Animal models also have been exploited for many years in attempts to identify specific gene variations associated with increased sensitivity to alcohol’s effects. These gene-mapping studies, which commenced in the early 1990s, have used methods similar to those described above for human studies (e.g., linkage analyses). They primarily have sought to identify quantitative trait loci (QTLs)—DNA regions that are associated with characteristics (i.e., quantitative traits) which vary in the degree to which they are present (e.g., sensitivity to alcohol or height). Such traits typically are determined by multiple genes and each QTL may contain one or more of these genes. Compared with humans, studies with rats and mice have the distinct advantage that researchers can use individuals with defined genotypes and control patterns of mating, making it much easier to localize the chromosome region of interest (i.e., the “locus” of the QTL). The most recent systematic review (Crabbe et al. 1999) of the alcohol-related QTL data for the various alcohol-related traits being mapped, which now is out of date, listed the likely locations of several genes affecting alcohol withdrawal severity,
