JUST EYES

Eye color is a polygenic trait and is determined by the amount and type of pigments in the eye’s iris. Humans and animals have many phenotypic variations in eye color. In human eyes, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris.

The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.

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. In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin. Color variations among different irises are typically attributed to the melanin content within the iris stroma.

The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium. OCA2 gene polymorphism, close to proximal 5′ regulatory region, explains most human eye-color variation.