What is heterozygote advantage (overdominance) and how does it affect allele frequencies?

• A. Aa has higher fitness than either homozygote, maintaining both alleles at balanced frequencies
• B. AA is always more fit than Aa and aa
• C. Alleles fix quickly to one allele
• D. Fitness is the same for all genotypes

Answer: (A)

Heterozygote advantage, or overdominance, is when the heterozygous genotype has higher fitness than either homozygous genotype. Because the Aa individuals have the highest fitness, selection favors producing both alleles in the population rather than letting one allele become fixed. This creates a stable balancing selection that maintains genetic variation, keeping both alleles present at intermediate, persistent frequencies rather than letting one allele drift to fixation.

A familiar example is the sickle cell trait in malaria-endemic areas: individuals with one normal and one sickle cell allele (heterozygotes) are more resistant to malaria and do not typically suffer severe disease, while normal homozygotes and sickle-cell homozygotes have lower overall fitness under those conditions. This illustrates why the heterozygote通常 has the highest fitness and why both alleles can be maintained.

The other scenarios would lead to different outcomes: if one homozygote were always more fit, one allele would eventually fix; if all genotypes were equally fit, there would be no selection to maintain variation.