In a cross between color-blind man (XbY) and carrier woman (XBXb), what is the probability that their son will be color blind?

• A. 0%
• B. 25%
• C. 100%
• D. 50%

Answer: (D)

Color blindness is inherited as an X-linked recessive trait. The man has XbY, so his only X is the color-blind one. The woman is a carrier with XBXb, so her eggs carry either X B or X b with equal likelihood. Sons receive the Y from their father and one X from their mother. They will be color-blind only if they inherit the X b from their mother. Since the mother passes X B or X b half the time, half of the sons will receive X b and be color-blind. Therefore, the chance their son is color-blind is 50%.