and two girls (Molly and Mary). Mark, Mike, Molly, and Mary all marry normal individuals and have children. The children (male and female) of Mark's or Mike's children were normal but the sons of Molly and Mary all display symptoms of hemophilia while their daughters do not.
Which of the following statement best explains the reason that Mark, Mike, Molly, and Mary do not display symptoms of hemophilia, even though their father, John, is a hemophiliac?

• A. Hemophilia is an X-linked disorder, and John can pass on only his Y chromosome.
• B. Hemophilia is an X-linked disorder and even though Molly and Mary received a hemophiliac X chromosome from father, mother gave them a normal X chromosome.
• C. Hemophilia is a Y-linked disorder and therefore cannot be displayed in females.
• D. Hemophilia is a Y-linked disorder and Mark and Mike must have received an X chromosome from John.
• E. Hemophilia is an X-linked disorder, and even though Mark and Mike received a hemophiliac X chromosome from John, Jane gave them a normal X chromosome.

Answer 1

Answer: (B)

Hemophilia is an X-linked disorder and even though Molly and Mary received a hemophiliac X chromosome from father, mother gave them a normal X chromosome.

Out of this family, the only members that express this condition are males. This is a tip-off for X-linked disorders, which are more common in males because they have only a single X chromosome.

John’s genotype is X${}^h$Y. He passed his Y chromosome to Mark and Mike; they also received a normal X from Jane, thus they do not have hemophilia, nor can they pass it on to their kids. Molly and Mary received X${}^h$ from John but also received a normal X from Jane, thus they are carriers of hemophilia but do not display its symptoms.

Therefore, the correct answer is option B.