For some observed measures, ‘D’ or non-additive genetic effects (correlated 1.0 in MZ and 0.25 in DZ twin pairs), which is defined as the influence of latent non-additive genetic influences on the observed variable, may be of importance. Due to available degrees of freedom (Neale and Cardon, 1992; Sham, 1998), either D or C, but not both, may be estimated in a model where information from only twin pairs is used. Comparing twin-cotwin correlations allows for a straightforward test for the presence or absence of D. If the DZ correlation is significantly less than half of the observed MZ correlation, then the modeling of D is preferred over C.
