Some of the eye-color genes include OCA2 and HERC2. As of 2010, as many as 16 genes have been associated with eye color inheritance. The genetics and inheritance of eye color in humans is complicated. Humans and other animals have many phenotypic variations in eye color. The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines, purines, and carotenoids. This is an example of structural color, which depends on the lighting conditions, especially for lighter-colored eyes. Neither blue nor green pigments are present in the human iris or vitreous humour.
The appearance of blue, green, and hazel eyes results from the Tyndall scattering of light in the stroma, a phenomenon similar to Rayleigh scattering which accounts for the blue sky. In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma. The irises of human eyes exhibit a wide spectrum of coloursĮye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.
For the shade of purple, see Iris (color).
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