Eye color is a polygenic phenotypic character determined by two distinct 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.
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 appearance of blue and green, as well as hazel eyes, results from the Tyndall scattering of light in the stroma, a phenomenon similar to that which accounts for the blueness of the sky called Rayleigh scattering.Neither blue nor green pigments are ever present in the human iris or ocular fluid.Eye color is thus an instance of structural color and varies depending on the lighting conditions, especially for lighter-colored eyes.
The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines, purines, and carotenoids.Humans and other animals have many phenotypic variations in eye color.
The inheritance of eye color was for generations erroneously understood; blue eye color, for example, was thought to be a simple recessive trait. This understanding is incorrect. The genetics and inheritance of eye color are complicated, eye color is determined by multiple genes, and a child can inherit blue or green eye color from one parent even if the other has no genes for light eye color, and do so at a rate of approximately 2% to 3%.
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Optometry : open access