Questions: Hemophilia A is an X-linked recessive disorder. What is the probability that a son will have hemophilia if the parents are a normal man and a woman who is a carrier?

Hemophilia A is an X-linked recessive disorder. What is the probability that a son will have hemophilia if the parents are a normal man and a woman who is a carrier?

Transcript text: Hemophilia $A$ is an $X$-linked recessive disorder. What is the probability that a son will have hemophilia if the parents are a normal man and a woman who is a carrier?

Solution

The answer is the first one: 50%.

Explanation for each option:

50%: This is correct. Hemophilia A is an X-linked recessive disorder. A normal man has an XY genotype, and a carrier woman has an X$^H$X$^h$ genotype (where X$^H$ is the normal allele and X$^h$ is the hemophilia allele). The possible combinations for their son (who inherits the Y chromosome from his father and one of the X chromosomes from his mother) are X$^H$Y (normal) and X$^h$Y (hemophilia). Therefore, there is a 50% chance that the son will inherit the X$^h$ chromosome and have hemophilia.
25%: This is incorrect. The probability of a son having hemophilia is not 25%.
0%: This is incorrect. There is a chance that the son could inherit the hemophilia allele.
100%: This is incorrect. Not all sons will inherit the hemophilia allele; there is only a 50% chance.
75%: This is incorrect. The probability of a son having hemophilia is not 75%.

Summary: The probability that a son will have hemophilia if the parents are a normal man and a woman who is a carrier is 50%.

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