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Heterochromia is a physical abnormality that is characterized by the irises of the eyes are not a uniform color. It comes in two main forms: complete heterochromia, where the irises are completely different colors, and partial heterochromia, where one of the irises has an area that is different from the rest.

The most common type of heterochromia is congenital. Heterochromia is a dominant trait, meaning that it will present itself if the gene is received from the parent. The condition runs in the family. This is the most common form of heterochromia, and is simply due to a single genetic mutation. Even then, less than one percent of the population has any form of heterochromia.

Acquired heterochromia is when the condition is caused by another disease or damage to the eye. Some triggers cause the iris to be darker, called hyperchromic, or be lighter, called hypochromic. Diseases that lead to pigment deposits normally cause Hyperchromic heterochromia. Ocular melanosis and oculodermal melanocytosis both cause melanocytes, cells that produce melanin which pigments the iris, to build up in the eye. This extra pigment causes the iris to darken. It also increases the risk of contracting uveal melanoma. Other abnormal pigment deposits can cause glaucoma.

Developmental disorders can cause a lack of melanin production, causing the iris to be lighter than normal; the iris is normally blue in these cases. Horner’s syndrome is a nerve palsy that causes damage to the upper part of the body. Besides for heterochromia, Horner’s can also present itself with ptosis, abnormal pupil dilation, and red eyes. Waardenburg syndrome is another neurological disorder that causes ocular problems, including heterochromia and displaced orbit or eyelids.

Iron poisoning can cause buildup of the metal in the eye, making the iris appear darker. Excess iron can also buildup from blood due to chronic hemorrhaging. Certain eye drops can inhibit the ability of the iris to open and close. This tightening causes central heterochromia, which is dark discoloration on the perimeter of the pupil. Tumors and cancers of the eye can also cause discoloration, normally darker.

Certain bacterial and viral infections may cause the eyes to appear lighter.

Technically, there is no way to reverse heterochromia, especially in the congenital case. Treating infection, iron poisoning, or any of the other underlying causes could cause the discoloration to disappear. In the early stages of life, eye color can change dynamically. Those born with heterochromia rarely lose it after childhood.

Heterochromia is also found in other animals, most notably in cats and dogs. The Siberian husky is famously the most common animal to have complete heterochromia, normally having one brown and one blue eye. Only a small number of other dog breeds show complete heterochromia, while a larger group has shown the partial counterpart.  Some cat breeds also show complete heterochromia. Horses, cows, sheep, buffalo, and ferrets have all shown different levels of heterochromia.