Heterochromia is an anatomical condition in which the irises in the eyes of a person or animal are two different colors. This condition can also occur in the color of hair or of the skin, but is most common in the eyes. Heterochromia occurs when a person or animal has too much or too little of the compound melanin in the body. Melanin is a compound found in plants and animals that creates pigment.

Heterochromia can occur from disease, injury, genetic mosaicism, or an inherited genetic trait. Different colored eyes can also occur due to a hemorrhage or foreign object in the eye, glaucoma or some medications to treat it, or neurofibromatosis. Even mild inflammation in one eye can cause this condition.

When it affects the eyes, this condition may also be called heterochromia iridis or heterochromia iridum. The eyes can have either complete heterochromia, meaning each eye is a different color, or partial or sectoral heterochromia, meaning that there are two different colors within one iris. Complete heterochromia is the more common variety. The partial or sectoral version usually comes from inherited conditions, such as Waardenburg syndrome and Hirschsprung's disease.

Complete heterochromia is most common in cats, though it can occur in humans. Most frequently, the "odd-eye" cat has one blue eye and one normal eye. It also occurs among dogs, particularly in Siberian husky and Dalmatian breeds, some horses, cattle, water buffalo, and certain ferrets. Partial heterochromia is most common in dogs from specific breeds, including the border collie and Australian shepherd.

Central heterochromia is when the central pupillary part of the iris is a different color than the mid-peripheral or ciliary part, forming a central "ring" around the eye's pupil. This type is most common in irises that contain low levels of melanin. The true color is actually the outer ring, whereas the central ring displays the color affected by the heterochromia.

Medical advice from a health care provider should be sought when any person notices a changes in the color of one eye. He or she will need a complete eye examination and a report of any other symptoms to determine the underlying problem. If an infant has two different colored eyes, a pediatrician should be consulted, who will likely want the child to see an ophthalmologist.