Physical cause[change | change source]
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.
Genetics[change | change source]
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. 
References[change | change source]
- Wielgus A.R. & Sarna T (2005). "Melanin in human irides of different color and age of donors". Pigment Cell Res. 18 (6): 454–64. . .
- Prota G. et al (1998). "Characterization of melanins in human irides and cultured uveal melanocytes from eyes of different colors". Exp. Eye Res. 67 (3): 293–9. . .
- Fox, Denis Llewellyn (1979). Biochromy: natural coloration of living things. University of California Press. p. 9. . http://books.google.com/books?id=c2xyxwlm2UkC&pg=PA9.
- Huiqiong Wang et al (2005). "Separating reflections in human iris images for illumination estimation". Tenth IEEE International Conference on Computer Vision 2: 1691–1698. . . http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.87.418.
- Mason, Clyde W. (1924). "Blue eyes". Journal of Physical Chemistry 28 (5): 498–501. .
- Oliphant LW (1987). "Pteridines and purines as major pigments of the avian iris". Pigment Cell Res. 1 (2): 129–31. . .
- No single gene for eye color, researchers prove. Sciencedaily.com 2007. Retrieved on 2011-12-23.
- "Genotype–phenotype associations and human eye color", Journal of Human Genetics 2011. White, Désirée; Rabago-Smith, Montserrat (2011). "Genotype-phenotype associations and human eye color". Journal of Human Genetics 56 (1): 5–7. . .
- Duffy, David L. et al (2007). "A three-single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation". Am. J. Hum. Genet. 80 (2): 241–52. . . .