Hemophilia is an X-linked recessive disorder. If a woman with hemophilia has a child with a man who does not have hemophilia, what is the chance the child will be a carrier?

• A. 0%
• B. 25%
• C. 50%
• D. 100%

Answer: (C)

In X-linked recessive inheritance, the gene for hemophilia sits on the X chromosome. An affected woman has two copies of the mutated X (XhXh), while a man without the disease has one normal X and a Y (XHY).

Every child gets one X from the mother and either an X or a Y from the father. Daughters receive X from the father and X from the mother, so a daughter would be XH Xh (one normal and one mutated X), which makes her a carrier. Sons receive Y from the father and X from the mother, so a son would be Xh Y (mutated X and Y), which makes him affected, not a carrier.

Since half the offspring are daughters (carriers) and half are sons (affected), the chance a random child will be a carrier is half.