Heterochromia irides is a disorder of the iris characterized by difference of the iris, the most visible part of the eye, in different areas of the same iris. It is uncommon in humans, but quite common in dogs, cats, and horses. Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin pigment within the iris tissues. In the heterochromic eye, the iris is characterized by at least two distinct and solidly colored areas or differently colored patches or spots. In some cases, it may be accompanied by thinning of the iris, or holes in the iris. Combinations of brown, blue and white are common. Most cases of heterochromia are hereditary, caused by a disease or an injury. Heterochromia irides is seen in the Old English Sheepdog, Siberian Husky, American Foxhound, Alaskan Malamute, and Shih Tzu.
In many breeds, a completely blue iris of one eye is acceptable; in other breeds it is rejected for registration. In some breeds, such as Siberian Husky, both irises can be blue. Blue irises in certain breeds, such as Dalmatian, have been associated with deafness and cardiac defects. Heterochromia irides is also associated with microphthalmia, cataracts, retinal dysplasia, optic nerve hypoplasia (underdeveloped optic nerve) and other eye anomalies. In many breeds heterochromia irides is associated with merling gene and coat color. Some breeds in which uveodermatologic syndrome has been reported, for example, Siberian Husky, Old English Sheepdog, Australian Shepherd, and Shetland Sheepdog, often have heterochromia irides.