Wahlund effect

In population genetics this is known as Wahlund effect when the heterozygosity of a population is reduced by structuring in subpopulations. In particular, if two or more sub- populations have different allele frequencies, the overall heterozygosity is reduced, even if there are sub- populations in Hardy - Weinberg equilibrium. The reasons why a population is split into several sub- populations, for example, can be geographic barriers that prevent the exchange of genetic material. If then uses genetic drift, gives the Wahlund effect.

The Wahlund effect was first recognized in 1928 by the Swedish geneticist Sten Wahlund.

A simple example

If a population with allele frequencies of A and a (given by and respectively) into two equal-sized populations and is broken down, and all of the A allele in the subpopulation all a alleles are subpopulation, which can easily occur with genetic drift are, it, even if the overall population still is no more heterozygotes in Hardy - Weinberg equilibrium.

Example with two alleles and two subpopulations

To generalize the example above, let us and the allele frequencies of A in and represent. ( as well as represent and accordingly a).

The allele frequencies in each population are now different, mathematically expressed as: .

If one now imagines that each population is balance in an internal Hardy-Weinberg, does that mean the genotypes AA, Aa and aa are p2, 2pq, and q2 for each population.

Then, the heterozygosity () is around the population is given by the average of the two:

And is always less than (=) applies unless:

Summary

Wahlund the effect may be applied to a wide variety of subpopulations of different sizes. The heterozygosity of the whole population is given by the average of the subpopulations heterozygosities, weighted by the size of the subpopulations.

• Population Genetics