Abstract
Covariances between traits can be partitioned into additive and dominance genetic components and between- and within-family environmental components, using a method analogous to that used in the analysis of single traits. The problem arises as to whether all additive genetic components simply reflect a single additive component, in the sense that, given an appropriate rescaling of the breeding values, a single additive genetic component would adequately describe the additive genetic variation. The statistical procedure for testing this hypothesis is discussed in detail. Similar considerations apply to the dominance variation. The approach is applied to twin data given by Loehlin and Vandenberg (1968) on covariation between five of Thurstone's Primary Mental abilities. Although the data do not permit a reliable separation of additive and dominance components, it is shown that a single genetic component will account for almost all the genetic variation and covariation. Unless there is marked linkage disequilibrium, this implies that most of the genetic variation for the five traits can be attributed to the pleiotropic action of genes at a common set of loci.
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