Fixed allele

In population genetics, a fixed allele is an allele that is the only variant that exists for that gene in a population. A fixed allele is homozygous for all members of the population.^[1] The process by which alleles become fixed is called fixation.

A population of a hypothetical species can be conceived to exemplify the concept of fixed alleles. If an allele is fixed in the population, then all organisms can have only that allele for the gene in question. Suppose that genotype corresponds directly to the phenotype of body color, then all organisms of the population would exhibit the same body color.

An allele in a population being fixed necessarily entails the phenotypic traits corresponding to that allele to be identical for all organisms in the population (if those genotypes correspond directly to a certain phenotype), as it follows logically from the definition of relevant concepts. However, identical phenotypic traits exhibited in a population does not necessarily entail the allele(s) corresponding to those traits to be fixed, as exemplified by the case of genetic dominance being apposite in a species' population. ^[2]

Low genetic diversity is accompanied by allele fixation, which can potentially lead to lower adaptibility to changing environmental conditions for a population as a whole. For example, often having certain alleles make an organism more susceptible to a disease than having other alleles; if an allele highly susceptible to a disease with a prevalent cause is fixed in a population, most organisms of the population might be affected. Hence, generally, populations exhibiting a significant range of fixed alleles are often at risk for extinction. ^[3][4]

Fixed alleles were first defined by Motoo Kimura in 1962.^[5] Kimura discussed how fixed alleles could arise within populations and was the first to generalize the topic. He credits the works of Haldane in 1927^[6] and Fisher in 1922^[7] as being important in providing foundational information that allowed him to come to his conclusion.