The human eye is different from its nearest relative, the chimpanzee eye. The eyes of chimps and also gorillas 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 of melanin in the iris. The appearance of blue, green and hazel eyes result from the Rayleigh scattering of light in the iris. A similar process accounts for the blueness of the sky. Neither blue nor green pigments are ever present in the human iris or fluid. Eye colour is an example of structural colour which varies according to the lighting conditions, especially for lighter-coloured eyes.
The view that blue eye colour is a simple recessive trait is incorrect. Two main gene loci code for the eye pigment: OCA2 and HERC2. They sit next to each other on chromosome 15. Mutations (SNPs) to these genes account for much of the variation seen in eye colour. OCA2 mutations close to the 5′ regulatory region explain most human eye-color variation. An intron in HERC2 contains the promoter region for OCA2, affecting its expression. 
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