Murine models of substance and alcohol dependence: Unraveling genetic complexities

Most behavioral traits operate on a phenotypic and genetic continuum, i.e., the phenotypic output is quantitative based on the genetic input. No one gene is either necessary or sufficient to account for the observed phenotype; rather, a collection of genes is responsible. This phenotypic and genetic complexity is particularly evident in psychological disorders. For instance, first-degree relatives of schizophrenics have a 9% risk for a diagnosis, whereas the risk drops to 2% for a third-degree relative (1. These findings suggest that many genes contribute, and as the proportion of shared genes increases among relatives, so does the likelihood of shared diagnosis. Regardless of commonalities among genotypes, phenotypic expression may vary significantly in the frequency and severity of symptoms. This further supports the contention that several genes contribute to the trait, each with small effects.