These analyses illustrate the importance of incorporating measured aspects of the environment into genetically informative twin models to understand how specific environments act and interact with genetic predispositions. They may also have implications for studying the risk associated with specific genes. For example, a recent study47 aimed to characterize the pathway of risk associated with GABRA2 (for each known human gene the Human Genome Organization [HUGO] Gene Nomenclature Committee approves a gene name and symbol [short-form abbreviation]. Details on this process can be found on the HUGO Web site at http://www.hugo-international.org/comm_genenomenclaturecommittee.php), a gene previously associated with adult alcohol dependence,48 in a community sample of children followed longitudinally from childhood to young adulthood. Association between GABRA2 and trajectories of externalizing behavior was tested from adolescence to young adulthood and moderation of genetic effects by parental monitoring was also tested. Two classes of externalizing behavior emerged: a stable, high externalizing class and a moderate, decreasing externalizing-behavior class. The GABRA2 gene was associated with class membership, with subjects who showed persistent increased trajectories of externalizing behavior more likely to carry the genotype previously
