Why do we have black, brown, blue or even violet eyes?

Despite the fascination that the different colors of human eyes cause us, 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 appear blue it is because of the white collagen fibers in the connective tissue of the iris, which scatter light and make the iris appear 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.

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 of founding father of modern genetics is hardly an exaggeration considering that as a result of those experiments, Mendel was able to conceive of more than 29,000 species of peas.

To achieve this, he crossed, for example, pea species that always produced round 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 descending 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.

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 chromatism.

All blue-eyed people on the planet are descendants of a single European who lived some 6,000 to 10,000 years ago, and who was the first to develop a specific mutation that accounts for the now widespread coloration of the iris. 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, led to different shades of brown.

However, the blue color 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 dark-skinned cave-dwelling man.

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