Multiple genetic variants, whose effects vary in direction and magnitude, likely influence manifestation of and variation in depression and alcohol dependence. Furthermore, these genetic variants likely interact with one another (epistasis), may be involved in multiple phenotypes (pleiotropy), and are subject to environmental influences. Typically, genes associated with a particular complex trait are of small effect, individually accounting for only a very low proportion of total variance (Flint, 2003; Plomin and Davis, 2009).
