Genes that affect multiple, apparently : unrelated, phenotypes are called pleiotropic genes. Pleiotropy should not be confused with polygenic traits, in which multiple genes converage to result in a single phenotype. There are many examples of pleiotropic genes in humans, some of which are associated with disease. For instance, Marfan syndrome is a disorder in humans in which one gene is responsible for a constellation of symptoms, including thinness, joint hypermobility, limb elongation, lens dislocation and increase: susceptibility to heart disease.