Though multiple causes have been posited, the scientific consensus is that a lack of genetic diversity is the primary reason behind heterochromia, at least in domestic animals.

[3] Though common in some breeds of cats, dogs, cattle and horses due to inbreeding, heterochromia is uncommon in humans, affecting fewer than 200,000 people in the United States, and is not associated with lack of genetic diversity.

The term is derived from Ancient Greek: ἕτερος, héteros "different" and χρῶμα, chrôma "color".

[10] Most cases of heterochromia are hereditary, or caused by genetic factors such as chimerism, and are entirely benign and unconnected to any pathology, however, some are associated with certain diseases and syndromes.

[11][12][13] Acquired heterochromia is usually due to injury, inflammation, the use of certain eyedrops that damage the iris,[19] or tumors, both benign and malignant.

[12] Notable historical figures thought to have heterochromia include the Byzantine emperor Anastasius the First, dubbed dikoros (Greek for 'having two pupils').

[33][34][35] A more recent example is the German poet, playwright, novelist, scientist, statesman, theatre director, and critic, Johann Wolfgang Goethe.

The Roman jurist and writer Cicero also mentions the same feature of 'double pupils' as being found in some Italic women.

[39] The twelfth-century scholar Eustathius, in his commentary on the Iliad, reports a tradition in which the Thracian Thamyris (son of the nymph Argiope), who was famed for his musical abilities, had one eye that was grey, whilst the other was black.

The blue eye occurs within a white spot, where melanin is absent from the skin and hair (see Leucism).

These species include the cat, particularly breeds such as Turkish Van, Khao Manee and (rarely) Japanese Bobtail.

It also occurs in certain breeds that do not carry the merle trait, such as the Siberian Husky, Dalmatian, and rarely, Shih Tzu.