Parallel to the way that evidence for heritability from twin studies for a given outcome was used to justify searching for specific genes involved in that outcome, evidence for gene-environment interaction from twin studies can also be used to develop hypotheses to test gene-environment interactions associated with specific, identified genes. Change in the overall heritability across environmental contexts does not necessarily dictate that any one specific susceptibility gene will operate in a parallel manner; however, a change in heritability suggests that at least a good portion of the involved genes (assuming many genes of approximately equal and small effect) must be operating in that manner for a difference in heritability by environment to be detected. In this sense, you are loading the dice when you test for specific candidate gene-environment interaction effects with an environment that has already been shown to moderate the overall importance of genetic influences on that outcome. This is the strategy we have used to further characterize the risk related to genes associated with alcohol dependence in the Collaborative Study on the Genetics of Alcoholism. On
