Eye color
Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris[1][2] and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.[3]: 9
"Iris color" redirects here. For the shade of purple, see Iris (color).
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.[4] 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.[5] Neither blue nor green pigments are present in the human iris or vitreous humour.[3][6] This is an example of structural color, which depends 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.[7] Humans and other animals have many phenotypic variations in eye color.[8]
The genetics and inheritance of eye color in humans is complicated. As of 2010, as many as 16 genes have been associated with eye color inheritance. Some of the eye-color genes include OCA2 and HERC2.[9] The earlier belief that blue eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur.[10][11] However, OCA2 gene polymorphism, close to proximal 5' regulatory region, explains most human eye-color variation.[12] Sex also appears to play a significant role in eye color, as several studies have shown that women tend to have darker eyes than men, even when they have similar SNP profiles.[13]
Changes in eye color
A 1997 study of White Americans found that eye color may be subject to change in infancy, and from adolescence to adulthood.[24]
17% of children experienced a change of eye color from early childhood to adulthood, and of those children, 50% of developed lighter eyes as they got older. The other 50% developed darker eyes.[25]
Generally, children with hazel and light brown eyes tended to experience a lightening of their eye color by adulthood.[25] Children with green eyes often experienced a darkening of their eye color.[25]
It was also found that 11% of the children's mothers experienced an eye color change during the same period, with most developing lighter eyes, relative to their original color at the time of their child's birth.[26]
Impact on vision
Although people with lighter eye color are generally more sensitive to light because they have less pigment in the iris to protect them from sunlight, there is little to no evidence that eye color has a direct impact on vision qualities such as visual acuity.[92] However, there is a study that found that dark-eyed people perform better at "reactive-type tasks", which suggests they may have better reaction times.[93] People with light-colored eyes, however, performed better at so-called "self-paced tasks", which include activities like hitting a golf ball or throwing baseballs.[93] In another study, people with darker eyes performed better at hitting racquetballs.[94] There are also other studies that challenge these findings.[95] According to scientists, more study is needed to verify these results.[92]