 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

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.