A previous investigation from the Collaborative Study on the Genetics of Alcoholism (COGA) identified a region on Chromosome 1 that was linked to both alcohol dependence and depressive syndrome (Nurnberger et al., 2001). More recent studies have identified specific genes that are associated with both depression and alcohol dependence, including CHRM2 (Edenberg and Foroud, 2006; Wang et al., 2004), SLC6A4 (Dick et al., 2007b; Gokturk et al., 2008), COMT (Baekken et al., 2008; Sery et al., 2006), and DRD2 (Dick et al., 2007c; Koks et al., 2006), although others report failures to replicate these associations (e.g., Cohen-Woods et al., 2009; Furlong et al., 1998; Gillespie et al., 2005; Serretti et al., 2006). Furthermore, DRD2, CHRM2, SLC6A4, and MAOA have all been associated with comorbid conditions involving alcohol use and internalizing symptomatology in adolescents (Saraceno et al., 2009). Thus, findings from molecular genetics studies, in conjunction with twin studies on genetic correlation between the phenotypes, represent converging evidence that comorbidity of these traits is genetically influenced. One would expect that genes associated with comorbidity could fall into one of several categories:
