Heterochromia is a condition best recognized by the different coloring of one's eyes, hair or skin. The most common type is heterochromia iridis or heterochromia iridium, where the irises can either have an entirely different hue from one another (complete heterochromia) or only differently discolored spots (partial heterochromia or sectoral heterochromia).
These differences in color are determined by variations of the melanin pigment within an iris, due to genetic causes (like Horner's syndrome), mosaicism (mutation occurred during the development of the fetus), disease or injury. Heterochromia can also be hereditary. The affected iris can be either hyperpigmented (hyperchromic) or hypopigmented (hypochromic), which is usually related to hyperplasia (overgrowth) or hypoplasia (underdevelopment) of the iris tissues.
The melanocytes (cells of the iris that contain melanin) do not synthesize in the absence of innervations and unilateral congenital or birth injuries of these nerves induce heterochromia. But the presence of this different eye discoloration can be causes by a wide range of conditions such as iris coloboma (gaps in one of the eye structures), which also results in heterochromia, the darker iris being the affected one.
There's also microphthalmia (small eyes), determined by mutation or fetal alcohol syndrome, neoplasms (benign tumors) in the iris that can change the color of just one eye (e.g. Lisch nodules) and melanosis (the abnormal increase of melanocytes). Individuals suffering from albinism, piebaldism (partial albinism), incontinentia pigmenti (hypopigmentation) are all prone to heterochromia. But heterochromia can also be induced by means of external factors, like accidental iron deposits in the eye, iritis (iris infection) and the use of certain eye-drops.
Heterochromia is much more common in animals than in humans, particularly in some breeds of cats (with one copper/orange eye and one blue eye), dogs and horses ("wall-eyed" with one brown and one white/blue eye), as well as in water buffalos. In cats, the most likely to have the disease are the Japanese Bobtails, the Turkish Vans and the Turkish Angoras while in dogs the Siberian Huskies, Australian Shepherds and Border Collies.
Partial or sectoral heterochromia is a lot less frequent than complete heterochromia and it's usually associated with genetic diseases like Hirschsprung's and the Waardenburg syndrome.