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Eye color: The myth One of the oldest myths in human genetics is that having blue eyes is determined by a single gene, with the allele for blue eyes recessive to the allele for non-blue eyes (green, brown, or hazel). Many people who know nothing else about genetics think that two blue-eyed parents cannot have a brown-eyed child. The reality Eye color as a character The color of the iris is determined by the amount of melanin, the ratio of eumelanin (which is dark brown) to pheomelanin (which is reddish), and the way the melanin is distributed in the eye. Irises with little melanin appear blue due to scattering of light by collagen fibers in the iris. Blue, gray, green and hazel eyes are only common in people of European ancestry; other people's eyes are various shades of brown. Many studies divide eye colors into three categories: blue (or blue and gray); green and hazel; and brown. This has been criticized as an oversimplification (Brues 1975), and eye colors have been divided into nine categories (Mackey et al. 2011) or the hue and saturation values quantified (Liu et al. 2010). Eye color can change dramatically in the first few years of life, as many babies are born with blue eyes but then develop green or brown eyes (Matheny and Dolan 1975), and changes can also occur later in life (Bito et al. 1997, Liu et al. 2010). Some people have a blue or green iris with a brown ring around the pupil (Sturm and Larsson 2009), which makes the classification of eye color even more complicated. Family studies Davenport and Davenport (1907) were the first to suggest that blue eye color was caused by a recessive allele. They claimed that whenever both parents had blue eyes, all of the children have blue eyes, but their data actually included two hazel-eyed offspring of blue-eyed parents. The authors said “we suspect [these] to be of a blue type,“ whatever that means. Hurst (1908) divided eyes into just two types, “simplex“ (blues and some grays, with no pigment on the outer surface of the iris) and “duplex“ (all other colors). He found the following results: Parents Duplex offspring Simplex offspring Duplex x Duplex 240 18 Duplex x Simplex 187 137 Simplex x Simplex 0 101 Because there are no “duplex“ (non-blue-eyed) offspring of two blue-eyed parents, these data fit the model of blue eyes being caused by a recessive allele at one gene. Holmes and Loomis (1909) criticized the earlier work, saying that eye color varies continuously, and dividing it into categories is arbitrary. Out of 52 offspring of two blue-eyed parents in their data, one had brown eyes and two had gray eyes, which does not fit the idea that blue eyes are caused by a recessive allele. Boas (1918) found an even larger number of non-blue-eyed offspring of two blue-eyed parents, 26 out of 223. Surprisingly, there don't seem to have been any parent-offspring studies of eye color since then, at least none that I could find. Molecular genetics A number of groups surveyed associations of single-nucleotide polymorphisms with eye color, with fairly consistent results: variation in the HERC2 and OCA2 genes, which are next to each other on chromosome 15, plays a major role in determining eye color. However, variation in at least 10 other genes, plus complicated interactions between these genes, also influences eye color (reviewed in Sturm and Larsson 2009, with more recent results in Liu et al. 2010 and Pospiech et al. 2011). Conclusion Eye color is not an example of a simple genetic trait, and blue eyes are not determined by a recessive allele at one gene. Instead, eye color is determined by variation at several different genes and the interactions between them, and this makes it possible for two blue-eyed parents to have brown-eyed children. References Bito, L. Z., A. Matheny, K. J. Cruickshanks, D. M. Nondahl, and O. B. Carino. 1997. Eye color changes past early childhood: the Louisville Twin Study. Archives of Ophthalmology 115: 659-663. Boas, H. M. 1918. Inheritance of eye color in man. American Journal of Physical Anthropology 2: 15-20. Brues, A. M. 1975. Rethinking human pigmentation. American Journal of Physical Anthropology 43: 387-391. Davenport, G. C., and C. B. Davenport. 1907. Heredity of eye color in man. Science 26: 589-592. Holmes, S. J., and H. M. Loomis. 1909. The heredity of eye color and hair color in man. Biological Bulletin 18: 50 65. Hurst, C. C. 1908. On the inheritance of eye-colour in man. Proceedings of the Royal Society of London B 80: 85-96. Liu, F., et al. (20 co-authors). 2010. Digital quantification of human eye color highlights genetic association of three new loci. PLOS Genetics 6: e1000934. #NikolaysGeneticsLessons #colour #iris #melanin #dominantAndRecessive #eyeColorPhenotype
