In what scenario do both alleles of a gene remain in the population?

The scenario where both alleles of a gene remain in the population is best described by the condition where heterozygotes have higher fitness. This phenomenon is known as heterozygote advantage, and it occurs when individuals with two different alleles for a particular gene (heterozygotes) exhibit a higher level of fitness compared to individuals who are homozygous for either allele.

This advantage allows both alleles to persist in the gene pool since the heterozygous individuals have an enhanced survival or reproductive success. A classic example of this is seen in the case of sickle cell anemia and malaria resistance, where individuals who are heterozygous for the hemoglobin gene have a survival advantage against malaria, ensuring that both alleles remain in the population.

In other scenarios, such as frequent mutations, while it may introduce new alleles, it does not guarantee that both alleles will persist long-term. When one allele is always dominant, typically the recessive allele can be driven out of the population over time. Similarly, if selection pressure is absent, alleles may drift in frequency due to random processes rather than maintaining both alleles in a stable state. Hence, the presence of heterozygote advantage is the key factor for