Nonpolymorphism

Nonpolymorphism is a term used in genetics to describe a genetic locus, gene, or trait that shows no variation among individuals within a given population. Such a locus is considered monomorphic, meaning that all sampled individuals carry the same allele or variant.

Nonpolymorphic loci occur when a single allele has become fixed in the population due to genetic drift, natural selection, or functional constraint that disfavors variation. They are often contrasted with polymorphic loci, where multiple alleles are present at appreciable frequencies.

Implications include reduced heterozygosity at the locus and limited usefulness for studying population structure or ancestry at that site. However, nonpolymorphism can indicate conserved regions, possibly essential genes or regulatory elements under purifying selection.

Detection typically involves sequencing or genotyping across a sample; a nonpolymorphic locus shows identical alleles in all individuals, and allele frequency data report a frequency of 1 for a single allele and 0 for others. Sample size and population scope influence whether a locus appears nonpolymorphic.

Notes: The term nonpolymorphism is often used interchangeably with monomorphism, though some authors reserve monomorphism for the fixed condition across all populations studied rather than in a specific sample. See also genetic variation and monomorphism.