The classical twin design was applied comparing the similarity of MZ and DZ twins using the ACE model which partitions the variance into: additive genetic (A), common environment (variance due to environmental effects shared within twin pairs)(C) and unique environment (environmental effects not shared within twin pairs)(E). As all twin pairs included in the study visited the clinic in pairs and MZs share 100% of their genes, any differences arising between them in these circumstances are unique (E). The correlation observed between MZ twins provides thus an estimate of A + C. In contrast, DZs have a common shared environment but share on average only 50% of their genes: so the correlation between DZs is a direct estimate of ½A + C . Consequently, twice difference between MZ and DZs gives the genetic additive effect (A) and the common environment (E) is the MZ correlation minus our estimate of the genetic effect (A). A standard linear mixed model was used to estimate these variance components as previously described 19. The following covariates were included in the model: age and experimental
