When we look at different eye colours, we often find people with brown eyes, blue eyes, amber eyes, or other colours. Though it is very rare, there are also people born with multi-coloured eyes. This condition is called heterochromia. Before investigating this phenomenon, it is important to understand why our iris, the coloured part of the eye, has colour at all.
Melanin is the natural pigmentation found in human hair, skin, as well as the eyes. The amount of melanin in a person’s iris is determined by genetics and correlates to what eye colour they will have. The more pigment there is, the darker the colour of the iris will be. For example, blue eyes tend to have smaller amounts of melanin, as blue itself is a lighter colour. In contrast, brown eyes have more pigment because of its darker colour. The level of melanin a person’s eye contains does not have any significant effect on the quality of their vision, but it does slightly affect how well your iris can block supplemental light. In usual circumstances, a person will have the same amounts of melanin in each eye (same colour). In the case of heterochromia, there are multiple levels of pigment.
There are three main types of heterochromia: complete heterochromia, central heterochromia, and sectoral heterochromia, with complete heterochromia being the most common form.
Complete heterochromia happens when one iris colour is completely different from the other. For example, Australian Shepherds, a breed of dog, often have a blue eye and a brown eye. Meanwhile, less than 1% of the human population has this condition.
Central heterochromia is the most common type of heterochromia among people. Instead of two separate eye colours between both eyes, central heterochromia occurs when there is a different colour surrounding the pupil. This is present in hazel eyes; a gold colour borders the pupil, while the rest of the iris is green coloured.
The final type of heterochromia is sectoral heterochromia. It is very similar to central heterochromia, except no ring is formed around the pupil. An irregular spot of colour takes its place. An example of this would be if a person’s iris was half blue, half brown. It can still be classified as sectoral heterochromia, even if it occurs in only one eye as well.
It is possible that there are many other eye conditions as unique as heterochromia that exist out in the world: some that humans have already discovered, and some we have yet to find. These unique phenomena make us truly see how complex the eye really is and how every eye colour adds to our individuality.