A method for analyzing the genetic basis of covariation.

Covariances between traits can be partitioned into additive and dominance genetic components and betweenand within;.jamily 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 rejlect a single additive component, in the sense that, given an appropriate rescaling oj the breeding values, a single additive genetic component would adequately describe the additive genetic variation. The statistical procedure jor 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 jive oj Thurstone's Primary Mental abilities. Although the data do not permit a reliable separation oj additive and dominance components, it is shown that a single genetic component will account jor almost all the genetic variation and covariation. [/nless there is marked linkage disequilibrium, this implies that most oj the genetic variation jor the jive traits can be attributed to the pleiotropic action oj genes at a common set oj loci.