Why Are Eyes Blue?
- Pieter Maas
Some eyes change color in different lighting — here’s why – Since blue eyes get their color from the light that’s coming in and being reflected back out, they really can appear as different colors depending on the lighting conditions. Green and hazel eyes are a mixture of pigment color and color from scattered light, so they can also look different in different lighting conditions.
Who was the first blue eyed person?
Behind Blue Eyes: A Look at the Genetic and Cultural Components that Propelled the Spread of Blue-Eyed Humans
- This thoroughly-researched piece is by Sarah Henry, an instructor at Delaware County Community College and tour guide at the Mütter Museum.-KI
I have blue eyes and I have always been interested in exploring my own genetic origins, but I’m not the only one interested in this genetic trait. Countless songs and poems reference people with blue eyes, whether considered a mark of beauty, a representation of sadness, or, in certain cultures, a sign of the oppressor.
But blue eyes, so popular in art, are relatively new in human evolution, as new as the invention of writing itself. My interest in this subject was sparked, in part, by a unique archaeological discovery; in 2006, researchers uncovered the world’s oldest confirmed blue-eyed person, dating to approximately 7,000 years ago.
This discovery helped to confirm theories regarding the familial relationship of nearly all blue-eyed individuals. This article will exam the genetic origins of blue eyes in humans, the spread of the blue-eyed gene, and the future of this genetic trait.
- Genetics: How Do They Work? The basic explanation of eye-color works like this: a person needs only one dominant brown-eyed gene (from one parent) to be brown-eyed but needs to have two recessive blue-eyed genes (one from each parent) to be blue-eyed.
- You have probably seen this explanation accompanied by a simple Punnet square (Image 1) in your science textbooks.
However, new studies illustrate that the genetics behind eye color are not so straightforward. There are actually two separate genes that control eye color in humans. In his article, “Blue Eye Color in Humans,” Hans Eiberg writes, “Blue/Brown eye-color are known to the public as a school example of inheritance of monogenetic inheritance, however, the variation in pigment concentration and the iris suggest the eye color genetics to be far more complex as supported by recent data.” In other words, eye color is controlled not by one gene passed from parent to offspring, but by two genes working in tandem; a more complex chart would take both of these genes into consideration (Image 2).
These genes are called OCA2 and HERC2 (represented as O, o, H and h in Image 2). The simplified explanation is that the OCA2 gene controls pigment in the stroma (the tissue and blood vessels) of the iris (the colored part of the eye around the pupil) and the HERC2 gene is needed to help turn on the OCA2 gene to cause it to produce this pigment, resulting in brown eyes.
- If a person has a non-functioning OCA2 gene, they will always have blue eyes, because the HERC2 gene can’t make the broken OCA2 gene work.
- Likewise, if a person has a HERC2 gene which doesn’t work, the OCA2 gene will “underachieve,” failing to produce enough pigment to make brown eyes, resulting in blue eyes.
These two genes aren’t directly related to each other, yet they affect each other. In this dependent relationship, both of these genes must work to give an individual brown eyes, a genetic relationship known as “epistasis.” Because of this process, it is actually possible (although rare) for two blue-eyed parents to have a brown-eyed child.
- How Do We Know All Blue-eyed People Are Related?
Homo sapiens (modern humans) emerged around 200,000 years ago in Africa, but the mutation that causes blue eyes did not appear until sometime around 10,000 years ago. In a study conducted by Professor Hans Eiberg and a team from the University of Copenhagen, researchers examined mitochondrial DNA from 155 blue-eyed subjects from Denmark, two from Jordan, five from Turkey, and 45 brown-eyed candidates, looking at the locus (specific location or position of a gene) responsible for brown or blue eyes.
- The result was the discovery that more than 97% of blue-eyed people share the single H-1 haplotype (a group of genes within an organism that was inherited together from a single parent).
- Eiberg and his team write, “A shared haplotype among blue-eyed individuals is almost perfect and suggests the blue color phenotype is caused by a founder mutation.” This means that the vast majority of people with blue eyes share a single inherited genetic mutation, rather than each person with blue eyes possessing a unique mutation.
The study also tested seven blue-eyed Mediterranean individuals unrelated to the Danish participants as a control group. They, too, carried the H-1 haplotype. These individuals with the H-1 haplotype all inherited the same switch at the same location in their genetic coding, whereas, brown-eyed individuals have a number of variations in melanin production and DNA, with brown-eyed phenotypes being spread out between haplotypes H-5 and H-10.
In short, almost all blue-eyed people came from a single ancestor, which is proven by the possession of the exact mutation at the same location in their genetic coding. That leads us back to the blue-eyed man from the article that sparked this entire investigation. Why is This Stone Age Body in Spain so Important? In 2006, researchers discovered a 7,000 year old body from the Stone Age in the La Brana cave system in Leon in Northern Spain (Image 4).
Genetic testing determined that this man had blue eyes. It was not in itself unusual, but what is remarkable is that he is the earliest known person with blue eyes. Far from being a fair-haired, far-skinned man that we may have expected, his genetics reveal he’s a mixture of other traits. In order to answer this question, we need to delve into Stone Age migratory patterns. According to Pickrell and Reich, there are two theories of cultural migration: Demographic Stasis vs. Demographic Change. In Demographic Stasis, inhabitants living in a particular region are the descendants of the first people to arrive in that region, meaning the people in a certain area were never integrated into or replaced by people from a second migration. Specifically, we can see this during the Neolithic (New Stone Age) Revolution, a period of time where humans began to cultivate crops, domesticate animals, and use polished stone tools. Prior to the Neolithic Revolution, almost all the world’s inhabitants subsisted primarily by hunting and gathering, but after the Neolithic Revolution, small pockets of farming emerged, first in the Fertile Crescent, China, and India and then spreading across Eurasia.
- The Neolithic Revolution occurred between 6,000-10,000 years ago, and because people were better able to procure a steady source of food, the population increased significantly.
- The technologies which emerged during this time allow archaeologists and researchers to track cultural migration from the northwestern part of the Black Sea region (where the first humans with blue eyes lived) into the rest of Europe.
A study of Armenian haplotypes determined, “.hospitable climatic conditions and the key geographic position of the Armenian Highland suggest that it may have served as a conduit for several waves of expansion of the first agriculturalists from the Near East to Europe and the North Caucasus.” People migrated out of the Caucuses (modern-day Georgia, Azerbaijan, and Armenia) and into other parts of Europe (Image 6). Another study, focusing specifically on the genetics of residents of the Iberian Peninsula (excluding the Basques), indicates a mixture of genetic traits from the Caucuses, Central Asia and North Africa, probably related to migration during the Neolithic Era.
A study of eight Bronze Age individuals dated to between 5,500 and 3,500 years ago shows an admixture between existing hunter-gatherer groups and people from later migrations, meaning people who migrated to this area began to blend into the peoples that already lived there by blending both their culture and genetics.
Why Did this Recessive Genetic Trait Survive for Thousands of Years? How did the blue-eyed gene persist if there’s no overt evolutionary advantage to possessing it? One argument would be that those original groups of people who possessed blue eyes produced offspring with other blue-eyed people in their own group, leading to a population where blue eyes were the norm.
- However, there are both objective and subjective benefits to possessing blue eyes.
- Subjectively, possessing blue eyes may just make one individual more sexually attractive to another.
- Objectively, blue eyes filter light differently than dark eyes (dark eyes, like dark skin, possess more pigment which can protect those organs from sun damage), which make them especially advantageous in the low light of Northern European winters.
Because people with light eyes are more sensitive to light, they can see better in areas that lack sufficient sunlight for large portions of the year. Conversely, while light sensitivity (photophobia) proved useful in a world prior to electricity, it actually opens blue-eyed people up to a host of medical problems including an increased risk of macular degeneration, which can ultimately lead to blindness because light eyes are worse at filtering out harmful UV light.
- What is the future of blue eyes? At the turn of the 20 th century, 50 percent of people living in the United States had blue eyes.
- Now, however, people are more likely than ever to marry outside of their ethnic group, leading to more genetically diverse offspring and a decline in blue eyes due to the dominance of the brown-eyed genes.
Currently, in the U.S., only 17 percent of the population (1 in 6) has blue eyes and only between 5-8% of people worldwide possess the trait. (Green eyes are even more rare, but they are a topic for another article.) Even though they are new in human history, blue eyes are already on the decline.
Whether used to convey beauty, as one writer notes about the poetry of Longfellow and Romanticism, “It delighted in sentimental musings amid the ruins, in pathetic legend, in dreamy pictures of monks and harpers and knights and radiant maidens with soft blue eyes” or to convey sadness like in the Who song “Behind Blue Eyes,” where Roger Daltry sings, “No one knows what it’s like/ To be the bad man/ To be the sad man/ Behind blue eyes,” or as Kristina Richardson writes in her article regarding the perception of blue eyes in the Islamic Middle Ages, “My preliminary archival work suggests the Medieval Muslim male writers overwhelmingly accepted the characterization of blue and green eyes as unattractive and deviant,” a line of thinking fueled by the brutality of European crusaders who raped, pillaged and murdered in an attempt to reclaim the Holy Land.
Blue eyes have been a notable trait in literature across cultures for centuries. Though the future of blue eyes is unclear, nearly all living and dead blue-eyed individuals share a familial relationship through a single genetic mutation. If you have blue eyes or know someone with blue eyes, they are more than likely related to that 7,000 year old man whose remains that researchers found in a remote cave in Spain.
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- Different waves and directions of Neolithic migrations in the Armenian Highland.
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- Eye Color.” TheTech.
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- How Blue Eyed Parents Can Have Brown Eyed Children: Two Different Ways to Get Blue Eyes.” TheTech.F.L.
Patty. Sidelights on American Literature. “The Shadow of Longfellow.” Century Company, 1922: p.237.H. Eiberg, J. Troelsen, M. Nielsen, A. Mikkelsen, J. Mengel-From, K.W. Kjaer, L. Hansen. Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression.
- Human Genetics 123 (2008): 177-187.I.
- Lazaridis, et all.
- Ancient human genomes suggest three ancestral populations for present-day European.
- Nature 513 (18 September 2014): 409-413.K.
- Blue eyes in Islamic Middle Ages.” Medievalists.
- Medievalists, 16 February 2014.J. Bryner.
- One Common Ancestor Behind Blue Eyes.” LiveScience.
LiveScience, 31 January 2008.J.K. Pickrell and D. Reich. Toward a new history and geography of human genes informed by ancient DNA. Trends in Genetics Vol.30, No.9 (Sept 2014): 377-389.J. Mengel-From, C. Borsting, J.J. Sanchez, Hans Eiberg, Neils Morling. Human eye colours and HERC2, OCA2 and MATP.
Forensic Science International: Genetics 4 (2010): 323-328.S. Connor. “Revealed: First Ol’ Blue Eyes is 7,000 years old and was a caveman living in Spain.” IndependentUK. IndependentUK, 26 January 2014.T. Günther, C. Valdiosera, H. Malmström, I Urena, et all. Ancient genomes link early farmers from Atapuerca in Spain to modern-day Basques.
PNAS 112 (2015): 11917-11922 University of Copenhagen. “Blue-eyed humans have a single, common ancestor.” ScienceDaily. ScienceDaily, 31 January 2008. : Behind Blue Eyes: A Look at the Genetic and Cultural Components that Propelled the Spread of Blue-Eyed Humans
Can 2 blue-eyed parent have a brown eyed child?
Can two parents with blue eyes have a child with brown eyes? Yes, blue-eyed parents can definitely have a child with brown eyes. Or green or hazel eyes for that matter. If you stayed awake during high school biology, you might find this answer surprising.
We were all taught that parents with blue eyes have kids with blue eyes. Every time. This has to do with the fact that blue eyes are supposed to be recessive to brown eyes. This means that if a parent has a brown eye gene, then that parent will have brown eyes. Which makes it impossible for two blue-eyed parents to have a brown-eyed child – they don’t have a brown eye gene to pass on! In fact, this is the model we used for our eye color calculator.* And that we talk about extensively here at Ask a Geneticist.
Blue-eyed parents can have kids with brown eyes. (Image via Shutterstock) Now we aren’t being dishonest or trying to hide anything by presenting this model. It works great most of the time. But as with anything genetic, there are always exceptions. For example, DNA can and does change between generations.
So if a change happened that turned a blue eye color gene into a brown one, then blue-eyed parents could have a brown-eyed child. As you might guess, this sort of thing is pretty rare. Too rare to explain all the exceptions we see with eye color. So something else must be going on. That something is most likely other genes involved in eye color that we don’t know about.
Eye color used to be presented as a fairly simple trait. A big part of the model was the idea that we had an eye color gene that came in two varieties – brown and blue. Geneticists represented the brown version as “B” and the blue version as “b”. The model also said that blue (b) was recessive to brown (B).
- This matters because it is an explanation for how brown-eyed parents can have a blue-eyed child.
- See, we have two copies of each of our genes – one from each biological parent.
- This means there are three possible combinations for this eye color gene: BB, Bb, and bb.
- BB is of course brown and in this model, bb would be blue.
Since blue is recessive to brown, Bb people have brown eyes. But they can pass a “b” down to their kids, who might end up with blue eyes. Now eye color is obviously more complicated than this. This model doesn’t explain green eyes for example. Scientists added a second gene to try to explain green eyes but we don’t need to go into that here ( to learn more about the two-gene model).
|Genes||What it Means|
|B b||Brown eyes|
|bb||Not brown eyes|
Again, bb people should not be able to pass on brown eyes to their kids. But we know they can. Which means that this model is incomplete (or wrong). The results I just put into the previous table are theoretical and based on the model I talked about. Here are some actual results I adapted from ‘s website:
|Genes||What it Means in Europeans|
As you can see, the original model holds up pretty well for BB and bb people. Most BB people have brown eyes and most bb people don’t. But the model clearly doesn’t explain the following:
- 1% of bb people have brown eyes
- 1% of BB people have blue eyes (and 14% have green)
- 44% of Bb people do not have brown eyes
The biggest disconnect is with Bb people. Only 56% have brown eyes. If this holds up, I am not sure we can even call blue and green recessive to brown. Whatever the reason, these data give some clues about how two blue-eyed parents might have a brown-eyed child.
For example, imagine two parents are Bb and have blue eyes. They each pass a B down to one of their children. That child will be BB and most likely have brown eyes. This example uses known data to show how blue-eyed parents might have a child with brown eyes. But it doesn’t explain why a Bb person has blue eyes in the first place.
To do this, we need to guess what other genes may be doing. And how they might be affecting the original eye color gene. Going into detail about these possibilities would need more space than I have here! And in the end, the truth is that eye color is a complex trait that we don’t fully understand yet.
Where do most blue-eyed people live?
Eye Color by Country: Blue – Those with blue eyes make up approximately 10% of all eye colors, clearly placing them in a less dominant position. It is also that rarity that has always made blue eyes something of an attraction in the Western culture. The most prevalent place to find people with blue eyes (and where those percentages are considerably higher) is in the Scandinavian region.
Do blue eyes hurt more in the sun?
Why are Blue Eyes More Sensitive to Light? – Lighter colored eyes like blue, hazel and green have less of a pigment called ‘melanin’ than brown eyes do. Melanin helps protect the retina from UV damage and blue light, putting those with blue eyes at a higher risk of developing UV-related eye damage.
Is blue eye color the rarest?
Most Common and Rarest Eye Colors – The conventional eye colors have generally been thought of as:
BrownBlueHazel (sometimes grouped with amber)Green
Of those four, green is the rarest. It shows up in about 9% of Americans but only 2% of the world’s population. Hazel/amber is the next rarest of these. Blue is the second most common and brown tops the list with 45% of the U.S. population and possibly almost 80% worldwide.