Two parents who both carry the recessive cystic fibrosis allele (Cc × Cc) have a child who does not have the disorder. What is the probability that this child is a carrier?
Answer and explanation
Correct answer: B. 2 in 3
- Step 1: The possible children are CC, Cc, Cc and cc.
- Step 2: The child is unaffected, so cc is ruled out, leaving CC, Cc, Cc.
- Step 3: Two of the three remaining outcomes are carriers: 2 in 3.
Answer
2 in 3.
Why not the others: 1 in 2: this ignores that the child is known to be unaffected. 1 in 4: this is the chance of an affected child. 3 in 4: this is the chance of an unaffected child.
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