Hardy Weinberg Equilibrium Disturbances in Case-Control Studies Lead to Non-Conclusive Results

Recently, it has been proposed the association of a common deletion affecting toll-like receptor 2 promoter (-196 to -177) to type 2 diabetes mellitus risk. However, genotyping results show a significant deviation from the Hardy- Weinberg Equilibrium (HWE). The law of Hardy-Weinberg shows that for an autosomal biallelic marker with allele frequencies f_A=p and f_a=q, the proportion of subjects with genotypes AA, Aa, and aa should follow the following: f_AA=p², f_Aa=2pq, and f_aa=q². Departure from HWE or Hardy-Weinberg Disequilibrium (HWD) in a human control population can be caused by natural factors such as selective pressure against a certain genotype. However their prevalence is scarce and magnitude of effect over the HWE are small. Other factors such as inbreeding caused by consanguinity, population stratification, or technical problems in genotyping are more usual. Nevertheless, if the control population follows a perfect HWE, the presence HWD among patients might be explained by the genetic association and evidencing a real link between the locus and the trait under study. However, HWD affecting both cases and controls, such as the one reported might be explained by one of the aforementioned issues.