Populations Exam 6 Practice

In a recessive deleterious allele scenario with selection against aa, what is the typical assumption about the fitness of Aa relative to AA?

Aa has lower fitness than AA

Aa has higher fitness than AA

Aa fitness is zero

Aa has fitness equal to AA

With a recessive deleterious allele and selection acting against the homozygous recessive genotype, the heterozygote doesn’t express the harmful trait. That means Aa has essentially the same fitness as AA. In standard terms, W_AA = W_Aa > W_aa, and it’s common to set W_AA = W_Aa = 1 and W_aa = 1 − s. The deleterious effect shows up only in aa, so the heterozygotes are not disadvantaged and share the same fitness as non-carriers. This is why the correct description is that the fitness of Aa is equal to that of AA. The other possibilities would imply some advantage or disadvantage to heterozygotes, which would not match the usual recessive deleterious scenario.