Figure 1.

Study design overview and the hypothetical relationship between genetic variant, exposure, and outcome of the Mendelian randomization design. Allowed relationships between the variables are indicated by solid arrows, while dashed lines and red cross indicate relationships that are not permitted for G to qualify as a valid instrumental variable. The G–X and X–Y arrows are parameterized by γ and β, with the latter denoting the causal effect of X on Y.