Genetic admixture is the presence of DNA in an individual from a distantly-related population or species, as a result of interbreeding between populations or species who have been reproductively isolated and genetically differentiated. Admixture results in the introduction of new genetic lineages into a population. It has been known to slow local adaptation by introducing foreign, unadapted genotypes (known as genetic pollution). It also prevents speciation by homogenizing populations and increasing heterozygosity.
Climatic cycles facilitate genetic admixture in cold periods and genetic diversification in warm periods. Natural flooding can cause genetic admixture within populations of migrating fish species. Genetic admixture may have an important role for the success of populations that colonise a new area and interbreed with individuals of native populations.
Admixture mapping is a method of gene mapping that uses a population of mixed ancestry (an admixed population) to find the genetic loci that contribute to differences in diseases or other phenotypes found between the different ancestral populations. The method is best applied to populations with recent admixture from two populations that were previously genetically isolated for tens of thousands of years, such as African Americans (admixture of African and European populations). The method attempts to correlate the degree of ancestry near a genetic locus with the phenotype or disease of interest. Genetic markers that differ in frequency between the ancestral populations are needed across the genome.
Admixture mapping is based on the assumption that differences in disease rates or phenotypes are due in part to differences in the frequencies of disease-causing or phenotype-causing genetic variants between populations. In an admixed population, these causal variants occur more frequently on chromosomal segments inherited from one or another ancestral population. The first admixture scans were published in 2005 and since then genetic contributors to a variety of disease and trait differences have been mapped. These include hypertension, multiple sclerosis, BMI, and prostate cancer in African Americans. By 2010, high-density mapping panels had been constructed for African Americans, Latino/Hispanics, and Uyghurs.