In population genetics, the Wahlund effect refers to reduction of heterozygosity(or when a person has two different alleles) in a population caused by subpopulation structure. Namely, if two or more subpopulations have different allele frequencies then the overall heterozygosity is reduced, even if the subpopulations themselves are in a Hardy-Weinberg equilibrium. The underlying causes of this population subdivision could be geographic barriers to gene flow followed by genetic drift in the subpopulations.
The Wahlund effect was first documented by the Swedish geneticist Sten Wahlund in 1928.
Read more about Wahlund Effect: Simplest Example, Case of Two Alleles and Two Subpopulations, Generalization, F-statistics
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