Douglas Harper's Etymology Dictionary
Wiktionary
n. 1 (context genetics English) The influence of a single gene on multiple phenotypic traits; pleiotropism 2 (context pharmacology English) The existence of drug effects other than the one for which the drug was designed; usually implies additional beneficial effects.
Wikipedia
Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously. An example is phenylketonuria, a human disease that affects multiple systems but is caused by one gene defect. Pleiotropic gene action can limit the rate of multivariate evolution when natural selection, sexual selection or artificial selection on one trait favours one specific version of the gene ( allele), while selection on other traits favors a different allele. Genetic correlations and hence correlated responses to selection most often exemplify pleiotropy.
In pharmacology, pleiotropy refers to a drug's actions, usually unanticipated, other than those for which the agent was specifically developed. It may include adverse effects which are detrimental ones, but is often used to denote additional beneficial effects.
For example, statins are HMG-CoA reductase inhibitors that primarily act by decreasing cholesterol synthesis, but which are believed to have other beneficial effects, including acting as antioxidants and stabilizing atherosclerotic plaques.
Usage examples of "pleiotropy".
But in the highly organized human body with its pleiotropy, the results were usually unsatisfactory, with the DNA often latching onto inappropriate parts of the embryo's chromosomes.