Fixation index

Fixation index (F_ST) is a measure of population differentiation due to genetic structure. It is frequently estimated from genetic polymorphism data, such as single-nucleotide polymorphisms (SNP) or microsatellites. Developed as a special case of Wright's F-statistics, it is one of the most commonly used statistics in population genetics.

Two of the most commonly used definitions for F_ST at a given locus are based on the variance of allele frequencies between populations, and on the probability of Identity by descent.

If ${\displaystyle {\bar {p}}}$ is the average frequency of an allele in the total population, ${\displaystyle \sigma _{S}^{2}}$ is the variance in the frequency of the allele between different subpopulations, weighted by the sizes of the subpopulations, and ${\displaystyle \sigma _{T}^{2}}$ is the variance of the allelic state in the total population, F_ST is defined as

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