Wahlund effect

In population genetics, the Wahlund effect refers to reduction of heterozygosity (that is when an organism has two different alleles at a locus) 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.

Suppose there is a population ${\displaystyle P}$, with allele frequencies of A and a given by ${\displaystyle p}$ and ${\displaystyle q}$ respectively (${\displaystyle p+q=1}$). Suppose this population is split into two equally-sized subpopulations, ${\displaystyle P_{1}}$ and ${\displaystyle P_{2}}$, and that all the A alleles are in subpopulation ${\displaystyle P_{1}}$ and all the a alleles are in subpopulation ${\displaystyle P_{2}}$ (this could occur due to drift). Then, there are no heterozygotes, even though the subpopulations are in a Hardy-Weinberg equilibrium.

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