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Eye color is a polygenic trait and is determined primarily by the amount and type of pigments present in the eye's iris. Humans and animals have many phenotypic variations in eye color. In humans, 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.
Blue eyes came with time, through millennia of environment adaptation to colder climates. Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of light tend to be absorbed by the underlying iris pigment epithelium and shorter wavelengths are reflected and undergo Rayleigh scattering. The type of melanin present is eumelanin. The inheritance pattern followed by blue eyes is thought to be similar to that of a recessive trait.
Blue eyes are relatively common throughout Europe and other areas with populations of European descent, such as Canada, the United States, Australia and some countries of South America, like Argentina, Uruguay or Brazil. Throughout the rest of the world they are relatively rare. Around 8% of the world's population has blue eyes.
Finland and Lithuania have the highest proportions of blue-eyed people; they are also very common in other countries around the Baltic Sea. One survey estimated that nearly 90% of Icelanders have blue or green eyes. A 2002 study found the prevalence of blue eye color among non-Hispanic whites in the United States to be 33.8% for those born between 1936 and 1951 compared to 57.4% for those born between 1899 and 1905.
As melanin production generally increases during the first few years of life, the blue eyes of some babies may darken as they get older. Wikipedia |
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