Underdominance

Underdominance, also known as heterozygote disadvantage, is a genetic phenomenon where the heterozygous genotype has lower fitness than either of the homozygous genotypes. This means that individuals with two different alleles for a particular gene are less likely to survive or reproduce than individuals with two identical alleles. This can occur for various reasons, including the disruption of a beneficial gene interaction in heterozygotes or the expression of detrimental traits.

In a population, underdominance can lead to the selection against heterozygotes. If the fitness of the heterozygous genotype is consistently lower than that of both homozygous genotypes, natural selection will favor individuals with homozygous genotypes. Over time, this can result in a decrease in heterozygosity within the population and potentially the fixation of one of the homozygous genotypes, provided there are no other evolutionary forces acting to maintain variation.

Underdominance is considered a form of frequency-dependent selection, although it's a specific case where the fitness advantage lies with homozygotes, regardless of allele frequency. It contrasts with overdominance, where heterozygotes have higher fitness, and is a significant factor in understanding the evolutionary dynamics of genetic variation within populations. Detecting underdominance can be important for conservation genetics and understanding the genetic basis of traits.