<p>I saw this site (MCAT</a> Question of the Day) and signed up to get their questions emailed to me, but I'm worried about the quality of the questions now. I'm pretty sure the answer (or at least the explanation) for the January 13, 2009 question is wrong. Could somebody who is good with genetics check it for me? </p>
<p>Question:
A particular species of monkey is in Hardy-Weinberg equilibrium. The monkeys notably have a dominant trait of white hair, and a recessive trait of black hair. In a field study, it was found that out of 500 monkeys, 400 had white hair. How many monkeys are heterozygous? </p>
<p>Answer and Explanation:
(b) About 150
From Hardy-Weinberg Equilibrium, we know that p^2+2pq+q^2 = 1, where p = 0.8 and q = 0.2 (The respective percentages of the population which are dominant and recessive). Thus, the total number of heterozygotes is equivalent to 2pq = 2(.8)(.2) = 0.32. 32% of 500 is roughly 150, thus we get (B) as the correct answer. </p>
<p>Why I Disagree:
q is not the proportion of monkeys expressing the phenotype of the recessive allele, it is the proportion of gametes with this allele. If q=.2, q^2=.04 and .04*500=20 monkeys would have white hair, so the explanation starts from faulty assumptions.<br>
In fact, q=sqrt(.2) and p=1-sqrt(.2) so 2pq=2sqrt(.2)(1-sqrt(.2)=~.5 so about 250 of the monkeys are heterozygous.</p>