Polymorphisms in the genes encoding the GABAA receptor subunits have been linked to ethanol response and risk for alcoholism in humans. Genes encoding the GABAA receptor subunits are clustered in several chromosomal regions including 4p13–q11 (α2, α4, β1, and γ1), 5q34–q35 (α1, α6, β2, and γ2), 15q11–q13 (α5, β3, and γ3), and Xq28 (α3, β4, and ε1) (Enoch 2008). Results from the Collaborative Study on the Genetics of Alcoholism (COGA) found an increased allele sharing among ethanol-dependent individuals in a region on chromosome 4p that includes a cluster of four genes: GABRA2, GABRA4, GABRB1, and GABRG1, encoding for α2, α4, β1, and γ1 subunits, respectively (Reich 1996; Reich et al. 1998). Since then, several studies have shown that GABRA2 is a key gene affecting the risk for alcoholism. Edenberg et al. (2004) reported that single nucleotide polymorphisms (SNPs) in the GABRA2 gene, but not other members in the gene cluster, were associated with both ethanol dependence and changes in β-frequency electroencephalogram (EEG). Brain oscillations measured by EEG are a key phenotype in ethanol dependence: alcoholics and the offspring of male
