Hemophilia A is an X-linked recessive trait in which the blood does not clot properly. An unaffected mother and a father with hemophilia have a daughter with hemophilia. Their next offspring is a son named Dan. What is the likelihood that Dan has hemophilia?

Question

BlackTom AI · Accepted Answer

X-linked recessive inheritance: males are affected if they inherit one mutant X (XhY), while females must inherit two mutant Xs (XhXh) to be affected. An affected daughter (XhXh) requires that the father contributed Xh and the mother contributed Xh, so the mother must be a carrier (XHXh) even though she is phenotypically unaffected.

Parental genotypes implied by the information:
- Father: XhY (affected)
- Mother: XHXh (unaffected carrier, inferred because they have an affected daughter)

List parental gametes:
- Mother produces eggs XH and Xh (each with probability 1/2).
- Father produces sperm Xh and Y (each with probability 1/2).

Full Punnett square (all offspring, but we will focus on sons):
- XH (egg) × Xh (sperm) → XHXh (carrier daughter, unaffected)
- Xh (egg) × Xh (sperm) → XhXh (affected daughter)
- XH (egg) × Y (sperm) → XHY (unaffected son)
- Xh (egg) × Y (sperm) → XhY (affected son)

For a son (Dan) the father must have supplied the Y chromosome. That restricts outcomes to the two male outcomes:
- Mother XH + Father Y → XHY (unaffected son)
- Mother Xh + Father Y → XhY (affected son)

Each of these male outcomes is equally likely because the mother’s egg is equally likely to be XH or Xh. Therefore probability Dan is affected = 1/2 = 50%.

Why other choices are incorrect:
- 0%: impossible because the mother must be a carrier (we observe an affected daughter).
- 25%: would be a typical probability for an autosomal recessive child when both parents are carriers, not applicable here.
- 100%: would require the mother to be affected herself (XhXh), which she is not.

Final answer confirmed: 50%.

Hemophilia A is an X-linked recessive trait in which the blood does not clot properly. An unaffected mother and a father with hemophilia have a daughter with hemophilia. Their next offspring is a son named Dan. What is the likelihood that Dan has hemophilia?单项选择题

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