A normal man marries a woman whose father had haemophilia. The probability that their first son will have haemophilia is
- A. zero
- B. one quarter
- C. one half
- D. certain
Explanation
The woman inherited her haemophiliac father's X, making her a carrier. She passes that X to half her sons, who will be affected, and the father's normal X contribution is irrelevant to sons, who receive his Y.
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