Recent advances in the field of genetics have shown that there may be limitations regarding a second assumption of the twin model, which is that MZ twins actually have identical genetic material. Examples of genetic differences have been described (Machin, 1996), and more recently epigenetic features such as methylation status have been shown to become increasingly divergent between MZ twin pairs with age (Fraga et al., 2005). More recently, researchers found a significant incidence of differences in copy number variations in MZ twin pairs, providing evidence for the presence of somatic mosaicism and the possible contribution of posttwinning structural variation in chromosomal architecture to phenotypic differences between identical twins (Bruder et al., 2008). Such work speaks to the potential utility of integrating modern genome-wide scans with quantitative statistical methods to begin identifying specific sources of variation.
