often vary on a number of important dimensions, including age of onset, course of illness, and the presence of comorbid conditions [Cloninger, 1987; Babor et al., 1992; Hesselbrock and Hessel-brock, 1994; Finn et al., 1997]. Evidence from twin and family studies suggests that alcohol dependence with comorbid disinhibitory disorders may represent a more heritable form of the disorder [Pickens et al., 1991, 1995; Johnson et al., 1996; Ohannessian et al., 2004] suggesting that comorbid, or combined, phenotypes may be particularly relevant for gene identification efforts.
