The discussion so far has considered G as an abstract index of genetic risk; however, this approach to modeling GxE interaction effects can also be used to analyze the effects of specific genes5 on risk of alcohol dependence. Several investigators have discussed circumstances where an environmental risk factor may have an effect only in the presence of a particular variant of a gene (i.e., allele), or vice versa (e.g., Khoury et al. 1993; Ottman 1990). In practice, however, most risk factors likely contribute to risk even in the absence of another risk factor, although their joint effects might be greater (or less) than would be predicted if interaction effects are ignored.
