Horizontal vs Vertical pleiotropy¶
Pleiotropy means one variant or gene affects more than one phenotype. Vertical pleiotropy is a *chain*: effect on trait A mediates or lies on the causal path to trait B (e.g. gene → biomarker → disease). Horizontal pleiotropy (biology sense) is *parallel*: distinct pathways to multiple traits without a simple mediator chain. Mendelian randomization borrows the same words but uses horizontal pleiotropy mainly for *invalid instruments* (independent pathways unrelated to the exposure of interest)—see the [MR-specific entry](../../terms/h/horizontal-pleiotropy.md).
How they differ¶
| Horizontal pleiotropy (biology) | Vertical pleiotropy | |
|---|---|---|
| Structure | Multiple outcomes via different mechanisms from the same variant. | Multiple outcomes on the same causal pathway (mediation). |
| Example | One locus affects height and hair color through unrelated biology. | LDL pathway variant affects lipids and coronary disease through a lipid-mediated route. |
| MR angle | Problematic if the instrument is pleiotropic for other reasons than the exposure pathway. | Often more compatible with a clean causal story if intermediates are modeled. |
Rule of thumb: Ask whether the second trait is plausibly downstream of the first (vertical) or a parallel outcome (horizontal). In MR papers, “horizontal pleiotropy” often means instrument validity, not the biology textbook sense.
Related terms¶
References¶
- Jee J, et al. (2026). The pleiotropic landscape of the human genome. Nat Rev Genet.
- Solovieff N, et al. (2013). Pleiotropy in complex traits: challenges and strategies. Nat Rev Genet.