Why do some people, like David Bowie, have different-colored eyes?

There are hundreds of reasons why someone might have two different-colored eyes, known as hetero-chromia, says Dr. Richard Hertle, a pediatric opthamologist at the National Eye Institute in Bethesda, Maryland. Two of these are direct injury to the eye or an underlying disease that affects the eyes.

Just as skin cells contain varying levels of pigment that give people different skin color, cells in the iris of the eye do too. The number of pigment-containing cells and the amount of pigment each cell produces—which are inherited traits—determine a person’s eye color. Any injury that damages pigment-producing cells, or the cellular pigment-making machinery, will change the color of the eye.

David Bowie is said to have two different eye colors because he damaged one eye during a fight. Different-colored eyes are also a common manifestation of Waardenburg syndrome—a genetic disease first identified in the 1950s generally characterized by facial abnormalities. In Waardenburg syndrome, the inner folds of the eyelids or the tear duct may be displaced (type I of the disease), there may be congenital deafness (type II of the disease), and often there’s abnormal pigmentation of the iris, the skin, and/or the hair.

Typically, people with the syndrome have one brown eye and one blue, and their hair may gray prematurely or they might have a white forelock. But not all of the gene carriers have these distinctive markings, and the majority of people with two different-colored eyes do not have a medical condition. Sometimes damage to the pigment-containing cells of the eye may be caused by something as simple as an allergic reaction to eyedrops or another eye irritant.