FunWhat determines the color of the eyes?

What determines the color of the eyes?

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

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

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

The pigment that makes our eyes look dark

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

Thus, dark eyes (jet-black or almost black) have the most pigment (eumelanin) and, on the contrary, light blue eyes have the least amount of pigment. Light blue eyes are more prevalent in individuals of European descent.

However, there is no blue pigment in our eyes. Why are they blue then? Due to 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 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, hazelnut, and gray.

But it’s not just the color that makes our eyes unique; the physical topography of the iris also plays an important role . When we examine our eyes closely, we can see various patterns. The easiest to detect is the pigmented ring, which is a colored ring that surrounds the pupil.

Areas where collagen fibers are less dense are seen as depressions or grooves and are called Fuchs stromata. White spots, or so-called Wolfflin’s nodules, are caused by conflict points in collagen fibers. And Nevi, on the other hand, are dark spots that occur as a result of increased pigment production by a group of melanocytes.

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

Genes and eyes

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 parents with brown eyes can have blue-eyed children.

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

When it comes to eye color, the total number of responsible genes currently stands at 11. A group of researchers, led by Manfred Kayser, professor of forensic molecular biology at Erasmus University Medical Center Rotterdam in the Netherlands, analyzed recently variants in these genes in more than 3,000 people from seven European countries.

By comparing these genetic profiles with a new method for evaluating eye color in photographs, the scientists were able to reliably predict eye color in most cases . However, they believe that “future genome association studies are likely to find new pigmentation genes and new pigmentation predictive DNA variants.”

Thus, the genetics of the eye pattern is still in its infancy, with some of the several thousand genes involved in the development of the iris under investigation.

As the search for all the genetic players involved in eye color and pattern continues, we can continue to marvel that such simple tools are capable of producing such a wide and spectacular variety of individual eye colors in our population. .

Reference: Novel quantitative pigmentation phenotyping enhances genetic association, epistasis, and prediction of human eye color. Scientific Reports 2017

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