The Genetics of Human Eye Color Inheritance

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Eye Colors in Humans

Human eyes are usually classed as being either brown, blue or green/hazel, but there is a huge variety of shades and patterns, ranging in a continuum from very dark brown to light blue.  The differing colors and patterns of human eyes are determined by the amount and distribution of melanin in the iris.  Melanin is a light-absorbing pigment produced by cells called melanocytes found in the top layer of the iris.  Brown eyes contain more melanin than green eyes, while blue eyes have very little melanin. 

Infants of European descent have very little melanin in their eyes at birth, and hence their eyes are usually blue or grey.  It takes around 6 months to 1 year for levels of melanin to build up in the body, so a baby’s eye colour will generally change around this time, however it may take up to three years for a child’s final eye color to develop.  African, Asian and Hispanic babies are often born with brown or black eyes that stay dark. 

The subtle differences in eye color seen between individuals arise because eye color is a complex, polygenic trait.  This means that there are multiple genes that interact to produce the color.  How many genes there are, where they are and what they do exactly is only partially known.

Early Genetic Models of Eye Color Inheritance

Geneticists have been modeling the inheritance of eye color since the late 19^th century, but originally used described inheritance using simple Mendelian dominant/recessive models, where brown eyes are dominant to green, and both are dominant to blue.  However, this model could not explain all patterns of inheritance (like two blue-eyed parents having a brown-eyed child), and it soon became obvious there must be more than one gene involved.

In the 1980s chromosome mapping techniques were developed which enabled researchers to identify particular regions on chromosomes (called loci) that are associated with inheritance of particular traits.  A locus associated with green eye color (named Gey) was mapped to chromosome 19, and a locus associated with brown/blue eye color (named Bey) was mapped to chromosome 15.  These “loci” are not genes as such, but they are regions of the chromosome that contain the genes likely to play a role in eye color.  

Many of the textbook explanations for eye color will tell you that there are 2 variants (or alleles) for each of these loci - a green (dominant) and blue (recessive) allele at Gey, and a brown (dominant) and blue (recessive) allele at Bey.  This is a useful model for demonstrating how inheritance of a polygenic trait works, and in the case of eye color it does explain some common patterns of inheritance.  Most online calculators for eye color are based on this model.  However, this model is still somewhat simplistic and there are clearly additional alleles and additional genes at work. 

Genes Responsible for Eye Color Inheritance

With the availability of human genome sequences, the genes associated with eye color inheritance are now being identified.  The Bey locus has now been identified as a gene called OCA2.  This gene codes for a protein that stimulates the melanin-producing cells in the eye to mature and produce melanin.  OCA2 now looks to be the major gene that determines eye color, with recent research estimating that this gene alone is responsible for about 74% of the variation in human eye color. 

There are far more than just two variant forms of OCA2 – dozens of alleles have now been identified, many differing by only a few changes in their DNA sequence.  A 2006 study found that some of these changes are highly diagnostic for particular eye colors – in fact, eye color can be predicted reasonably well (but not entirely) from a person’s genotype at OCA2.  Additional genes associated with eye color have also been recently discovered, but how these genes interact with OCA2, and exactly how much they contribute to eye color is yet to be determined.

Resources About The Genetics of Human Eye Color Inheritance

Scientific articles about eye color genetics that are free to access:

Sturm RA and Larsson M (2009) Genetics of human iris color and patterns. Pigment Cell and Melanoma Research 22: 544-562.

Duffy DL, Montgomery GW, Chen W, et al (2006).  A three single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation. American Journal of Human Genetics 80: 241-52.

Simple eye color inheritance calculators, based on the two-gene model:

http://www.athro.com/evo/inherit.html

http://museum.thetech.org/ugenetics/eyeCalc/eyecalculator.html

Resource by H.Miller

I am a biologist and freelance science writer based in Wellington, New Zealand. My research interests include evolution, genetics and genomics of New Zealand’s native birds and reptiles. I also write a popular science blog at www.chickenoreggblog.wordpress.com, and in my spare time enjoy biking and hiking.

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