Eye colour

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Close up of a human iris
Humaniris.jpg
Brown eye, pupil dilated
Close up image of a mantis' face (Archimantis latistyla) showing its camouflaged eyes

Eye color is a visible character caused by two factors: the pigmentation of the eye's iris,[1][2] and way light is scattered as it hits the top layer of the iris.[3]

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.[4] 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.[3][5] Eye colour is an example of structural colour which varies according to the lighting conditions, especially for lighter-coloured eyes.

The brightly coloured eyes of many birds, reptiles and insects is caused by other pigments, such as pteridines, purines, and carotenoids.[6]

Genetics[change | change source]

The view that blue eye colour is a simple recessive trait is incorrect.[7] Two main gene loci code for the eye pigment: OCA2 and HERC2. They sit next to each other on chromosome 15.[8] 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. [9]

There are about 15 other genes which have some effect on eye colour inheritance. This is an effect called epistasis, and is not unusual.[8]

References[change | change source]

  1. Wielgus A.R. & Sarna T (2005). "Melanin in human irides of different color and age of donors". Pigment Cell Res. 18 (6): 454–64. doi:10.1111/j.1600-0749.2005.00268.x . PMID 16280011 .
  2. 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. doi:10.1006/exer.1998.0518 . PMID 9778410 .
  3. 3.0 3.1 Fox, Denis Llewellyn (1979). Biochromy: natural coloration of living things. University of California Press. p. 9. ISBN 0-520-03699-9 . http://books.google.com/books?id=c2xyxwlm2UkC&pg=PA9.
  4. Huiqiong Wang et al (2005). "Separating reflections in human iris images for illumination estimation". Tenth IEEE International Conference on Computer Vision 2: 1691–1698. doi:10.1109/ICCV.2005.215 . ISBN 0-7695-2334-X . http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.87.418.
  5. Mason, Clyde W. (1924). "Blue eyes". Journal of Physical Chemistry 28 (5): 498–501. doi:10.1021/j150239a007 .
  6. Oliphant LW (1987). "Pteridines and purines as major pigments of the avian iris". Pigment Cell Res. 1 (2): 129–31. doi:10.1111/j.1600-0749.1987.tb00401.x . PMID 3507666 .
  7. No single gene for eye color, researchers prove. Sciencedaily.com 2007. Retrieved on 2011-12-23.
  8. 8.0 8.1 "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. doi:10.1038/jhg.2010.126 . PMID 20944644 .
  9. 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. doi:10.1086/510885 . PMC 1785344 . PMID 17236130 .