MCAT Biology Question 177: Answer and Explanation

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Question: 177

12. In a nonevolving population, there are two alleles, R and r, which code for the same trait. The frequency of R is 30 percent. What are the frequencies of all the possible genotypes?

  • A. 49% RR, 42% Rr, 9% rr
  • B. 30% RR, 21% Rr, 49% rr
  • C. 0.09% RR, 0.42% Rr, 0.49% rr
  • D. 9% RR, 42% Rr, 49% rr

Correct Answer: D

Explanation:

Let's use the information provided by the question stem to set up our equations. We are told that the frequency of R equals 30%, and as such, p = 0.30. The frequency of the recessive gene r = 100% – 30% = 70%; thus, q = 0.70. The frequency of the genotypes, according to the Hardy–Weinberg equilibrium, are given by p2 = RR, 2pq = Rr, and q2 = rr. Therefore, the frequency of the genotypes are (0.3)2 = 0.09 = 9% RR, 2 × 0.3 × 0.7 = 0.42 = 42% Rr, and (0.7)2 = 0.49 = 49% rr.

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