You observe a population of snow leopards before and after a very cold, long winter.

Question

You observe a population of snow leopards before and after a very cold, long winter. The survival rates of three genotypes is as follows: AA: 30%; Aa: 90%; aa: 70%. What can you say about this population?

(A) This population shows selection against the dominant allele

(B) After the terrible winter, this population will have genotype frequencies in the next generation of f(AA):

0.3, f(Aa): 0.9, and f(aa): 0.7 for the three genotypes.

(C) This population shows heterozygote advantage; both A and a alleles will be maintained at a

polymorphism in the long run (i.e., at an equilibrium).

(D) This population shows selection against the recessive allele.

(E) None of these statements is true.

Answer 1

(C) is the first answer that jumps out at me. We see that Aa > aa > AA, meaning that the heterozygote has the highest survival rate, which here is our proxy for fitness in general. Because the heterozygote is fittest, it requires that both alleles will persist in some ratio. So (C) is correct.

It's also worth noting that while (A) looks plausible, I don't think it's correct: we see that A is dominant over a and that AA has the lowest fitness overall. This would seem to suggest that there is selection against the dominant allele. However, because the heterozygote Aa has the highest survival rate, we know there is also selection pressure in favor of the heterozygote, including that dominant A allele, which will keep it from going extinct. So we know there is actually pressure in both directions. so (A) is incorrect.

(D), like (A), is incorrect. The fact that the heterozygote (Aa) and recessive (aa) genotypes have higher survival rates than the dominant homozygote (AA) tell us that if anything, selection is in favor of the recessive allele. thus (D) is incorrect.

(B) also looks plausible: If those are the survival rates, then surely that just tells us the ratios among the population come spring! But not so fast: this only follows if they had equal sizes beforehand. Without knowing the sizes of each group before winter, we only know how each group changed, not their sizes, and so we can't know the frequencies after winter. So (B) is also incorrect.

So at the end of the day, I think only (C) is correct.

Hope that helps!