Pleiotropy refers to the ability of one single gene to influence several other body characteristics. This gene interaction in chickens can cause the frizzle trait—a gene that causes malformed feathers. This mutation makes it so the chicken cannot keep warm, causing several changes in various organ systems. In Siamese cats, the allele responsible for the animal’s color patterns of light fur on the body and dark at the extremities is the same one responsible for many Siamese cats being cross-eyed.

In humans, an example involves sickle cell anemia, a disorder in which a single point mutation in the amino acid sequence for hemoglobin (red blood cells) results in many different medical problems. In this case, red blood cells produce abnormal hemoglobin molecules which, because of their odd shape, tend to stick together and crystallize. This causes the normal disk shape of red blood cells to change to a sickle shape (hence the name of the disorder). Sickle-shaped red blood cells can clog small vessels, causing pain and the possibility of brain damage and heart failure. In addition, since some of the hemoglobin in these cells is abnormal, less oxygen is available, leading to physical weakness and anemia. If left untreated, the anemia can impair mental function.