Pleiotropy refers to genes that can affect more than one character. A gene may make just one polypeptide, but that polypeptide can affect a number of different properties of an organism. The hemoglobin gene, sickled cells cause a breakdown of red blood cells and a higher likelihood of red blood cell clumping, which can block blood vessels and cause impaired mental function and kidney failure, and increase the risk of pneumonia. These are different phenotypic characteristics caused by a defect in a single, pleiotropic gene.
