Why Are Eyes Brown?

Why Are Eyes Brown
Is eye color determined by genetics? A person’s eye color results from pigmentation of a structure called the iris, which surrounds the small black hole in the center of the eye (the pupil) and helps control how much light can enter the eye. The color of the iris ranges on a continuum from very light blue to dark brown.

Most of the time eye color is categorized as blue, green/hazel, or brown. Brown is the most frequent eye color worldwide. Eye color is determined by variations in a person’s genes. Most of the genes associated with eye color are involved in the production, transport, or storage of a pigment called melanin.

Eye color is directly related to the amount of melanin in the front layers of the iris. People with brown eyes have a large amount of melanin in the iris, while people with blue eyes have much less of this pigment. A particular region on plays a major role in eye color.

Within this region, there are two genes located very close together: and HERC2,
The protein produced from the OCA2 gene, known as the P protein, is involved in the maturation of melanosomes, which are cellular structures that produce and store melanin.
The P protein therefore plays a crucial role in the amount and quality of melanin that is present in the iris.

Several common variations (polymorphisms) in the OCA2 gene reduce the amount of functional P protein that is produced. Less P protein means that less melanin is present in the iris, leading to blue eyes instead of brown in people with a polymorphism in this gene.

A region of the nearby HERC2 gene known as intron 86 contains a segment of DNA that controls the activity (expression) of the OCA2 gene, turning it on or off as needed. At least one polymorphism in this area of the HERC2 gene has been shown to reduce the expression of OCA2 and decrease P protein production, leading to less melanin in the iris and lighter-colored eyes.

Several other genes play smaller roles in determining eye color. Some of these genes are also involved in skin and hair coloring. Genes with reported roles in eye color include ASIP, IRF4, SLC24A4, SLC24A5,, TPCN2,, and, The effects of these genes likely combine with those of OCA2 and HERC2 to produce a continuum of eye colors in different people.

Researchers used to think that eye color was determined by a single gene and followed a simple inheritance pattern in which brown eyes were dominant to blue eyes.
Under this model, it was believed that parents who both had blue eyes could not have a child with brown eyes.
However, later studies showed that this model was too simplistic.

Although it is uncommon, parents with blue eyes can have children with brown eyes. The inheritance of eye color is more complex than originally suspected because multiple genes are involved. While a child’s eye color can often be predicted by the eye colors of his or her parents and other relatives, genetic variations sometimes produce unexpected results.

Several disorders that affect eye color have been described.
Is characterized by severely reduced pigmentation of the iris, which causes very light-colored eyes and significant problems with vision.
Another condition called affects the pigmentation of the skin and hair in addition to the eyes.
Affected individuals tend to have very light-colored irises, fair skin, and white or light-colored hair.

Both ocular albinism and oculocutaneous albinism result from mutations in genes involved in the production and storage of melanin. Another condition called heterochromia is characterized by different-colored eyes in the same individual. Heterochromia can be caused by genetic changes or by a problem during eye development, or it can be acquired as a result of a disease or injury to the eye. Sturm RA, Duffy DL, Zhao ZZ, Leite FP, Stark MS, Hayward NK, Martin NG, Montgomery GW. A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color. Am J Hum Genet.2008 Feb;82(2):424-31. doi: 10.1016/j.ajhg.2007.11.005.

Epub 2008 Jan 24. PubMed:, Free full-text available from PubMed Central:, Sturm RA, Larsson M. Genetics of human iris colour and patterns. Pigment Cell Melanoma Res.2009 Oct;22(5):544-62. doi: 10.1111/j.1755-148X.2009.00606.x. Epub 2009 Jul 8. Review. PubMed:, White D, Rabago-Smith M. Genotype-phenotype associations and human eye color.

J Hum Genet.2011 Jan;56(1):5-7. doi: 10.1038/jhg.2010.126. Epub 2010 Oct 14. Review. PubMed: : Is eye color determined by genetics?

Why do eyes become brown?

Story highlights – All human eyes are brown at their core, due to the presence of melanin Varying levels of the pigment melanin determine how much light is reflected CNN — When you’re next staring deep into the eyes of your partner, the moment may soon be ruined by the knowledge that, regardless of whether these windows to their soul appear piercingly blue or a shimmering green, the reality is that they are brown.

That’s right. All human eyes are brown. As the owner of a sparkling set of deep brown eyes, I see no disappointment in the knowledge that all human eyes are in fact a wonderful shade of brown, but for anyone feeling misled or confused, a mix of biology and physics should help explain this reality. It all comes down to the presence of the pigment melanin, also found in skin and hair, within your eye’s iris – the colored part that surrounds the pupil.

“Everyone has melanin in the iris of their eye, and the amount that they have determines their eye color,” said Dr. Gary Heiting, a licensed optometrist and senior editor of the eye care website All About Vision, “There’s really only (this) one type of pigment.” Melanin – made up of melanocyte cells – is naturally dark brown in color but has the ability to absorb different amounts of light, depending on how much of it there is.

The more melanin inside the iris, the more light is absorbed, meaning less light is reflected out, leaving the iris appearing brown.
But when someone has blue eyes, they have less melanin in their iris, resulting in less light being absorbed and more light reflecting, or scattering, back out.
When this light is scattered, it reflects at shorter wavelengths along the blue end of the light color spectrum – leaving you seeing blue.

Green and hazel eyes are somewhere in the middle, with differing quantities of melanin resulting in different levels of light absorption and therefore different colors reflecting out. Hazel is considered a mixture of eye colors, according to Heiting. Different light settings can also make some eyes appear to change color depending on where the person is standing. Why Are Eyes Brown “It’s an interaction between the amount of melanin and the architecture of the iris itself,” added Heiting. “It’s a very complex architecture.” This part of the eye is therefore unique to most individuals and can act as something like a fingerprint, due to the existence of various textures and patterns.

Blue eyes have the least amount of pigment of all eye colors. When babies are born, their eyes may sometimes appear blue early on, while their melanin is still forming. Their eye color may then darken as they develop. “As a baby develops, more melanin accumulates in the iris,” said Heiting. Like skin color, one theory behind the evolution of eye color is the migration of our early ancestors toward cooler parts of the world.

While high levels of melanin – in eyes, hair, and skin – help protect people in hotter climates, like Africa, from UV radiation, the need for the protective pigment decreases as people move to locations with less sun. “There was less need for all that melanin,” Heiting said.

Another theory conceived by professor Hans Eiberg at the University of Copenhagen was that a mutation once switched off the ability of someone’s eye to produce melanin.
This would lead to light eyes in the affected individual; their rarity may have made them more attractive and aided their natural selection within the population.

In one study, he analyzed genes for eye color and identified what he believed to be a common mutation causing blue eye color. “It’s believed that’s how blue eyes came about, but it may just be the de-emphasis on the need for all the melanin,” Heiting said.

It’s long been believed that if someone has brown eyes – or what appear to be brown eyes – their chances of having a child with lighter eyes are slim.
Following suit is the theory that two people with blue eyes will automatically have a child with blue eyes due to the gene being recessive, rather than dominant.

But this is also not quite true.

Why do most humans have brown eyes?

Why Are Brown Eyes Most Common? – NewView Eye Center Ever found yourself wondering just why brown eyes are so common? As it turns out, there’s a reason ; in fact, it’s estimated that more than 50% of the world’s population has brown eyes. If you think this number’s big, consider this: all humans used to have brown eyes, until an ancestor with a genetic mutation developed what we now know as blue, green, and hazel eyes. There’s a lot more cool info about brown eyes and why they happen. Consider busting out these fun facts during your next trivia night – your friends will be amazed!

Are brown eyes really blue?

Story highlights – Blue eyes have long been associated with movie star good looks, but why they make hearts throb is open to conjecture An estimated 17% of the world’s population has blue eyes. The color is an illusion created by light refracting in clear eyes Stroma Medical has developed a laser system that agitates the pigmentation in irises to reveal the clear blue eyes underneath While the company says tests have shown the treatment to be safe, critics say that there may be a risk of developing glaucoma CNN — From the piercing blue eyes of Paul Newman to the steely gaze of Daniel Craig, blue eyes have always been a measure of attractiveness.

But exactly why they make hearts throb and catapult actors and models to stardom is a point of conjecture.
Theories range from evolutionary psychologists who say that blue-eyed women in Palaeolithic societies had a better chance of standing out in the crowd, others posit that pupil dilation – a signifier of attraction – is easier to see in lighter eyes.

Either way, just 17% of the world’s population has blue eyes. For the majority of the world’s population – an estimated 80% – those elusive movie star eyes can usually only be obtained with the aid of colored contact lenses. But today there’s a medical procedure that can permanently turn your brown eyes blue.

Pioneered by Stroma Medical, the laser procedure works by eliminating the brown melanin that’s present in the anterior layers of the iris.
The fundamental principle is that under every brown eye is a blue eye,” Dr Gregg Homer told CNN, adding that there is no actual blue pigmentation in the eye.
The only difference between a brown eye and a blue eye is this very thin layer of pigment on the surface.

“If you take that pigment away, then the light can enter the stroma – the little fibers that look like bicycle spokes in a light eye – and when the light scatters it only reflects back the shortest wavelengths and that’s the blue end of the spectrum.” He said the effect is similar to the Rayleigh scattering of sunlight in the sky – the physics that makes our sky appear blue.

The company says it has developed a laser treatment that disrupts the layer of pigment, causing the body to begin removing the tissue naturally. While the procedure takes all of 20 seconds, the blue eyes lurking underneath do not emerge for several weeks. He said that Stroma Medical wanted to develop a procedure that was safer, cheaper and more convenient than any of the alternatives on the market.

While it has yet to get the green light from regulatory bodies in the United States, the company’s medical board has said that preliminary studies show the surgery is safe. So far, just 17 patients in Mexico and 20 in Costa Rica have undergone the treatment.

It’s difficult to work out a way to injure someone with this laser because the energy is so low,” he said. The laser treats only the iris and does not enter the pupil or treat any portion of the inside of the eye where the nerves affecting the vision are located. The company is still in the fundraising stage but hopes to have completed clinical trials within several years.

And the cost of turning your brown eyes blue? Dr Homer says Stroma Medical would charge around $5,000 (£3,120) for the procedure. Industry reaction to the process has been muted. Ophthalmologists who deal with people’s eyes, Homer concedes, have every right to be skeptical for the simple fact they are dealing with one of the most sensitive organs in the human body.

Saj Khan, an ophthalmologist at the London Eye Hospital, told CNN the treatment raised some red flags.
The main concern with any procedure that involves releasing pigment inside the eye is that the pigment can clog up the normal drainage channels which can in turn cause the pressure inside the eye to go up,” he said.

“If that happens significantly enough, for long enough, it’s how patients develop glaucoma.” He said that while Stroma Medical claims that the particles released by the process are too fine to cause glaucoma – and that any complications were likely to be short-term and easily remedied – a risk still remains.

“Theory has some sense to it, but without seeing long-term outcomes and without seeing patients that have been treated in this way I wouldn’t commit myself to it,” Khan said. In the meantime, Homer says there are no shortage of potential customers wanting to have the irreversible procedure. “It’s not a goal of our company to promote blue eyes,” he said.

“From my experience what most people are after is the translucence of the blue eye rather than the color of the blue eye. “The people who seem most vigilant about pursuing this always have a story about being young and in the presence of a sibling or a friend who had light eyes and the friend is being told how beautiful their eyes are and it sticks with them.

What is unique about brown eyes?

Brown Eyes and Optometry. – Now that we have looked at the aesthetics of brown eyes. Let’s examine brown eyes from an Optometry perspective. The pigmentation of brown eyes is due to the presence of melanin. Melanin is a pigment produced by the body in response to exposure to sunlight.

Do brown eyes fade with age?

The Claim: Eye Color Can Change as We Age (Published 2005) Really?

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THE CLAIM – Eye color can change as we age. THE FACTS – It can bend light, bring the world into focus, and next to the human brain may be our most complicated organ. But for many people the most intriguing feature of the human eye is simply its color. Can it really change for no apparent reason? In most people, the answer is no.

Eye color fully matures in infancy and remains the same for life. But in a small percentage of adults, eye color can naturally become either noticeably darker or lighter with age. What determines eye color is the pigment melanin. Eyes that have a lot of it in the connective tissue at the front of the iris, called the stroma, are darker, while those that have less tend to be lighter.

The levels of melanin generally remain the same throughout life, but a few things can change them permanently. The first is a handful of ocular diseases like pigmentary glaucoma. Another is a condition called heterochromia, or multicolored eyes, which affects about 1 percent of the population and is often caused by traumatic injuries.

An example of this can be seen in the rock star David Bowie, who attributes his contrasting eye colors, hazel and light blue, to a blow to the face as a child.
The third cause appears to be genetics.
A study in 1997, for example, looked at thousands of twins and found that 10 percent to 15 percent of the subjects had gradual changes in eye color throughout adolescence and adulthood, which occurred at nearly identical rates in identical twins.

THE BOTTOM LINE – Eyes can change color in some people because of genetics or injury. ANAHAD O’CONNOR Really? [email protected] : The Claim: Eye Color Can Change as We Age (Published 2005)