Heterochromia is the somatic characteristic of individuals who present an iris of a different color from the other. The reason is to be attributed to the different amount of melanin in the two eyes: if the pigment is not very concentrated, it will undergo a blue color, while in the opposite case the iris will turn to brown tones.
Heterochromia is rarely found in humans, while it is quite common in animals, such as dogs, cats and horses. Besides being congenital, heterochromia can also be acquired: eyes of different colors can signal traumas, reactions to drugs and eye diseases, as in the case of Fuchs' heterochromic iridocyclitis, Horner's syndrome and pigmentary glaucoma.
Heterochromia should not be confused with somatic mosaicism, that is, in the case in which different tonal variations are present in the same iris.
