Figure 6.

The genetic variant G is pleiotropic since it affects both phenotypic measures (X and Y). In this illustration, G exhibits both vertical pleiotropy (X and Y are influenced by G via the same causal pathway $G\to X\to Y$) and horizontal pleiotropy (X and Y are influenced by G via different causal pathways). The former is crucial to the application of MR, while the latter results here in a violation of the IV3 assumption.