Eye color is a visiblecharacter caused by two factors: thepigmentation of theeye'siris,[1][2] and way light is scattered as it hits the top layer of the iris.[3]
The human eye is different from its nearest relative, thechimpanzee eye. The eyes of chimps and alsogorillas are entirely brown. Humans have the white to their eyes. This difference is thought to be significant. In the apes, individuals do not usually look directly at other members of the family, but all the same they keep a close watch on what is going on with the family group. They tend to look out of the corners of their eyes, as if to keep the glance secret. Perhaps to them a direct look is aggressive.Jane Goodall used to be careful and avoid looking directly at the gorilla families in the bush. She would sit at a distance, appearing to nibble at grass and leaves, while making occasional glances at the gorilla family.
In humans, the pigment of the iris varies from light brown to black. This depends on the amount ofmelanin in the iris.[4] The appearance ofblue,green and hazel eyes result from theRayleigh scattering of light in the iris. A similar process accounts for the blueness of thesky. Neither blue nor green pigments are ever present in the human iris or fluid.[3][5] Eye color is an example of structural color which varies according to the lighting conditions, especially for lighter-colored eyes.
The brightly colored eyes of manybirds,reptiles andinsects is caused by other pigments, such as pteridines,purines, andcarotenoids.[6]
The view that blue eye color is a simplerecessive trait is incorrect.[7] Two main geneloci code for the eye pigment:OCA2 andHERC2. They sit next to each other on chromosome 15.[8] Mutations (SNPs) to these genes account for much of the variation seen in eye color.OCA2 mutations close to the 5′ regulatory region explain most human eye-color variation. Anintron inHERC2 contains the promoter region forOCA2, affecting itsexpression.[9]
There are about 15 othergenes which have some effect on eye color inheritance. This is an effect calledepistasis, and is not unusual.[8]
