Recessive risk: an unaffected sibling of an affected child
A woman's brother has cystic fibrosis. She and both parents are unaffected. She plans children with an unrelated man from a population where the carrier frequency is 1 in 25. What is the probability their first child will have cystic fibrosis?
Answer B: explanation for every choice
A
This assumes she is definitely a carrier.
B
Correct. As an unaffected child of two carriers, her carrier probability is 2/3. The probability is 2/3 × 1/25 × 1/4 = 1/150.
C
This uses 1/2 for her carrier probability. Being unaffected excludes the homozygous recessive outcome, so it is 2/3.
D
This applies the ¼ factor twice.
TakeawayAn unaffected sibling of an affected child has a 2/3 chance of being a carrier.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb164.