Why do we have black, brown, blue or even violet eyes: the science behind our gaze

Why do we have black, brown, blue or even violet eyes: the science behind our gaze

Despite the fascination with the different colors of human eyes, the underlying explanation is actually quite simple. Eye color is due to two types of pigment: eumelanin (brown-black) and phenomelanin (red).

Thus, in dark eyes there is a lot of eumelanin and in light ones, little. That is, if the eyes look blue it is because of the white collagen fibers in the connective tissue of the iris, which scatter the light and make the iris look blue. In turn, the different shades of brown, blue and green are determined by the thickness and density of the iris and the degree of accumulation of white collagen fibers.

Heredity and genes

The concept of "gene", which was also unknown to Charles Darwin, was developed by a Moravian monk named Gregor Mendel , who at that time (1856) began a series of experiments in the garden of the Augustinian monastery of St. Thomas from Brünn, currently in Brno, Czech Republic. His title as the founding father of modern genetics is no exaggeration when we consider that, as a result of those experiments, Mendel was able to conceive more than 29,000 species of peas.


To achieve this, he crossed, for example, pea species that always produced rounded seeds with species that produced wrinkled seeds, or long-stemmed plants with short-stemmed plants, and so on with many other characteristics.

What Mendel discovered is that, contrary to what Darwin believed, the characteristics of the descendant peas were not a mixture of the characteristics of the two original peas, but only one of the traits appeared, which predominated over the others .

He also discovered that some characteristics that did not appear in one generation could appear in the next. Thus, Mendel concluded that there were both dominant and recessive mating "factors" (what we now know as dominant genes and recessive genes).

But now we know that things are a lot more complicated. Let’s go back to the eyes: a couple in which both have blue eyes, for example, will not necessarily have children with blue eyes, because there is more than one gene dedicated to regulating ocular chromaticism.

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75 percent depend on a gene called OCA2 (oculocutaneous albinism type 2), responsible for controlling the amount of pigment made in the iris, as well as the expression of this gene depends on changes in a single letter of DNA in three different regions , and there are other specific regions for the color green and … well, all this enormous complexity results in an eye with different textures and shades that we simply call "blue eyes" or "brown eyes" as if to an impressionist painting define it as a bunch of spots.

The rarity of blue eyes

All blue-eyed people on the planet are descendants of a single European who lived about 6,000 to 10,000 years ago, and who was the first to develop a specific mutation that accounts for the now-widespread iris coloration. Originally, all humans had brown eyes, although a genetic variation in a gene called OCA2, which changed the amount of pigment found in different individuals, resulted in different shades of brown .

However, the color blue responds to another gene, HERC2. The alteration in HERC2 causes the OCA2 gene, which determines the amount of pigment, to ‘turn off’. Although the identity of the initial mutant remains a mystery, the remains of the first blue-eyed person date back 7,000 years, a skeleton that was discovered in Spain: a man who lived in a cave and was dark-skinned.

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We do not retain his eyes, obviously, but a genetic analysis of his bones revealed that this person is, to date, the most primitive blue-eyed human in history. Now 10% of the population has blue eyes . They are relatively frequent in Europe, there are areas in the Nordic and Eastern countries in which a large part of the population has eyes of this color. Only 3% of the world’s population has green eyes .

So all people had brown eyes until a genetic mutation in the OCA2 gene triggered a process that literally "turned off" the ability to produce brown in the iris.

There are also people with violet eyes, like those of the film actress Liz Taylor. This color, in fact, is produced due to a mixture of red tones with blue reflections, which result in a very intense "unreal" blue. It is frequent in people affected by albinism , although there have also been cases in other people who do not suffer from this condition.

That’s how fascinating, as well as pedestrian, is the color of our eyes: a few simple genes, a mixture of pigments, and the unique result in each canvas.