What Determines Eye Colour? Why Blue Eyes Have No Blue Pigment

Brown, hazel, green, grey, blue: human eyes come in a remarkable range of colours. You might expect each colour to come from its own pigment, a blue dye for blue eyes and a green one for green eyes. In fact, almost all of the variety comes from just one pigment, and how much of it there is.

One pigment, many colours

The coloured ring of the eye, the iris, has several layers. The back layer is packed with dark pigment in almost everyone. What makes the difference is the front layer, called the stroma, and how much melanin it contains. Melanin is the same brown-black pigment that colours skin and hair.

• Brown eyes have a lot of melanin in the stroma, which absorbs most light.
• Hazel and green eyes have less melanin, plus a yellowish pigment and some light scattering.
• Blue eyes have very little melanin in the stroma.

Why blue eyes are blue

If blue eyes have almost no pigment in front, why aren’t they colourless? Because the tissue of the stroma scatters light. Short, blue wavelengths are scattered more strongly than long, red ones, so blue light bounces back out of the eye while other colours pass through and are absorbed by the dark layer behind. It is a similar effect to the one that makes the sky blue. There is no blue pigment in a blue eye.

That is also why eye colour can seem to change with lighting or clothing: different light conditions change how much is scattered back.

The genes behind it

Many people learn at school that brown is “dominant” and blue is “recessive”, controlled by a single gene. The reality is more complicated. More than ten genes influence eye colour. The biggest effect comes from a gene called OCA2, which helps make melanin in the iris, and a nearby region of another gene, HERC2, which acts as a switch that turns OCA2 up or down.

In 2008 a team led by Hans Eiberg in Copenhagen found that almost all blue-eyed people they studied shared the same single-letter change in this HERC2 switch, which dials down melanin production in the iris. That suggests blue eyes trace back to a single ancestor who lived perhaps 6,000 to 10,000 years ago.

Brown is the world’s eye colour

Worldwide, brown is by far the most common eye colour, found in most people in Africa, Asia and the Americas. Blue eyes are most common in northern Europe, especially around the Baltic Sea. Green eyes are one of the rarest colours globally.

Why babies’ eyes change

Many light-skinned babies are born with blue or grey eyes because their irises contain little melanin at birth. Over the first months and years, melanin builds up, and the eyes may turn green, hazel or brown. Babies with darker skin are usually born with brown eyes that stay brown.

Two colours at once

Some people have heterochromia: two differently coloured eyes, or patches of different colours in one iris. It is usually harmless and present from birth, but a change in eye colour in adulthood should be checked by a doctor.

In short

• Eye colour depends mainly on how much melanin is in the front of the iris.
• Brown eyes have lots of melanin; blue eyes have very little.
• Blue eyes have no blue pigment; the iris scatters blue light, like the sky.
• More than ten genes are involved, especially OCA2 and HERC2.
• Most blue-eyed people may descend from one ancestor who lived 6,000–10,000 years ago.
• Brown is the most common eye colour worldwide.