variances of a1 and a2 to be equal, the genetic covariance in MZ twins will be twice as large as in DZ twins. The variance of a1 and a2 together, VAR(a1) + VAR(a2) = 2 * VAR(a1) can then be interpreted as the variance due to additive genetic effects. We assume that a1 ∼ N(0, ½ σ2a), a2 ∼ N(0, ½ σ2a), c ∼ N(0, σ2c), and e ∼ N(0, σ2e).
