Figure 2.
Transgenerational changes in epigenetic variation in twins. The figure provides illustrative examples of epigenetic heritability estimates in hypothetical families that include pairs of MZ or DZ twins across three generations. This highlights the idea that epigenetic heritability estimates from twin studies are expected to be higher than those obtained from transgenerational families. At each of three generation (G1, G2, G3) we represent MZ (blue) and DZ (red) intra-class correlation distributions for genome-wide DNA methylation patterns, and these are consistent with reported correlation estimates [44]. We compare within-generation correlation distributions to hypothetical transgenerational correlations in DNA methylation from parent–offspring (G1 versus G2) and grandparent–grandchild (G1 versus G3) pairs to illustrate the fact that epigenetic heritability becomes diluted over generations. In addition, the figure also emphasizes the time-specific aspect of epigenetic heritability estimates and specifically the reduction in correlation of genome-wide DNA methylation patterns for one pair of twins (at generation G1) with age, where genome-wide methylation correlations are slightly lower at later (TN) than at earlier (T1) stages in life. This is consistent with the observed increase in epigenetic variance in older twins [37,38].