Estimates of the genetic and environmental influences on CD and AAB were obtained with standard univariate twin models (Neale & Cardon, 1992). The twin design is based on the premise that differences in resemblance between monozygotic (MZ) twin pairs, who share 100% of their genes, and dizygotic (DZ) twin pairs, who share approximately 50% of their segregating genes, can be used to parse variance in a trait into additive genetic (A), shared environmental (C), and non-shared environmental influences (E). Several assumptions underlie the twin model, namely: 1) MZ and DZ twins are equally correlated in their exposure to trait-relevant environments (i.e, the equal environments assumption), 2) there is no assortative mating for the trait, and 3) there are no gene-environment correlations or interactions for the trait (see Rijsdijk & Sham, 2002).
