Heterochromia iridium is the condition of having two irises of different colors.
Many dogs and cats (particularly Siamese cats) have two differently coloured eyes, but it is less common in humans. One cause of heterochromia iridium is trauma to the eye, but it can also be a hereditary trait. Other causes of heterochromia iridium are certain types of diseases and the medication taken to treat them, such as glaucoma.
Singer David Bowie is often described as an example, but in fact has two blue irises, with one pupil permanently dilated due to a childhood injury. By comparison, actress Kate Bosworth was born with one blue eye and one hazel eye, and is thus heterochromatic.
A related condition is heterochromia iridis. In this condition, different parts of a single iris vary in color. This is caused by areas within the iris having increased or decreased melanin production, due to tumors, clump cells, or birthmarks.
In fiction
Differently-coloured irises appear at least as often in fictional characters as in life, and are often employed by an author as being evocative of some particular quality.
The character Yuna from the video games Final Fantasy X and Final Fantasy X-2 is known for being heterochromatic, but her condition is due to her mixed Al Bhed heritage.
The character Montgomery from the 1997 film Nowhere was shown to be heterochromatic, with a blue eye and a green eye.
