In addition, results from quantitative trait locus (QTL) studies in animals suggest that specific loci may influence different alcohol-related behaviors, such as acute/chronic withdrawal, preference drinking, and blood level after acute ethanol dose (see Crabbe et al., 1999). One such alcohol-related trait is the level of response to ethanol, measured by the dose of ethanol needed to produce a specified effect. Level of response at the time of initial use is a risk factor for the development of alcoholism (Schuckit and Smith, 2001). Other alcohol-related traits, including tolerance and withdrawal, reflect physiological adaptation to chronic alcohol use (Schuckit et al., 1998). Withdrawal is an especially pivotal physical symptom for AD diagnosis (Langenbucher et al., 2000) and is associated with worse prognosis (Hasin et al., 2000). In addition to these physiological traits, variation in age at onset of AD may also reflect differences in the magnitude of the underlying AD liability (Johnson et al., 2000). These quantitative alcohol-related traits represent different aspects of the dependence process, and may provide greater insight into the genetic contributions to the risk to develop AD.
