As mentioned earlier, alcohol dependence and smoking co-occur more often than by chance alone; moreover, twin studies have supported the role of common genetic factors in the development of both disorders. Accordingly, the COGA investigators also examined the association between the DRD2 gene and co-occurring alcohol dependence and habitual smoking. Again, those analyses found no evidence that the DRD2 gene is linked to the combined phenotype (Bierut et al. 2000). In summary, although the dopamine neuro-transmitter system clearly plays a role in alcohol and other drug use and related disorders, the COGA study detected no association between a genetic variation in the DRD2 gene and the development of alcohol dependence and/or habitual smoking. The investigators are currently using the COGA sample to examine additional candidate genes for alcohol dependence and habitual smoking.
