MZ and DZ twins of the FinnTwin12 and FinnTwin16 cohorts were used to provide heritability estimates for NMR. The intraclass correlation for NMR was 0.80 for MZ and 0.26 for DZ pairs. The pattern of correlations suggested that in addition to additive genetic effects, dominance effects may be present but shared environmental were unlikely to be present. The data were consistent with both AE and ADE models, but not the ACE model as the MZ correlation was much larger than twice the DZ correlation (in such a situation the C component will be zero). In the AE model, A effects accounted for 0.81 (95% CI 0.70–0.88) of variance in NMR. The ADE model fit difference was not statistically significant from the more parsimonious AE model (p = 0.087, Δχ2 = 2.94, Δdf = 1); in the ADE model A effects accounted for 0.20 (95%CI 0.00–0.85) and D effects for 0.62 (95%CI 0.00–0.88) of the variance in NMR.
