Next, we selected the most predictive p-value threshold (pT) to test for PRS-by-age, PRS-by-sex and PRS-by-family history interactions. Each interaction was tested in an independent model with all other cross-term covariates included (Keller, 2014). Finally, since the strongest and most robustly validated genetic signal for alcohol consumption and dependence is rs1229984 in ADH1B, we examined the extent to which including rs1229984 genotype as a covariate attenuated the variance explained by the PRS, for traits where alcohol consumption PRS were significantly predictive. The potential attenuation was computed by contrasting a model that included covariates and rs1229984 (coded in the direction of the effect allele, i.e., those homozygous for the protective allele were coded as 2) with a model that included those terms as well as the PRS (i.e. Null model: Phenotype ~ rs1229984 + covariates vs. Full model: Phenotype ~ PRS + rs1229984 + covariates).
