A large collaborative research program on the genetics of alcoholism (COGA), involving several institutions in the USA, seeks to identify genes contributing to alcoholism and related traits (ie, phenotypes), including comorbid psychiatric conditions. COGA investigators have found an increased prevalence of depressive syndromes in alcoholics. In particular, the combination of alcoholism and depression tends to cluster in families (Bierut et al 2002; Nurnberger et al 2002). Comorbid alcoholism and depression occurred substantially more often in first-degree relatives of COGA participants with alcoholism than in relatives of nonalcoholic control participants. Based on these data, COGA investigators defined three phenotypes: alcoholism (ALC); alcoholism and depression (AAD); and alcoholism or depression (AorD). The data were analyzed to determine whether the phenotypes were linked to specific chromosomal regions. These analyses have identified several chromosomal regions, particularly on chromosomes 1 and 4 that appear to be linked to alcohol-related phenotypes (Bierut et al 2002; Nurnberger et al 2002). In addition, increased allele sharing was seen near two markers called D1S1648 and D1S1588 between 100 and 110 centi-Morgan (Nurnberger et al 2002). The same portion of
