SAT Subject Biology Practice Question 39: Answer and Explanation

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Question: 39

3. A recessive allele accounts for 60% of the alleles of a gene in a population. What proportion of the individuals in this population are heterozygous?

A. 0.16
B. 0.24
C. 0.36
D. 0.48
E. 0.6

Correct Answer: D


The frequency of heterozygotes in the population can be determined using the Hardy-Weinberg equation, p2 + 2pq + q2 = 1. The proportion of heterozygotes equals the middle term, 2pq, where p is the dominant allele frequency and q is the recessive allele frequency. In this example, q is given as 0.6 and p must be 0.4 (because p + q = 1). The heterozygote frequency is therefore 2 × 0.4 × 0.6 = 0.48.

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