Although some family studies have suggested that BD has an autosomal dominant genetic determinant, the vast majority of genetic studies suggest that BD has a high level of genetic heterogeneity and a substantial polygenic component (5, 6). Linkage studies have identified a number of loci with inconsistent replication. While previous linkage studies were highly divergent (7), recent meta-analyses of linkage studies have found consistent supportive evidence for linkage to a few potential BD susceptibility loci (8-10), notably 6q, 8q, 13q, and 22q. A number of polymorphisms in a variety of candidate genes have been tested for association with BD, but most of the polymorphisms have shown no statistically compelling associations with BD; those that appear to be associated show odds ratios of 1.1 to 1.3, which is again consistent with a polygenic basis for BD (11).
