Brown, blue, green, and hazel: What is the secret behind eye color?

Eyes come in many shades, ranging from dark to light brown, and from green, hazel, and gray, to blue. But despite the many variations that we perceive, there are actually only two different pigments in our eyes: brown and red.

The colored area at the front of the eye is called the iris. It is around 12 millimeters in diameter and has an opening in the middle, which is called the pupil. The iris is made from connective tissue and a thin muscle that allows it to open and close in response to light.

Our individual eye color is made up of different amounts of pigment and the connective tissue that forms part of the iris.

Pigment makes eyes look dark

The cells in the iris that make pigment are called melanocytes, and they are also responsible for the color of our hair and our skin. Melanocytes can make two different types of pigment: eumelanin, which is brown-black, and pheomelanin, which is red.

Dark eyes have the most pigment, particularly brown-black eumelanin. In contrast, light blue eyes have the least amount of pigment. Light eye color is most prevalent in individuals of European descent. 

However, there is no such thing as blue pigment in our eyes. Instead, an individual's eyes are blue because of the white collagen fibers in the connective tissue in the iris. These fibers scatter light and make the iris look blue. 

Eye colors that fall in-between the extremes of dark brown and light blue have varying amounts of pigment and areas without any pigment. This leads to the unique colors that we see in the form of green, hazel, and gray.

But it's not just the color that makes our eyes unique; the physical topography of the iris also plays a big part.

When we examine our eyes closely, we can see several patterns. The easiest one to spot is the pigmented ring, which is a ring of color surrounding the pupil.

Areas where the collagen fibers are less dense look like troughs or furrows and are called Fuchs' crypts. White spots — or so-called Wolfflin nodules — are due to hotspots of collagen fibers. Nevi, on the other hand, are dark spots that occur as the result of increased pigment production by a cluster of melanocytes.

So, what regulates this incredible array of colors and patterns in our eyes?

Many genes make one eye color

For many years, geneticists believed that a single gene was responsible for deciding an individual's eye color, with brown eyes dominating blue eyes. However, two brown-eyed parents can have blue-eyed children.

While eye color is an inherited trait, today we know that it is much more complex: several genes contribute to the spectrum of colors that we see in the population.

As far as eye color is concerned, the total number of responsible genes currently stands at 11. A group of researchers — led by Manfred Kayser, who is a professor of forensic molecular biology at Erasmus University Medical Centre Rotterdam in the Netherlands — recently analyzed genetic variants in these genes in more than 3,000 people from seven European countries.

When they compared these genetic profiles with a new method of assessing eye color in photographs — which was developed as part of the study — the team could reliably predict eye color in most cases. However, they did comment that "[...] future genome-wide association studies will likely deliver new pigmentation genes and new pigmentation predictive DNA variants."

The genetics of eye pattern is very much still in its infancy, with a handful of the several thousand genes involved in iris development under investigation.

While the search for all the genetic players that are involved in eye color and pattern continues, we can continue to marvel in the fact that two pigments and bundles of collagen can produce such a vast and spectacular array of individual eye colors in our population.