Types of pleiotropy. (a) Biological pleiotropy indicates that a gene regulates multiple traits; therefore, the gene is responsible for the association of the traits. (b) Mediated pleiotropy refers to when a gene regulates Trait B while fully or partially mediated by Trait A. Testing whether an indirect effect exists can distinguish (b) from (a). (c) Unmeasured confounding between genes may initiate a spurious pleiotropic gene (Gene 2) due to a biological pleiotropic gene (Gene 1). The solid line indicates authentic causation, the dashed line indicates potential authentic causation, and the dotted line indicates spurious causation.