the chromosomal regions showing linkage in human and animals studies are often broad and imprecise (at least initially) in terms of their localization of the underlying susceptibility variant(s). Simulation studies have demonstrated considerable variation in the linkage peaks observed for complex phenotypes (Roberts et al., 1999). This has also been borne out in ongoing gene identification projects; for example, the COGA detected linkage on chromosome 4 (Reich et al., 1998; Williams et al., 1999; Saccone et al., 2000) and has subsequently identified several genes in the region as associated with alcohol dependence (Edenberg et al., 2004, 2006, 2008). However, these genes are located throughout the QTL region, with the strongest associations (Edenberg et al., 2004) not localized directly under the strongest linkage peak with alcohol dependence (Williams et al., 1999).
