Additive genetic variance (A) refers to the additive genetic effects of alleles at every contributing locus. Shared environmental variance (C) is the variance that is due to effects shared by a twin pair. Unique environmental variance (E) is the variance due to effects not shared by a twin pair and also includes measurement error. The total variance of a measure (σ2P) is partitioned into additive genetic (σ2A), non-shared or unique environmental effects (σ2E), and shared environmental effects (σ2C). The total variance is parameterized as: σ2P=σ2A + σ C + σ2E, while the covariance terms are parameterized as: CovMZ= σ2A + σ2C and CovDZ = .5σ2A + σ2C. Univariate ACE, AE, CE and E only models were fitted to test the significance of additive genetic and shared environmental effects.
