We genotyped 10 single nucleotide polymorphisms (SNPs) in GABRA2, which were selected based on previous evidence for an association with alcohol dependence in the Collaborative Study on the Genetics of Alcoholism (see Dick et al., 2009 for a description of selection criteria and further genotyping details including chromosomal position and minor allele frequency). Linkage disequilibrium across these markers was very high, with an r2 average of 0.91, indicating that the SNPs do not reflect independent tests of association (Dick et al., 2009). As in previous developmental and gene-environment interaction (G × E) studies of GABRA2 (Dick, Agrawal, et al., 2006; Dick, Bierut, et al., 2006; Perry et al., 2013), we selected the SNP with the single most significant association from the COGA sample (Edenberg et al., 2004), rs279871, to represent the risk-associated haplotype block in the present analyses. Genotypic information for this SNP was available for 97% of the sample. Genotyping was done on the minus strand, and the minor allele frequency (MAF) for the G allele was 0.43.
