A baby girl is born with hemophilia, which is an X-linked recessive disorder. What are the most likely genotypes of her parents?
Correct answer: C. The mother is carrier and the father is hemophiliac
- A. The mother is a carrier and the father is normal
- B. The mother is hemophiliac and the father is normal
- C. The mother is carrier and the father is hemophiliac
- D. Both are normal
Explanation
A girl has two X chromosomes and needs the recessive allele on both, one from each parent, so the father must be haemophiliac himself and the mother must carry at least one copy. A carrier mother with a normal father could only produce affected sons, which is why the first option fails. This is why affected females are so rare.
This question appeared on the UHS MDCAT 2025 paper, which you can sit online with every answer explained.
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About X-linked Recessive Inheritance
X-linked recessive inheritance involves genes on the X chromosome, so males express a recessive allele with no corresponding allele on their single X chromosome, while females may be carriers. Pedigrees cover haemophilia and red-green colour blindness, including carrier mothers, affected sons and the absence of father-to-son transmission.
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