To address this controversy, COGA investigators studied two polymorphisms in separate regions of the DRD2 gene in the stage II COGA families using family based association study methods (Edenberg et al. 1998). Neither of those polymorphisms showed any evidence of linkage with alcohol dependence, even when the analysis was limited to more severe forms of alcoholism, where associations should be detectable more easily. Similarly, no significant linkage existed between the DRD2 gene and habitual smoking (Bierut et al. 2000).
