Fig. 2: Diagram of the concept and assumptions for Mendelian randomisation.
The presence of an arrow indicates a causal effect. The genetic variant Z is a valid instrument when it is strongly associated with the risk factor of interest X (relevance assumption) and when it has an effect on the outcome of interest Y exclusively through X, and not via another pathway or a direct pathway (absence of horizontal pleiotropy or exclusion restriction). So ΦZ should be 0. This diagram uses the genetic variant rs880315 (located at chromosome 1, position 10796866) as an example, as it is known to be related to SBP and DBP [38].
