Why/how do some people have eyes of 2 different colours?
Three main elements within the iris contribute to its color: the melanin content of the iris pigment epithelium, the melanin content within the iris stroma, and the cellular density of the iris stroma.
When an individual has different amounts of melanin in each of their irises, their eyes are different colors.
Heterochromia iridium (the scientific name for two different color eyes in the same individual) is relatively rare in humans but common in some animals, such as horses, cats, and certain species of
dogs. A variation on the condition is
heterochromia iridis, in which an individual has a variety of colors within one iris.
Heterochromia iridium is thought to result from an alteration to one of the genes that controls eye color. This can be an inherited trait, although trauma and certain medications may result in increased or decreased pigmentation in one of the irises. Certain medical syndromes, such as
Waardenburg syndrome, may also cause someone to have two different colored eyes.