Phenotypes are observable characteristics or behaviors of an organism that are genetically determined, such as hair color (at least in animal models) or drinking behavior. Related phenomena are endophenotypes—traits or characteristics that are not a direct symptom of the condition under investigation (e.g., alcoholism) but which have been shown to be associated with the condition; for example, neurobiological characteristics such as reduced P3 amplitude have been noted in people with alcoholism and may be used as endophenotypes to identify people at risk for alcoholism. Brain oscillations also provide a rich source of useful endophenotypes for psychiatric genetics. They represent traits that are less complex than, for example, drinking behavior and likely are to be more directly related to the function of individual genes than the diagnosis of alcoholism. Moreover, as described above, brain oscillations can be used to differentiate affected and unaffected members of an affected family, including offspring at risk, providing a more direct connection with underlying biological vulnerability. Most importantly, these brain oscillations are highly heritable; thus, 76 percent of the variation in δ waves, 89 percent of
