In a randomly mating population, the birth incidence of cystic fibrosis is approximately 1 in 2500. Assuming Hardy-Weinberg equilibrium, the carrier frequency for the CFTR mutation is approximately:

• A 1 in 50
• B 1 in 25 ✓
• C 1 in 2500
• D 1 in 500

Explanation

For an autosomal recessive disease, q squared equals disease incidence, so q equals the square root of 1/2500, which is 1/50. Carrier frequency is 2pq, which approximates 2q when q is small, giving 2/50 or 1 in 25. Option A comes from forgetting to double q, a common error. This arithmetic is applied routinely in counseling couples about recessive diseases such as cystic fibrosis, sickle cell disease, and thalassemia.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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