Simple heterochromia is symptomless and does not require any treatment. Other types may require ocular treatment i.e. Fuch's heterochromia cyclitis.
Although people may have this condition, it is very prevalent in certain types of animals. Cats and dogs, in particular, can be prone to heterochromia, and certain breeds may be more likely to have different colored eyes (within a single specimen) than others. One example of an animal (breed of dog) that very commonly exhibits heterochromia is the old English Sheepdog. These sheepdogs often have one eye much lighter than the other, or one blue eye and one brown eye. Domestic cats are also often seen with different colored eyes, as are horses.
Heterochromia of hair is the presence of more than one distinct color of hair in the same person. A color difference between scalp hair and a mustache or sideburns is not uncommon. Pubic and axillary hair and eyebrows and eyelashes are often darker than scalp hair in a fair-haired person. Rarely, a circumscribed patch of hair of different colors occurs.
Heterochromia iridis is listed as a "rare disease" by the Office of Rare Diseases (ORD) of the National Institutes of Health (NIH). This means that Heterochromia iridis, or a subtype of Heterochromia iridis, affects less than 200,000 people in the US population.
This is usually inherited as an autosomal dominant trait. An autosomal dominant trait is a gene on one of the non sex chromosones that is always expressed even if there is only one copy of it present. A single copy of the mutation is enough to cause a difference in eye colouration.
When the eyes of an individual are different colors, the reason almost certainly has something to do with that person’s melanocytes, the cells that manufacture pigment. What can go wrong with melanocytes? They can get lost. Most of the melanocytes in our bodies are produced in one location in utero and have to journey to their individual destinations. One example is Waardenburg syndrome, where genetic mutation results in wayward melanocytes that never find their way to the iris for which they were intended. In some cases, only some of an eye’s melanocytes get lost, resulting in patches of different colors in the same eye. Heterochromia can also be the result of an individual’s receiving different eye color genes. This can happen if two fertilized eggs become fused in utero.
Iris color develops during the first few months after birth, with the levels of the pigment melanin determining how dark eyes will become. The less melanin expressed in the iris, the lighter a person's eyes look, and vice versa. Sometimes, though, the concentration and distribution of melanin isn't uniform, which leads to a condition known as heterochromia.
This condition can present itself in different ways. There's complete heterochromia, when each eye is a distinctly different color, say, one blue and one brown. Central heterochromia is when the eyes show various colors, such as a blue iris with a golden-brown ring around the pupil. And sectoral heterochromia is when one iris has a splash of color that's different from its overall hue, a trait that actress Kate Bosworth has.
Most cases of heterochromia are hereditary, caused by a disease or syndrome, or due to an injury. Sometimes one eye may change color following certain diseases or injuries. Specific causes of eye color changes include bleeding (hemorrhage), familial heterochromia, foreign object in the eye, glaucoma, or some medications used to treat it, injury, mild inflammation affecting only one eye, neurofibromatosis and Waardenburg syndrome.
Alexander the Great is rumored to have had a hazel eye and a green eye, and according to some stories, may have purposely sought out a horse with unique peepers, too.