Evidence for linkage has also been demonstrated in genome scans for other drugs of abuse in Native Americans including marijuana, stimulants, and tobacco. A genome scan using a marijuana dependence phenotype that also included externalizing traits uncovered a LOD score of 4.4 on chromosome 16q24.1, as well as a LOD score of 6.4 on chromosome 19q13.33 [23]. Genome scans have also been conducted to map loci associated with stimulant dependence (amphetamine, cocaine) phenotypes in California Indians. In that study linkage analysis revealed a locus with a LOD score of 3.02 on chromosome 15q22.3 near the nicotinic receptor gene cluster. A genome scan for loci associated with tobacco usage in California Indians identified a region with a LOD score that exceeded 3.0 on chromosome 4q22.1 in a bivariate analysis with an alcohol drinking severity phenotype [25]. This finding suggests that a region of chromosome 4q22.1 harbors a gene(s) that jointly influences alcohol dependence and tobacco use phenotypes.
