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Table of Contents
Genetics of eye color
Summary
You've probably heard that blue eyes are recessive, so two blue-eyed parents cannot have a brown-eyed child. That's not true! Eye color is actually determined by several different genes, which makes the genetics of eye color much more complicated. Two blue-eyed parents do have non-blue-eyed children fairly often. If you're a teacher, don't use eye color as an example of a simple genetic trait.
Can eye colors be divided into distinct categories?
Different people have different eye colors. This is unusual; wild species of mammals may show variation in eye color due to age, sex, or breeding status, but humans and some of our domesticated animals (cats, dogs, etc.) are among the only species to have major variation in eye color within populations that is unrelated to age, sex, or breeding status (Negro et al. 2017).
Some studies divide eye colors into two categories, light and dark or blue and non-blue. Other studies divide colors into three categories: blue (or blue and gray); green and hazel; and brown. This has been criticized as an oversimplification (Brues 1975), and some researchers have divided eye colors into five categories (Seddon et al. 1990), nine categories (Mackey et al. 2011), or fifteen categories (Matheny and Dolan 1975).
Several researchers have measured eye color in two dimensions (hue and saturation) or three dimensions (luminance, red vs. green, blue vs. yellow) on large samples of people. Their results show variation among individuals that doesn't fall neatly into discrete categories (Liu et al. 2010, Edwards et al. 2016, Norton et al. 2016). In addition, measuring eye color gives results that depend a lot on which technique you use (Wollstein et al. 2017).
An additional complication is that eye color can change dramatically in the first few years of life, as many babies of European ancestry are born with blue eyes but then develop green or brown eyes (Matheny and Dolan 1975), and some people's eyes change color 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
Galton (1886) collected data on a large number of British families. Out of 355 offspring of two light-eyed parents, 21 had dark eyes. Galton did not interpret his results in terms of dominant and recessive alleles (because he didn't know about Mendel's work yet), but the data do not fit the simple model that blue eyes are caused by a recessive allele.
After the rediscovery of Mendel's research in 1900, there was a fierce debate between "Mendelians" (who thought all genetic variation could be explained using Mendel's rules for one-gene, two-allele traits) and "biometricians" (who thought genetic traits were continuous and could be explained by numerical models of blending). Karl Pearson, an ardent biometrician, gave several examples of two blue-eyed parents having brown-eyed offspring, and two brown-eyed parents having blue-eyed offspring, as one of his arguments against Mendel's laws and in favor of Pearson's "law of ancestral heredity" (Pearson 1903). From the very dawn of Mendelian genetics, people recognized that eye color didn't fit the simple Mendelian model.
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×Duplex | 240 | 18 |
| Duplex×Simplex | 187 | 137 |
| Simplex×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 family studies of eye color since then, at least none that I could find.
Genome studies
Forensic geneticists would love to get DNA from a crime scene, analyze the sequence data, then issue an accurate description of the suspect. As a result, there has been a lot of research looking for associations of genes with visible characters, such as eye color. I'm not going to review it all, but the main points are:
- A large number of genes influence eye color (Frudakis et al. 2003, Liu et al. 2009, Liu et al. 2010, Lona-Durazo et al. 2020, Simcoe et al. 2022)
- Variation at these genes interacts in complicated ways, so you can't just add together the effects of different genes, you have to consider particular combinations (Branicki et al. 2009, Pospiech et al. 2011)
- There is a lot of geographic variation in the frequences of alleles that affect eye color, so the alleles associated with blue eyes in European populations are different from the alleles associated with blue eyes in central and east Asian populations (Kidd et al. 2020).
- Sophisticated mathematical models, using information from several genetic markers, are somewhat successful in predicting eye color, but the models still make a lot of mistakes (Walsh et al. 2012, Yun et al. 2014, Dario et al. 2015, Salvoro et al. 2019)
- Models that are developed using European populations are less accurate when used on non-European or mixed European/non-European populations (Dembinski et al. 2014, Edwards et al. 2016, Hohl et al. 2018, Hohl et al. 2022).
This is all fascinating, but it confirms what people have known since 1903, that blue vs. non-blue eye color is not determined by a single gene with two alleles.
Further research
Scientists have known for over 100 years that blue eyes are not caused by a recessive allele at a single gene; the genetics are more complicated, and there are changes in eye color with age. Despite this, blue eyes being caused by a recessive allele is one thing about genetics that "everyone" knows (everyone in a country that has some blue-eyed people, at least), and people incorrectly gossip about the parentage of non-blue eyed children with two blue-eyed parents. It would be interesting to trace the history of this myth in popular culture; when was the "scandal" of a brown-eyed child of two blue-eyed parents first used as a plot point in literature, a play or movie? Did magazine and newspaper articles for the general public use eye color to explain the exciting new science of genetics in the early 1900s? When did this myth first make it into textbooks, and is it still there?
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.
Branicki, W., U. Brudnik, and A. Wojas-Pelc. 2009. Interactions between HERC2, OCA2 and MC1R may influence human pigmentation phenotype. Annals of Human Genetics 73: 160-170.
Brues, A. M. 1975. Rethinking human pigmentation. American Journal of Physical Anthropology 43: 387-391.
Dario, P., H. Mouriño, A.R. Oliveira, L. Lucas, T. Ribeiro, M.J. Porto, J.C. Santos, D. Dias, F.C. Real. 2015. Assessment of IrisPlex-based multiplex for eye and skin color prediction with application to a Portuguese population. International Journal of Legal Medicine 129: 1191-1200.
Davenport, G. C., and C. B. Davenport. 1907. Heredity of eye color in man. Science 26: 589-592.
Dembinski, G.M., and C.J. Picard. 2014. Evaluation of the IrisPlex DNAbased eye color prediction assay in a United States population. Forensic Science International--Genetics 9:111-117.
Edwards, M., D. Cha, S. Krithika, M. Johnson, G. Cook, and E.J. Parra. 2016. Iris pigmentation as a quantitative trait: variation in populations of European, East Asian and South Asian ancestry and association with candidate gene polymorphisms. Pigment Cell and Melanoma Research 20: 141-162.
Frudakis, T., et al. (10 co-authors). 2003. Sequences associated with human iris pigmentation. Genetics 165: 2071-2083.
Galton, F. 1886. Family likeness in eye-colour. Proceedings of the Royal Society of London 40 402-416.
Hohl, D.M., B. Bezus, J. Ratowiecki, and C.I. Catanesi. 2018. Genetic and phenotypic variability of iris color in Buenos Aires population. Genetics and Molecular Biology 41: 50-58.
Hohl, D.M., R. González, G. Meztler, J. Patiño-Rico, C. Dejean, S. Avena, M. Gutiérrez, and C. Catanesi. 2022. Applicability of the IrisPlex system for eye color prediction in an admixed population from Argentina. Annals of Human Genetics 86: 297-327.
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., K. van Duijn, J.R. Vingerling, A. Hofman, A.G. Uitterlinden, A.C.J.W. Janssens, and M. Kayser. 2009. Eye color and the prediction of complex phenotypes from genotypes. Current Biology 19: R192-R193.
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.
Mackey, D. A., C. H. Wilkinson, L. S. Kearns, and A. W. Hewitt. 2011. Classification of iris colour: review and refinement of a classification schema. Clinical and Experimental Ophthalmology 39: 462-471.
Kidd, K.K., et al. (13 co-authors). 2020. The distinctive geographic patterns of common pigmentation variants at the OCA2 gene. Science Reports 10: 15433.
Lona-Durazo, F., R. Thakur, E. Pairo-Castineira, K. Funderburk, T. Zhang, M.A. Kovacs, J. Choi, I.J. Jackson, K.M. Brown, and E.J. Parra. 2022. Investigating the genetic architecture of eye colour in a Canadian cohort. iScience 25: 104485.
Matheny, A. P., and A. B. Dolan. 1975. Changes in eye color during early childhood: sex and genetic differences. Annals of Human Biology 2: 191-196.
Negro, J.J., M.C. Blazquez, and I. Galvan. 2017. Intraspecific eye color variability in birds and mammals: a recent evolutionary event exclusive to humans and domestic animals. Frontiers in Zoology 14: 53.
Norton, H.L., M. Edwards, S. Krithika, M. Johnson, E.A. Werren, and E.J. Parra. 2016. Quantitative assessment of skin, hair, and iris variation in a diverse sample of individuals and associated genetic variation. American Journal of Human Genetics 160: 570-581.
Pearson, K. 1903. The law of ancestral heredity. Biometrika 2: 211-236.
Pośpiech, E., J. Draus-Barini, T. Kupiec, A. Wojas-Pelc, and W. Branicki. 2011. Gene-gene interactions contribute to eye colour variation in humans. Journal of Human Genetics 56: 447-455.
Salvoro, C., C. Faccinetto, L. Zucchelli, M. Portoa, A. Marino, G. Occhi, G. los Campos, and G. Vazzaa. 2019. Performance of four models for eye color prediction in an Italian population sample. Forensic Science International--Genetics 40: 192-200.
Seddon, J.M., C.R. Sahagian, R.J. Glynn, R.D. Sperduto, and E.S. Gragoudas. 1990. Evaluation of an iris color classification system. The Eye Disorders Case-Control Study Group. Investigative Ophthalmology and Visual Science 31: 1592-1598.
Simcoe, M., et al. (46 co-authors). 2021. Genome-wide association study in almost 195,000 individuals identifies 50 previously unidentified genetic loci for eye color. Science Advances 7: eabd1239.
Sturm, R. A., and M. Larsson. 2009. Genetics of human iris colour and patterns. Pigment Cells and Melanoma Research 22: 544-562.
Walsh, S., et al. (14 co-authors). 2012. DNA-based eye colour prediction across Europe with the IrisPlex system. Forensic Science International--Genetics 6: 330-340.
Wollstein, A., et al. (14 co-authors). 2017. Novel quantitative pigmentation phenotyping enhances genetic association, epistasis, and prediction of human eye colour. Scientific Reports 7: 43359.
Yun, L., Y. Gu, H. Rajeevan, K.K. Kidd. 2014. Application of six IrisPlex SNPs and comparison of two eye color prediction systems in diverse Eurasia populations. International Journal of Legal Medicine 128: 447-453.
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This page was last revised October 26, 2026. ©2026 by John H. McDonald. Content is available under the Creative Commons Attribution-Noncommercial license.