Most of us have two eyes but two eyes of the same color. But for those with heterochromia iridis, the irises don’t follow convention – and with complete heterochromia the color of the eyes are completely different.
In anatomy, heterochromia (ancient Greek: ἕτερος, héteros, different + χρώμα, chróma, color[1]) is a difference in coloration, usually of the iris but also of hair or skin. Heterochromia is a result of the relative excess or lack of melanin (a pigment). It may be inherited, or caused by genetic mosaicism, chimerism, disease, or injury.[2]
What is “Heterochromia Iridis?” We looked it up on Wikipedia:
“Heterochromia of the eye (heterochromia iridis or heterochromia iridum; the common incorrect form “heterochromia iridium” is not correct Latin) is of three kinds. 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 and finally in “central heterochromia” there are spikes of different colours radiating from the pupil.
Though multiple causes have been posited, the scientific consensus is that inbreeding is the primary reason behind heterochromia. This is due to mutation of the genes that determine melanin distribution at the 8-HTP pathway, which usually only become corrupted due to chromosomal homogeneity.[3]
Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin.[4][5] The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic).[3] In humans, usually, an excess of melanin indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia.”
While the rate of congenital heterochromia iridis in humans is approximately six out of every 1,000, and many of those cases are very subtle, it is much more common in animals. It’s not wildly unusual to see this in dogs – and felines with it even have a moniker: odd-eyed cats.