Some of the strongest findings involved chromosomal regions—for example, on chromosome 2—that appear to contribute to both habitual smoking and alcohol dependence. The degree of allele sharing was highest when both disorders were evaluated simultaneously— that is, the strongest evidence for increased sharing was seen among sibling pairs with both alcohol dependence and habitual smoking. Additional analyses indicated that a region on chromosome 3 may contribute to both conditions (Bierut et al. 2004). Hence, linkage findings support the hypothesis that certain genes contribute to both alcoholism and habitual smoking.
