Consider a locus with two alleles, A and a. Under which of the following scenarios will the frequency of the A allele increase? A. The fitness of AA and Aa individuals are higher than the fitness of aa individuals. B. The fitness of heterozygotes is higher than the fitness of either homozygote. C. The fitness of heterozygotes is lower than the fitness of either homozygote. D. The fitness of aa individuals is higher than the fitness of Aa and AA individuals.

A. The fitness of AA and Aa individuals are higher than the fitness of aa individuals.

Explanation:

Frequency of allele A will increase if the fitness of AA and As individuals are higher than the fitness of aa individuals because this condition will select individuals who carry A allele, making that this allele will be more common.

If the fitness of heterozygotes is higher than the fitness of either homozygote, both allele frequencies A and a will be favored and there won't be significant differences between them.

In option C the fitness of heterozygotes is lower than the fitness of either homozygote, so individuals aa will have a higher fitness as well as individuals AA. In this situation, both allele frequencies (A and a) will be favored too. So, the correct answer is (A).