In quantitative genetics, Q_ST is a statistic intended to measure the degree of genetic differentiation among populations with regard to a quantitative trait. It was developed by Ken Spitze in 1993. Its name reflects that Q_ST was intended to be analogous to the fixation index for a single genetic locus (F_ST). Q_ST is often compared with F_ST of neutral loci to test if variation in a quantitative trait is a result of divergent selection or genetic drift, an analysis known as Q_ST–F_ST comparisons.

Q_ST represents the proportion of variance among subpopulations, and its calculation is synonymous to F_ST developed by Sewall Wright. However, instead of using genetic differentiation, Q_ST is calculated by finding the variance of a quantitative trait within and among subpopulations, and for the total population. Variance of a quantitative trait among populations (σ²_GB) is described as:

${\displaystyle \sigma _{GB}^{2}=(1-Q_{ST})\sigma _{T}^{2}}$

And the variance of a quantitative trait within populations (σ²_GW) is described as:

${\displaystyle \sigma _{GW}^{2}=2Q_{ST}\sigma _{T}^{2}}$

Where σ²_T is the total genetic variance in all populations. Therefore, Q_ST can be calculated with the following equation:

${\displaystyle Q_{ST}={\frac {\sigma _{GB}^{2}}{\sigma _{GB}^{2}+2\sigma _{GW}^{2}}}}$

Calculation of Q_ST is subject to several assumptions: populations must be in Hardy–Weinberg equilibrium, observed variation is assumed to be due to additive genetic effects only, selection and linkage disequilibrium are not present, and the subpopulations exist within an island model.