The contribution of GxE interaction effects to family resemblance for disorders such as alcohol dependence can be determined using some basic algebra. Assume that the total variation (V) in a population for a given index of alcoholism risk is the sum of all individual and interaction effects. This assumption is stated algebraically as V=G+SE+NSE+GxSE+GxNSE. The values of the various components in this sum can then be determined as follows (also see table 1):3
