Pleiotropy is the phenomenon whereby a single gene has multiple consequences in numerous tissues. Pleiotropic effects stem from both normal and mutated genes, but those caused by mutations are often more noticeable and easier to study. Pleiotropy is actually more common than its opposite, since in a complex organism, a protein from a single gene is likely to be expressed in more than one tissue, and the cascade of problems caused by a mutation is likely to lead to numerous complications throughout the organism. Single-gene defects with effects in only one tissue are more common for nonessential features such as hair texture or eye color.

Sickle cell disease is a classic example of pleiotropy. This disease develops in persons carrying two defective alleles for a blood protein, beta-hemoglobin. Mutant beta-hemoglobins are misaligned inside a blood cell and cause misshapen red blood cells at low oxygen concentrations. Deformed blood...