Eye color
Eye color is a polygenic trait shaped by melanin and light scattering.
Eye color comes down to two things: how much pigment is in the iris, and how light scatters through the iris's cloudy front layer (the stroma). In humans, the iris can range from light brown to nearly black, depending on the amount of melanin in the pigment epithelium at the back of the iris, the melanin in the stroma at the front, and how densely packed the stroma's cells are. Blue, green, and hazel eyes aren't caused by blue or green pigments—those colors don't exist in the human iris or vitreous humour. Instead, they result from Tyndall scattering, a process similar to why the sky looks blue. This makes eye color a type of structural color, meaning it can shift with lighting, especially in lighter eyes.
Many bird species get their bright eye colors from other pigments, like pteridines, purines, and carotenoids. Humans and other animals show a wide range of eye-color variations. Mammals are born with blue eyes, and any darker shades develop after birth.
The genetics behind human eye color are complex. By 2010, researchers had linked up to 16 genes to eye color inheritance, including OCA2 and HERC2. Scientists study tiny DNA changes called single-nucleotide polymorphisms (SNPs) to find these genes. A 2009 study in Rotterdam showed that just six SNPs could predict brown or blue eyes with over 90% accuracy. People of European descent have the most eye-color diversity of any population. Ancient DNA analysis, reported in a 2020 study in *Experimental Dermatology*, suggests the common blue-eye gene likely originated in the Near East and reached Europe about 42,000 years ago, after humans left Africa.
The main genes tied to eye color are OCA2 and HERC2, both on chromosome 15. A variant form of OCA2 causes the pink eyes and pale skin seen in oculocutaneous albinism type II. Different SNPs in OCA2 are also strongly linked to blue and green eyes, as well as freckles, moles, and hair and skin tone. These SNPs may sit in a regulatory region, affecting how much pigment the gene produces. A specific mutation in HERC2, which controls OCA2, partly explains blue eyes. Other genes like SLC24A4 and TYR also play a role. A 2010 study using high-resolution eye photos found three new gene loci, bringing the total to ten genes that explain about half of all eye-color variation. Blue eyes with a brown spot, green eyes, and gray eyes come from a completely different part of the
- field
- Human genetics and pigmentation biology
- known_for
- Polygenic inheritance; structural color via Tyndall scattering; OCA2 and HERC2 genes
- common_color_worldwide
- Brown (approximately 79% of people)
- rarest_color_worldwide
- Green (approximately 2% of people)
- number_of_associated_genes
- As many as 16 (as of 2010)
Lore & Background
Eye color is an inherited trait determined by multiple genes. These genes are sought by studying small changes in the genes themselves and in neighboring genes, called single-nucleotide polymorphisms or SNPs. The total number of genes that contribute to eye color is unknown, but there are some likely candidates. A study in Rotterdam (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs. People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in ancient DNA technology have revealed some of the history of eye color in Europe. Through the analysis of ancient DNA, a 2020 study published in Experimental Dermatology suggested that the common gene for blue eye color likely originated in the Near East and arrived in Europe around 42,000 years ago, after the exodus out of Africa.
Reader's Guide
Eye color serves as a key example of polygenic inheritance and structural color in humans. The genetics and inheritance of eye color in humans is complicated. As of 2010, as many as 16 genes have been associated with human eye color inheritance. Some of the eye-color genes include OCA2 and HERC2. The gene OCA2, when in a variant form, causes the pink eye color and hypopigmentation common in human albinism. Different SNPs within OCA2 are strongly associated with blue and green eyes as well as variations in freckling, mole counts, hair and skin tone. A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are SLC24A4 and TYR. A 2010 study of eye color variation in hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, allowing the explanation of about 50% of eye color variation. Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome. Eye color may be subject to change in infancy, and from adolescence to adulthood. A 1997 study of White Americans found that 17% of children experienced a change of eye color by adulthood. Of those children, 50% developed lighter eyes as they got older. The other 50% developed darker eyes.
Did You Know?
- Neither blue nor green pigments are present in the human iris or vitreous humour; blue and green eyes result from structural color via Tyndall scattering.
- A 1997 study found that 17% of White American children experienced a change in eye color by adulthood, with half developing lighter eyes and half darker.
- The common gene for blue eye color likely originated in the Near East and arrived in Europe around 42,000 years ago, after the exodus out of Africa.
- Around 18% of the US population and 5% of the world population have hazel eyes.
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