The Enigmatic Allure of Green Eyes: Unveiling Fascinating Facts About This Rare Eye Color

You've probably filled in an "eye color" box on a form, or leaned close to a mirror, and wondered whether to call your eyes green, hazel, or something in between. If they are green, you belong to a small group: about 2% of people worldwide, and about 9% of Americans. The odd part is that a green iris contains no green pigment at all. Its color comes from a modest amount of melanin, a yellowish pigment, and the way light scatters inside the eye. Here is how that works, and what it means for you.

There Is No Green Pigment in Your Iris

Eye color depends on the amount, quality, and distribution of melanin in the front layers of the iris. Brown eyes hold a lot of melanin. Blue and green eyes hold much less.

Green adds a twist. According to the American Academy of Ophthalmology, the green you see comes from light scattering through the iris's tissue, called the stroma. This is the same kind of scattering, known as Rayleigh scattering, that colors the sky. In green eyes, the stroma has a low-to-moderate level of melanin and also contains a yellowish pigment called lipochrome. The scattered light and the yellow tint combine, and the eye reads as green.

A popular idea holds that green eyes are "a mix of blue and yellow pigment." That isn't right. The iris has no blue pigment either. Blue and green are both effects of structure and light, not dyes.

It Isn't One Gene, and It Isn't Simply Recessive

Many people learned in school that eye color follows a neat dominant-and-recessive pattern. On this account, green-eyed children inherit two "recessive" copies from their parents. Real eye color is more complicated. MedlinePlus, the U.S. National Library of Medicine's health resource, describes it as a polygenic trait, which means variations in many genes work together to produce it.

The biggest single influence sits on chromosome 15, where two neighboring genes work as a team:

• OCA2 makes the P protein, which melanosomes need in order to mature. Melanosomes are the structures inside cells that produce and store melanin.
• HERC2 contains a stretch of DNA (intron 86) that acts like a switch, turning OCA2 activity up or down.

Several other genes fine-tune the result, including ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1. Because so many genes contribute, eye color runs along a continuum rather than falling into tidy boxes. That is why two brown-eyed parents can sometimes have a green-eyed child. It is also why "green or hazel?" can be a genuinely hard question.

Just How Rare Are Green Eyes?

Worldwide, green is one of the rarest eye colors, at an estimated 2% of the population. Brown dominates, at roughly 70% to 80% of people. Blue and green eyes are concentrated mainly in populations of European descent.

The picture changes a lot depending on where you look. In the United States, a 2014 Harris Poll cited by the American Academy of Ophthalmology found these figures:

• Brown: about 45%
• Blue: about 27%
• Hazel: about 18%
• Green: about 9%

So green eyes are uncommon even in a country with large European-descended populations. Globally, they are rarer still.

When Eyes Seem Different From What They Are

Because eye color depends on structure as well as pigment, eyes can appear to differ for reasons that have nothing to do with being green.

David Bowie is a famous example. Many people believe he had two different eye colors. In fact, his look came from anisocoria, meaning pupils of unequal size. A blow to the eye in his teens left his left pupil permanently dilated, which made that eye look darker. Unequal pupils are not rare: about 20% of people have a harmless, natural difference in pupil size.

True heterochromia is a different thing. It means having irises of two different colors, or patches of different color within one iris. People can be born with it, or it can develop later from conditions such as:

• Horner syndrome
• Waardenburg syndrome
• eye injury or bleeding inside the eye
• some glaucoma medications

If one of your irises changes color as an adult, that is worth showing to an eye doctor.

What Lighter Eyes Mean for Your Health

The low melanin that helps make eyes green also has a practical downside. The Centers for Disease Control and Prevention lists light eye color, such as blue or green, as a risk factor for skin cancer and melanoma, especially together with fair skin that burns easily. The risk applies inside the eye, too. The National Cancer Institute lists light eye color, fair skin, and an inability to tan among the risk factors for intraocular (uveal) melanoma. That is the most common primary eye cancer in adults. It forms in the iris, the ciliary body, or the choroid.

The simple defense is sunglasses. The American Academy of Ophthalmology recommends lenses that block 99% to 100% of UVA and UVB rays, often labeled UV400. It recommends wearing them all year, because UV damage to the cornea, iris, and retina builds up over time.

You may have heard other claims about green eyes: that they protect against some eye diseases, or that green-eyed people feel more pain, need more anesthesia, or are more prone to depression or alcoholism. None of these is established. The studies behind them are small, mixed, or inconclusive. They are best treated as unproven. The same goes for stories that Cleopatra or Van Gogh had green eyes, since no reliable record settles the question.

If You Want Green Eyes: Colored Contacts and the Law

Green eyes are rare, so plenty of people buy colored contact lenses to get the look. In the United States, those lenses are regulated more strictly than many buyers realize.

The U.S. Food and Drug Administration classifies all decorative and colored contact lenses as medical devices. That includes lenses that don't correct vision at all. You need a valid prescription from a licensed eye care professional. Selling them over the counter without checking a prescription is illegal under federal law. The FDA explains the reason: poorly fitted or unprescribed lenses can cause corneal scratches, infections, corneal ulcers, and even permanent blindness.

The Federal Trade Commission's Contact Lens Rule works in your favor. After your fitting, the prescriber must give you a copy of your contact lens prescription right away, at no extra charge. You can then buy lenses from any seller who verifies it.

A Color Made of Light

Green eyes are rare because so many conditions have to line up. Many genes must settle on a modest amount of melanin, the iris must hold a touch of yellow lipochrome, and light must scatter through the stroma in just the right way. Nothing in the eye is actually green. If yours are, enjoy the rarity, and protect them the way any light iris should be protected: with good sunglasses and regular eye checkups.

Sources (10)

• MedlinePlus Genetics, "Is eye color determined by genetics?": medlineplus.gov
• MedlinePlus, "Heterochromia": medlineplus.gov
• American Academy of Ophthalmology: aao.org
• American Academy of Ophthalmology, "The Sun, UV Light and Your Eyes": aao.org
• National Eye Institute: nei.nih.gov
• Centers for Disease Control and Prevention, "Skin Cancer Risk Factors": cdc.gov
• National Cancer Institute, "Intraocular (Uveal) Melanoma Treatment (PDQ)": cancer.gov
• U.S. Food and Drug Administration, "Decorative Contact Lenses for Halloween and More": fda.gov
• U.S. Food and Drug Administration: fda.gov
• Federal Trade Commission, "Contact Lens Rule": ftc.gov