Heterozygous advantage

Heterozygous (or heterozygote) advantage is the relative fitness of organisms with a heterozygous genotype when compared to either the homozygous dominant or homozygous recessive genotypes for the same locus. It is a key component of the overdominance hypothesis, one of the hypotheses used to explain heterosis, which states that some combinations of alleles working together in heterozygotes are genetically superior to when those alleles exist alone in homozygotes.

Heterozygous advantage is one way of maintaining genetic polymorphisms that might otherwise be selectively removed for being deleterious. For instance, the sickle-cell anaemia allele in a homozygote is fatal, but in a heterozygote it affords protection to malaria while not drastically reducing the oxygen-carrying capacity of the blood. This makes it advantageous not only to individuals homozygous for sickle-cell anaemia but also to individuals homozygous for normal red blood cells.