study candidate genes for both alcohol dependence and habitual smoking without running the risk of spurious findings resulting from population stratification by using family-based tests, such as the Transmission Disequilibrium Test (TDT).4 This approach eliminates the need for a separate, matched unaffected control population. As a result, the investigators can directly test for both genetic linkage and association between a gene of interest and the presence of alcohol dependence and habitual smoking.
