## Interact box Simulating the Wahlund effect

The Wahlund effect can be seen readily with a simple simulation that allows you to set allele frequencies for five subpopulations and the degree of non-random mating within populations. Launch PopGeneS2, and select Wahlund effect from the Gene Flow and Subdivision menu. The simulation computes genotype frequencies within each population depending on allele frequencies and the degree of non-random mating. It also shows the expected and observed genotype frequencies averaged over the subpopulations along with the average allele frequency and variance in allele frequency among subpopulations. The fixation indices FIS, FST, and FIT are also calculated. Try the first simulation with the default values of f = 0 and allele frequencies of 0.9, 0.8, 0.7, 0.5, and 0.5.

What is the relationship between the variance in allele frequency and the difference between the average observed and expected heterozygosities? How does Fst change with variance in allele frequency? What happens to the observed genotype frequencies when you try different levels of non-random mating within subpopulations? (Set f to a value other than zero.)

n n n n and var(p) > 0 then the total population will have a deficit of heterozygotes and an excess of homozygotes compared to the case of panmixia. This method also provides the prediction that the total deficit of heterozygotes will equal the total excess of homozygotes when var(p) > 0.

One consequence of the Wahlund effect is termed isolate breaking to describe the increase of heterozygote genotypes that occurs when previously subdivided populations with diverged allele frequencies experience random mating. In human populations, disease phenotypes caused by recessive alleles expressed in homozygotes include cystic fibrosis, Tay-Sachs disease, and sickle-cell anemia. These disorders are more common in relatively insular populations such as Ashkenazi Jews, native American groups, and the Amish, but rarer in human populations that have experienced greater amounts of genetic mixing and thereby have less of a heterozygosity deficit due to subdivision. To see the impact of isolate breaking, imagine two randomly mating populations of squirrels that initially do not share any migrants and have allele frequencies that have diverged over time (Fig. 4.11). Suppose that the population on the left has albino individuals and the basis of the albino phenotype is the completely recessive allele a with frequency q. The albino allele is completely absent in the population on the right. The average frequency of albino squirrels in the subdivided population is

Relying on Hardy-Weinberg, we can similarly determine the average frequency of dominant homozygotes zygotes

## Post a comment