Heterochromia is a difference in coloration, usually of the iris but also of hair or skin. Heterochromia is determined by the production, delivery, and concentration of melanin (a pigment). It may be inherited, or caused by genetic mosaicism, chimerism, disease, or injury. It occurs in humans and certain breeds of dogs and cats.
Heterochromia of the eye (heterochromia iridum or heterochromia iridis) is of three kinds. In complete heterochromia, one iris is a different color from the other. In segmental heterochromia or sectoral heterochromia, part of one iris is a different color from its remainder and finally in central heterochromia there are spikes of different colors radiating from the pupil.
Though multiple causes have been posited, the scientific consensus is that a lack of genetic diversity is the primary reason behind heterochromia. This is due to a mutation of the genes that determine melanin distribution at the 8-HTP pathway, which usually only become corrupted due to chromosomal homogeneity.
Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin.
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