on average, 50% of their segregating genes, as do ordinary siblings. Common/shared environmental effects, as defined in biometrical twin modeling, refer to all environmental influences that make siblings more similar to one another. By definition, these influences correlate 1.0 between both MZ and DZ twins. Unique/nonshared environmental influences are uncorrelated between co-twins and have the effect of decreasing the covariance between siblings. When data on multiple phenotypes are available, these models can be extended to evaluate the extent to which genetic and environmental contributions to the disorders are shared. This is calculated by comparing cross-twin, cross-trait correlations, with the logic extended from the basic twin model that comparison of the cross-twin, cross-trait correlations between MZs and DZs provides information about the extent to which a2, c2, and e2 contribute to the phenotypic correlations between traits.
