Every eye color, from near-black brown to pale blue, comes from the same pigment, melanin, present in different amounts.1PubMed. The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation Blue and green eyes contain no blue or green pigment at all; part of their color is a trick of light. Genes set how much pigment the iris makes, and one small genetic switch does much of the work in separating blue from brown.2PubMed Central. A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color
How Pigment in the Iris Creates Brown, Hazel and Light Eyes
The iris is the colored ring around the pupil, and its front layer holds the pigment cells. People of every eye color have about the same number of these cells.3PubMed. The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation What differs is how many melanin granules each cell carries, and that amount sets the color.4PubMed. The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation
Because the amounts change in small steps from person to person, eye colors grade into one another instead of falling into separate groups. Two people who both say they have hazel eyes can have quite different amounts of pigment, so the word covers a range.
Melanin itself comes in two forms: a dark brown-black one and a reddish-yellow one. Dark irises carry much more of the dark form, while light irises carry about as much of the reddish-yellow form as dark ones.5PubMed. Characterization of melanin in human iridal and choroidal melanocytes from eyes with various colored irides In one comparison, dark brown irises had nearly 15 parts dark pigment to every part reddish pigment, while blue-green had barely more than 1 to 1.6PubMed. Human iridal stroma melanosomes of varying pheomelanin contents possess a common eumelanic outer surface
There is also a back layer to the iris, and it is heavily pigmented in nearly everyone. It stops light from passing through the iris, so the differences you see between people come from the front layer.
Why Blue and Green Eyes Have No Blue Pigment
In animals, humans included, blue is almost always made by structure rather than pigment.7Wiley Online Library (Pigment Cell Research). On the blue coloration of vertebrates Tiny fibers in the iris tissue scatter blue light back out more than other colors.8Wiley Online Library (Pigment Cell Research). On the blue coloration of vertebrates With little pigment there to absorb it, the scattered blue is what reaches your eye. Green irises combine some yellowish pigment with this same blue scattering.9Wikipedia. Eye color Gray eyes are a variation of the same light effect.
The same logic explains why eyes seem to shift color. Because a scattered color depends on the light falling on it, the same blue or green eyes can look different in sunlight, indoor light or shade. Nearby colors, such as a blue shirt or dark eye makeup, change how the eyes read to the person looking at them.
The Genes Behind Eye Color
A gene called OCA2 controls how much pigment iris cells make. Right next to it sits another gene, HERC2, and a stretch of DNA inside HERC2 acts as a dimmer switch for the pigment gene.10PubMed Central. A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color A single-letter change in that switch, a C instead of a T, weakens the pigment gene in iris cells.11PubMed Central. A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color Less pigment gets made, which is thought to be the main cause of blue eyes.12The Tech Interactive. Do we know how the blue eye gene causes blue eyes?
Several more genes, including TYR, IRF4 and SLC24A4, also influence eye color.13PubMed Central. Investigating the genetic architecture of eye colour in a Canadian cohort These genes interact, with HERC2 and SLC24A4 together affecting blue and HERC2 with the pigment gene affecting hazel.14PubMed Central. Gene–gene interactions contribute to eye colour variation in humans Just six DNA markers predict brown and blue eyes correctly over 90% of the time, but in-between colors only about 72%.15Gene Health. How do genes determine eye color?
This is why the school model of one brown gene and one blue gene falls short. It treated brown as dominant and blue as recessive, which gets the broad pattern roughly right but leaves out most of the biology. Many genes contribute, so two blue-eyed parents can occasionally have a brown-eyed child, something the simple model would call impossible.
Brown-eyed parents can each carry the blue-eye version of the switch without showing it, so they can still have a blue-eyed child. Looking at both parents’ eyes and any grandparents’ eyes gives a better sense of the odds, but never a promise.
When Babies’ Eye Color Settles
Babies’ irises make little pigment before birth, so parents typically see changes in the first six months, continuing to about the first birthday.16Popular Science. Why do babies’ eyes change color? Sunlight, genetics, and more. Eyes that will darken often look muddier first.17Popular Science. Why do babies’ eyes change color? Sunlight, genetics, and more. Subtle shifts, like blue toward hazel, can continue for a couple of years.
Many babies with lighter skin are born with blue or gray eyes. Babies born with brown eyes usually stay brown, since pigment gets added over time and is not taken away. Eye color rarely lightens during infancy; it either darkens or stays as it is. Gradual change is a normal part of development and not a sign of a health problem.
Some things do deserve a doctor’s look. Two very different colored eyes from early on should be checked, and so should a very pale iris along with poor vision or jerky eye movements.
Unusual Eye Colors and Adult Changes
In albinism, pigment cells are present but can’t make melanin properly, giving a blue or light brown iris that lets light through.18PubMed Central. Ophthalmological Manifestations of Oculocutaneous and Ocular Albinism: Current Perspectives Two different colored eyes can also run in families as a separate inherited trait.19PubMed Central. Genetic Analyses of a Three Generation Family Segregating Hirschsprung Disease and Iris Heterochromia Damage to the nerves that supply the eye, called Horner syndrome when present from birth, can leave one iris lighter.20PubMed Central. Reversal of Iris Heterochromia in Adult-Onset Acquired Horner Syndrome
Prostaglandin eye drops for high eye pressure can darken the iris by adding pigment.21PubMed Central. Patient considerations in ocular hypertension: role of bimatoprost ophthalmic solution About 12% of people on latanoprost saw a color change, versus about 2% after a year on bimatoprost.22PubMed Central. Patient considerations in ocular hypertension: role of bimatoprost ophthalmic solution Mixed-color eyes like hazel or green change most, while evenly blue or dark brown eyes rarely do.23PubMed Central. Patient considerations in ocular hypertension: role of bimatoprost ophthalmic solution The darkening does not reverse after stopping the drops.24PubMed Central. Patient considerations in ocular hypertension: role of bimatoprost ophthalmic solution Eyelash serum with bimatoprost has rare reports of iris darkening.25PubMed Central. Enhanced Eyelashes: Prescription and Over-the-Counter Options
If you have hazel or green eyes, ask your eye doctor about the chance of color change before you begin, since mixed colors are the most likely to shift. If a drop is used in only one eye, that eye may darken while the other does not, and the eyes can end up looking mismatched.
A new change in the color of one eye that you cannot explain should be checked by an eye doctor. Small spots or freckles on the iris are common, but a spot that grows or changes should be looked at. Note roughly when you first noticed the change, since the timing helps narrow down the cause, and old photos can show when a difference first appeared.
Does Eye Color Affect Eye Health
Iris pigment helps shield the inner eye from UV light.26PubMed. Iris colour and uveal melanoma So light eyes are linked to a higher risk of uveal melanoma, a rare cancer inside the eye.27PubMed Central. Iris color and associated pathological ocular complications: a review of epidemiologic studies In one Dutch comparison, green or hazel eyes had about 3.6 times the odds and blue or gray about 1.4 times, versus brown.28PubMed Central. Iris Colour and the Risk of Developing Uveal Melanoma Darker eyes, on the other hand, are linked to more cataract,29PubMed Central. Iris color and associated pathological ocular complications: a review of epidemiologic studies and no consistent link exists with macular degeneration.30PubMed Central. Iris color and associated pathological ocular complications: a review of epidemiologic studies
Uveal melanoma is rare, so a higher chance for light-eyed people still means a low chance for any one person. Sunglasses that block UV light are a simple step that protects the eyes from the sun.
Regular eye exams can catch changes in the iris or the inside of the eye early. Tell your doctor if eye cancer runs in your family.
Where Blue Eyes Came From and Reading Color From DNA
Blue eyes likely trace to a single mutation in one person in Europe or the Near East more than 14,000 years ago.31PubMed Central. Why humans evolved blue eyes Later, lighter eye, hair and skin pigment spread fast in Europe over the last 5,000 years.32PubMed Central. Direct evidence for positive selection of skin, hair, and eye pigmentation in Europeans during the last 5,000 y Blue-eyed lemurs got their color differently, since other primates lack the human switch change.33PubMed Central. Intraspecific eye color variability in birds and mammals: a recent evolutionary event exclusive to humans and domestic animals
Forensic DNA tests can estimate eye color from tiny traces like blood or saliva.34PubMed. Developmental validation of the HIrisPlex system: DNA-based eye and hair colour prediction for forensic and anthropological usage In a German population, they caught nearly all brown and blue eyes, but they failed to identify in-between colors.35PubMed Central. Evaluation of the Prediction Potential of the HIrisPlex-S System in a North German Population
Consumer DNA eye-color guesses have the same limits. Results are given as probabilities, so a result may say blue is likely instead of saying your eyes are blue. Hazel and green predictions are often wrong, because those colors depend on several genes interacting.