A variation in coloration (and distribution of the color) of the eyes. Heterochromia iridum is determined by the production, delivery, and concentration of melanin in the iris. It may be inherited, or caused by genetic mosaicism, chimerism, disease, or injury.
Heterochromia iridum can be complete, sectoral or central. In complete heterochromia, one iris is a different color from the other. In sectoral heterochromia, part of one iris is a different color from its remainder. In central heterochromia, there is a ring around the pupil or possibly spikes of different colors radiating from the pupil.
An increase of melanin production in the eyes indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia. The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic).
In domestic animals, the scientific consensus is that heterochromia results from a lack of genetic diversity: the genes that determine melanin distribution mutate, corrupted by chromosomal homogeneity. Due to inbreeding, heterochromia is common in some breeds of cats, dogs, cattle and horses. However, it is uncommon in humans (fewer than 200,000 people in the United States for example) and not associated with lack of genetic diversity.