Figure 1:

Potential sources of genetic correlation. (a) Traits Y₁ and Y₂ share a common cause X under genetic control. (b) Trait Y₁ causes trait Y₂. (c) Traits Y₁ and Y₂ share two common causes, X₁ and X₂, one of which has the same directions of effect on both traits, the other of which has opposite directions. In this case, the genome-wide genetic correlation may be close to 0, although when restricted to the loci in G₁ or G₂ the magnitude could be away from 0.