4 Consequences for Variation and Covariation

In this section we will work through the matrix algebra to derive expected variance and covariance components for a simplified model of sibling interaction. We then show how this model can be adapted to handle the specific cases of additive and dominant genetic, and shared and non-shared environmental effects. Numerical examples of strong competition and cooperation will be used to illustrate their effects on the variances and covariances of twins and unrelated individuals reared in the same home.