Genome wide association is now increasingly the method of choice for identifying allelic variants that contribute to complex genetic disorders, especially those with polygenic genetic bases (eg derived from effects at many gene loci, each with modest effects, as well as from environmental determinants; see also Glossary)[1–9]. Substance dependence was one of the first complex phenotypes for which replicated association-based genome scanning data was reported [3, 10–14]. There is now a torrent of information from genome wide association studies of a number of other complex, brain-based phenotypes that both display substantial heritability and are unlikely (based on linkage study results) to manifest many common gene variants that produce large effects [1, 2, 4, 5]. A number of these other heritable, brain-based phenotypes co-occur with addictions and are thus good candidates to display genetic overlaps with addiction.
