Eye Colour and Genetics: Why Eyes Are Brown, Blue, Green or Grey

Eye colour is one of the first things people notice about a face, and it has fascinated scientists for more than a century. For a long time, school biology textbooks taught that eye colour was controlled by a single gene, with brown dominant over blue. It is a neat story, but it is wrong. Modern genetics shows that eye colour is shaped by several genes acting together, and that the colours we see depend as much on physics as on pigment.

In this guide we explain where eye colour comes from, why blue eyes contain no blue pigment at all, how eye colour is inherited, why many babies' eyes change during the first year, what heterochromia is, and, importantly, when a change in eye colour may signal a medical problem. If you are curious about how your own colour might be described or passed on, you can also explore our eye colour tool.

What gives the iris its colour

The iris, the coloured ring around the pupil, is made of two main layers. At the back is the iris pigment epithelium, a thin layer that is densely packed with dark brown pigment in almost everyone, whatever their eye colour. Its job is to absorb light and stop it from scattering through the iris. In front of it is the stroma, a loose meshwork of connective tissue fibres, blood vessels and pigment cells called melanocytes. The differences between eye colours come almost entirely from this front layer.

Melanin: the only real pigment

The main pigment in the iris is melanin, the same family of pigments that colours skin and hair. Melanocytes in the stroma produce two forms:

• Eumelanin, a brown-black pigment, which dominates in brown eyes.
• Pheomelanin, a red-yellow pigment, which contributes to warmer tones in hazel, amber and green eyes.

Interestingly, people with blue and brown eyes have roughly similar numbers of melanocytes in the iris. What differs is how much pigment those cells produce and store. A stroma rich in melanin absorbs most light and looks brown; a stroma with very little melanin lets light scatter and looks blue.

Structural colour: why blue eyes are not really blue

If you could extract the material from a blue iris, you would not find a blue pigment. Blue eyes owe their colour to the way light interacts with the structure of the stroma. When light enters a lightly pigmented stroma, the tiny fibres and particles scatter shorter (blue) wavelengths more strongly than longer (red) ones. This scattered blue light comes back out towards the observer, while longer wavelengths pass deeper and are absorbed by the dark pigment layer behind. The effect is similar to the reason the sky looks blue, and it is often described as Tyndall scattering (or Rayleigh-like scattering) of light.

This is why blue eyes can appear to shift between blue, grey and even greenish depending on lighting, clothing and the angle of view. The underlying iris does not change colour; the light conditions and the eye's surroundings change what you perceive.

The main eye colours compared

Truly red or violet eyes are not a normal eye colour. In some people with severe albinism, the iris contains so little pigment, even in its back layer, that light reflected from the blood vessels inside the eye can make the iris look pinkish or reddish under certain lighting. Albinism also affects the development of the fovea and the optic nerve pathways, causing reduced sharpness, light sensitivity and often nystagmus, so it is much more than a cosmetic difference.

How eye colour is inherited

Eye colour is a classic example of a polygenic trait, meaning it is influenced by many genes, each contributing a small or large effect. Large studies have linked a dozen or more genes to eye colour, and the total number with some influence is likely higher.

OCA2 and HERC2: the biggest players

The single most important region lies on chromosome 15 and involves two neighbouring genes:

• OCA2 produces a protein (often called the P protein) that helps melanosomes, the tiny structures inside melanocytes, produce and store melanin. Severe mutations in OCA2 cause a form of albinism (oculocutaneous albinism type 2).
• HERC2 is a neighbouring gene. A specific variation inside HERC2 acts like a switch that controls how strongly OCA2 is turned on in the iris. A particular version of this switch, very common in people of European ancestry, reduces OCA2 activity in the iris and is strongly associated with blue eyes.

Research suggests that this blue-eye variant may have arisen in a single ancestor some thousands of years ago, which is why many blue-eyed people share the same version of this genetic switch. However, it is not the whole story: people with the same HERC2 and OCA2 variants can still have different eye colours because other genes adjust the result.

Other genes involved

Genes such as SLC24A4, SLC45A2, TYR, TYRP1, IRF4 and others influence how much melanin is produced, the type of melanin and how it is distributed. Together they help explain the wide range of shades from light blue to almost black, and the many in-between colours such as green and hazel. Forensic scientists can now predict whether a person has blue or brown eyes from DNA with fairly high accuracy, but in-between colours remain much harder to predict, which shows how complex the genetics are.

What parents can expect

Because eye colour is polygenic, it cannot be predicted with certainty from the parents' eye colours. Some general patterns, however, hold true:

Why babies' eyes change colour

Many babies, especially those of European descent, are born with blue or grey eyes. This happens because melanocytes in the iris have not yet produced much melanin, so the structural blue colour dominates. During the first months, exposure to light and genetic programming stimulate melanin production, and the eyes may gradually turn green, hazel or brown.

• Most of the change happens in the first 6 to 12 months.
• Eye colour often settles by around age one to three years, though subtle changes can continue into early childhood.
• Babies of African, Asian, Middle Eastern or Latin American ancestry are usually born with brown eyes that stay brown, because their melanocytes are already active at birth.
• Eyes generally get darker over infancy, not lighter. A baby's eye that becomes noticeably lighter, or one eye that looks very different from the other, should be checked.

Small changes in adulthood can also occur. Some people notice their eyes look slightly lighter or darker over decades, and a greyish-white ring around the edge of the cornea (arcus senilis) is common in older adults. In younger people, especially under about 40 to 50, such a ring can sometimes be associated with high cholesterol and is worth mentioning to a doctor.

Heterochromia: eyes of different colours

Heterochromia means a difference in colour between the two eyes or within one iris. It is uncommon in humans and usually harmless, but sometimes it points to an underlying condition.

Types of heterochromia

• Complete heterochromia: one iris is a different colour from the other, for example one brown and one blue.
• Sectoral (partial) heterochromia: a segment of one iris is a different colour, like a wedge of brown in a blue eye.
• Central heterochromia: a ring of different colour around the pupil, for instance gold near the pupil and green towards the edge. This is fairly common and often considered a normal variation, especially in hazel eyes.

Causes

Heterochromia present from birth or early infancy (congenital) is most often a harmless quirk of pigment distribution, sometimes running in families. It can occasionally be part of a genetic syndrome such as Waardenburg syndrome, which may also involve hearing loss, or be linked to congenital Horner syndrome, in which the affected eye is lighter and often has a smaller pupil and slightly droopy lid. Any heterochromia in a baby should be examined by a paediatrician or eye doctor.

Heterochromia that appears later in life (acquired) is more likely to have a medical cause, including:

• Inflammation inside the eye, such as some forms of uveitis (for example Fuchs heterochromic iridocyclitis)
• Eye injury or bleeding inside the eye
• Some types of glaucoma, including pigment dispersion
• Certain glaucoma eye drops called prostaglandin analogues, which can permanently darken a light-coloured iris over months
• Iron or copper foreign bodies retained in the eye
• Iris freckles, naevi (moles) or, rarely, iris melanoma
• Damage to the sympathetic nerve supply of the eye (acquired Horner syndrome)

When a change in eye colour is a warning sign

In adults, the colour of the iris should be essentially stable. A new change, especially in one eye only, is not something to ignore. Doctors take note of:

• A new dark spot on the iris that grows, changes shape or distorts the pupil, which can very rarely be an iris melanoma; see our guide to eye tumours.
• One eye becoming lighter or darker than the other over weeks to months.
• Colour change with redness, pain, light sensitivity or blurred vision, which can signal inflammation.
• A white, grey or yellowish glow in the pupil, particularly in a child's photographs. This is different from iris colour but is sometimes noticed at the same time. A white pupil reflex (leukocoria) can be a sign of cataract, retinal problems or, rarely, retinoblastoma, and needs urgent assessment.
• A yellow tinge to the white of the eye, which suggests jaundice and needs medical review.

Most iris freckles and small moles are harmless and stay the same for life. Your optometrist or ophthalmologist can document them, sometimes with photographs, and monitor them during routine eye examinations.

Does eye colour affect vision or eye health?

Eye colour does not determine how sharply you see, but it may have small effects on comfort and on the risk of some conditions:

• Light sensitivity: people with lighter irises often report more glare and discomfort in bright sunlight, because less pigment means more stray light inside the eye.
• UV and uveal melanoma: lighter eye colour is associated with a somewhat higher risk of uveal melanoma, a rare cancer, although the overall risk remains low.
• Macular degeneration: some studies suggest lighter iris colour may be associated with a modestly higher risk of age-related macular degeneration, but the evidence is mixed.
• Cataract: some research has linked darker irises with a slightly higher risk of certain types of cataract, again with mixed findings.

Whatever your eye colour, good-quality sunglasses with full UV protection are a sensible habit, especially for people with light eyes.

Can you change your eye colour?

Coloured contact lenses are the safest way to change how your eyes look temporarily, provided they are properly fitted and prescribed by an eye care professional, even if you have no refractive error. Decorative lenses bought without a fitting are a recognised cause of serious corneal infections.

Permanent cosmetic procedures, such as artificial iris implants inserted through surgery or laser techniques claimed to remove pigment, carry significant risks, including glaucoma, cataract, corneal damage, chronic inflammation and permanent vision loss. Major ophthalmology organisations, including the American Academy of Ophthalmology, have warned against cosmetic iris implants. No diet, eye exercise, honey drops or supplement can change your natural eye colour.

When to see a doctor

A stable eye colour, even if the two eyes differ slightly, is usually nothing to worry about. Still, any change in the appearance of the eye deserves a professional look.