What determines your eye colour?

They say the eyes are the window to the soul…they come in all shades, from almost black to dark and light brown, grey, hazelnut, along with varieties of green and blue. However, despite the appeared variation in colours, the eyes only actually contain two possible pigments - brown or red. 

The coloured part of the eye is called the iris. The iris is approximately 12 millimetres in diametre and has an opening in the middle, the pupil. The iris is made up of connective tissue and a thin muscles that allow the pupil to open and close in response to light. Eye colour is determined by the various amounts of pigmentation in the iris, along with the way the iris scatters light that passes through it. It is the pigment or “melanocytes” in the iris that determine how dark the eyes are. The same pigments are also responsible for hair colour and skin. Melanocytes produce two different types of pigments: eumelanin, which is brown-black, and pheomelanin, which is red.

Dark eyes (black or dark brown) have the most pigment (eumelanin) and, on the contrary, light blue eyes have the least amount of pigment. Light blue eyes are more common in those of European descent.

Despite the presence of blue eyes in people across the world, a blue ‘pigment’ does not exist, so how can eyes be blue?

Eyes appear blue because of the white collagen fibres found in the connective tissue in the iris. These fibres scatter the light and make the iris appear blue.

Eye colours that lie on the spectrum between dark brown and light blue have varying amounts of pigment and some areas without any pigment. This pigment placement is what creates varying forms of eye colour from shades of green, hazelnut and grey.

It’s not the simple colour alone that makes our eyes unique; the natural structure or ‘topography’ of the iris also plays an important role. When you look at eyes closely, you will see numerous pattern. The easiest to find is the ‘pigmented ring’, which is a coloured circle that surrounds the pupil.

So what regulates the variety of colours and patterns in the eyes? 

Genes and eye colour 

For many years, geneticists believed that a single gene was responsible for determining a person’s eye colour, with brown eyes dominant over blue eyes. However, two brown-eyed parents can have blue-eyed children, which suggests things aren’t quite so simple. 

While eye colour is an inherited trait, there are actually 11 different genes that contribute to why people have different coloured eyes. A team of researchers led by Manfred Kayser, professor of forensic molecular biology at the Erasmus University Medical Center Rotterdam in the Netherlands, explored the correlation between genes and eye colour. The team analysed identified genes related to eye colour in a study involving more than 3,000 people from seven European countries.

The tests revealed that, for people with blue or brown eyes, testing just six of the 11 genes allowed the scientists to predict a person's eye colour with over 90% accuracy. However, the test was less accurate (around 75%) for people with eyes of an intermediate colour, and as the researchers noted, further investigation of the genetic variations associated with these subtle variations in colour is needed. The scientists said: “future genome-association studies will probably find new pigmentation genes and new variants of predictive pigmentation DNA.”

Currently, there are several thousand genes involved in the development of the iris under investigation, meaning it’s likely that further correlations between genes and eye colour will arise as research continues. 

Reference: ‘Novel quantitative pigmentation phenotyping enhances genetic association, epistasis, and prediction of human eye colour’, Scientific Reports, 2017