Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes. In frequency-dependant selection, the fitness of a phenotype declines it it becomes too common in the population.
Explanation;
The heterozygotes have both alleles, which is needed for genetic variation. Heterozygotes are essential for there to be genetic variation in a population. In other words it is when When a single copy of a disease allele doesn't result in a disease but instead is good for the person or organism that carries it, we say that allele has a heterozygote advantage. For example a sickle cell trait, which protects against malaria in heterozygotes, but causes a deadly disease in homozygotes.
-Frequency-dependent selection occurs when the fitness of a genotype depends on its frequency. It is possible for the fitness of a genotype to increase (positively frequency-dependent) or decrease (negatively frequency-dependent) as the genotype frequency in the population increases.
