Fst

Fst

[2021-01-08]

In simple terms, Fst measures the ratio between the observed heterozygosity (Ho) and the expected heterozygosity (He).

The expected heterozygosity can be computed following the Hardy-Weinberg equilibrium principle, e.g. for a biallelic locus this is 2p(1-p) where p is the frequency of one of the alleles.

On one extreme, if the ratio between observed and expected heterozygosities in a population is zero, i.e. Ho/He = 0, then this means that the individuals or subpopulations are not intercrossing (i.e. assortative mating and/or selfing and/or asexually reproducing). Hence the population is said to be highly structured. In terms of Fst:

Fst = 1 - (Ho/He).

Therefore, if Ho/He = 0 then Fst = 1, which means highly structure populations.

On the other extreme, a population with Ho/He = 1 or Fst = 0, means that the individuals or subpopulations are intercrossing freely as if they were a single panmictic population, hence the population is said to have no structure.