Additive disequilibrium and z statistic
Additive disequilibrium (D) is a statistic that estimates the difference between observed genotypic frequencies and the genotypic frequencies that would be expected under Hardy–Weinberg equilibrium. At a biallelic locus with alleles 1 and 2, the additive disequilibrium exists according to the equations f 11 = p 1 2 + D f 12 = 2 p 1 ( 1 − p 1 ) − 2 D f 22 = ( 1 − p 1 ) 2 + D {\displaystyle {\begin{aligned}f_{11}&=p_{1}^{2}+D\\[5pt]f_{12}&=2p_{1}(1-p_{1})-2D\\[5pt]f_{22}&=(1-p_{1})^{2}+D\end{aligned}}} where fij is the frequency of genotype ij in the population, p is the allele frequency in the population, and D is the additive disequilibrium coefficient.
Source: Wikipedia — Additive disequilibrium and z statistic (CC BY-SA 4.0)