nonpolymorphic

Nonpolymorphic is a term used in genetics and genomics to describe a genetic locus that shows no variation among individuals of a given population or species. When all sampled individuals carry the same allele at a locus, the site is described as nonpolymorphic or monomorphic. This contrasts with polymorphic loci, where two or more alleles or sequence variants are observed.

Causes and interpretation: Nonpolymorphism can result from functional constraint, where essential genes or regulatory elements tolerate little change, leading to purifying selection that removes alternatives. It can also reflect past selective sweeps or small effective population size, where genetic drift fixes a single allele. Conservation across related species can create nonpolymorphic sites within a species due to long-term constraint. Importantly, nonpolymorphism is often context-specific: a locus may be nonpolymorphic in one population or species but polymorphic in another.

Applications and examples: In population genetics, nonpolymorphic markers are less informative for distinguishing individuals but provide stable reference points for sequence alignment and phylogenetic analysis. In comparative genomics, nonpolymorphic, highly conserved sites help identify regions under strong functional constraint. Researchers assess nonpolymorphism by sequencing many individuals across populations and noting sites with zero observed variation; such sites are considered monomorphic for the sampled dataset. It is also possible for a locus to be nonpolymorphic in humans but polymorphic in other species or populations, highlighting the importance of the sampling context.