Animal models allow us to study environmental effects in genotypes with self-defined levels of alcohol drinking. In the first report of inbred mouse strain differences in two-bottle preference drinking, data on individual mice from 5 inbred strains were presented (McClearn and Rodgers, 1959). Even within the high-drinking C57BL strain, there were individual differences in alcohol preference, and within some other strains such as C3H, the individual differences were substantial. Such differences among individuals within a strain must reflect gene-environment interaction, but the source of the environmental effect is entirely unknown. A strong possibility is that the individual differences within a strain are due to differences in gene expression. Molecular technology now permits the genome-wide analysis of gene expression differences, and pronounced individual differences within inbred strains have been shown for genes and gene networks. For example, genes in a functional group may be up-regulated in one brain tissue and down-regulated in another in the same individual brain, in addition to differing from expression patterns across individuals of the same genotype (Cowley et al., 2009).
