MZ twin pairs are less than twice as similar as the DZ twin pairs, shared environment influence is implied; however, if the MZ twin pairs were at least twice as similar as DZ twin pairs it would indicate that the additive effects of genes are sufficient to explain their similarity. Finally, if the correlation between MZ twin pairs is less that 1, there must be an influence of non-shared environment on the phenotypic variance, since this is the only component that accounts for MZ twin pairs being different from one another. If MZ twin pairs correlation more than twice the DZ correlation, there is evidence of non-additive genetic effects, for instance dominance (interaction of genes at a single locus). However, in twins reared together the effects of shared environment and non-additive genetic effects are confounded and cannot be estimated simultaneously.
