CHRNA4 SNPs. We used the STRUCTURE program (Pritchard et al., 2000) to estimate the admixture proportion for each individual based on the AIMs. The log-likelihood of each analysis at a different number of population groups (k) was estimated from the average of 3 independent runs (20,000 burn in and 30,000 iterations) and, as expected, the result favored a two-ancestry population model. We included the admixture proportion of each individual as a covariate in the combined analysis and in each population specific subgroup analysis.
