The Secret of Green Eyes; A Rare Trait Possessed by Only 2 Percent of Humans

According to SAEDNEWS, until about 10,000 years ago, nearly all humans on Earth had brown eyes. At that time, a dark pigment called melanin was produced in large amounts in the human iris, which caused most people’s eyes to appear brown. But at some point in history, one or more random genetic mutations occurred that reduced melanin production in the irises of some individuals. The result of this genetic change was the emergence of lighter eye colors such as blue, gray, hazel, and green.

Among all these colors, green eyes are considered the rarest natural eye color in the world. According to statistics from the American Academy of Ophthalmology, only about 2 percent of the world’s population has green eyes. Of course, this percentage is higher in some countries; for example, about 9 percent of people in the United States have green eyes. Nevertheless, this color is still considered extremely rare among most human populations.

The mentioned statistics do not include people with albinism. In these individuals, due to a disorder in melanin production, the eyes may appear reddish, pink, or even violet-like. These colors are actually caused by visible blood vessels behind the iris and differ from natural green eyes.

The main reason for the rarity of green eyes is related to genetics. For years, it was believed that eye color was determined by only one gene. Many people still think based on old teachings that brown eyes are “dominant” and blue eyes are “recessive,” and therefore a child’s eye color can be predicted easily. However, new studies have shown that the issue is far more complex than this simple explanation.

Until about 10,000 years ago, nearly all humans had brown eyes, but genetic mutations led to the emergence of lighter eye colors

For example, in some cases, two blue-eyed parents have had a child with brown eyes; an event that cannot be properly explained by the simple “dominant and recessive” model. The reason is that eye color results from the cooperation and interaction of many different genes, not just a single gene.

All eye colors, from dark brown to blue and green, ultimately depend on the amount of melanin present in the iris. The iris is the colored part of the eye surrounding the pupil and regulates the amount of light entering the eye. The more melanin present in the iris, the darker the eye appears, and the less pigment there is, the lighter the eye color becomes.

Researchers believe that about 75 percent of differences in human eye color are related to a gene called OCA2, which plays a major role in melanin production. More specifically, this gene is involved in the formation and function of cellular structures called melanosomes. Melanosomes act like tiny reservoirs inside cells that produce and store melanin.

Another important gene called HERC2 also plays a role in this process. Some versions of this gene can reduce the activity of the OCA2 gene and cause less melanin to be produced. As a result, the eyes appear lighter. But the story does not end with these two genes. Dozens of other genes also contribute to eye color and interact with one another in highly complex ways.

Scientists explained in an article published in the journal Eye that eye color is actually a complex genetic trait formed by the interaction of several major genes and many minor genes. Therefore, accurately predicting children’s eye color is not always simple.

Eye color is not determined by a single gene, and dozens of different genes play a role in shaping it

In the case of green eyes, the situation becomes even more complicated. People with green eyes must inherit a very specific combination of genes; a combination that produces an exact amount of light brown pigment in the iris. If there is too much melanin, the eyes become brown, and if there is too little, the eyes tend toward blue. Green eyes actually lie somewhere in between.

Interestingly, green eyes do not actually contain any green pigment. Contrary to what many may think, the human body does not produce a substance called “green eye pigment.” The green color of the eyes is the result of a combination of several different factors.

One of the most important of these factors is a substance called lipochrome; a type of yellow, fat-soluble pigment that is also found in other parts of the body and nature. For example, the color of egg yolk or butter is partly due to this substance.

When a small amount of brown melanin in the iris combines with lipochrome and light is scattered and reflected in a particular way within the iris tissue, the eyes appear green. Therefore, green eye color is more of an optical and structural phenomenon than the result of an actual green pigment.

A similar phenomenon also occurs in blue eyes. In blue eyes, there is almost no melanin, and light is scattered as it passes through the layers of the iris. Under these conditions, shorter wavelengths of light, namely blue, are reflected more strongly. This is the same phenomenon that makes Earth’s sky appear blue.

The rarity of green eyes is because achieving the necessary genetic balance to create them is very difficult. To have green eyes, there must be an exact amount of brown melanin and yellow lipochrome alongside the proper iris structure. Such a precise combination of genes is inherited less often compared to other eye colors, which is why green eyes are rare worldwide.

Despite all these classifications, no two humans have exactly the same eye color. If one looks closely at people’s irises, a complex collection of shades, spots, lines, and different patterns can be seen. Even individuals who appear to have similar eye colors differ in the details of their iris patterns.

Contrary to popular belief, green eyes contain no green pigment, and their color results from a combination of a small amount of melanin, the yellow pigment lipochrome, and light scattering.

The unique characteristics of each person’s eyes are not limited to genes alone. Developmental and environmental factors also play a role in shaping the texture and structure of the iris. Very subtle changes in eye development during fetal stages and childhood can affect the final appearance of the iris.

These tiny differences are what make iris-scanning identity recognition technologies so accurate. Even identical twins, who have nearly identical DNA, do not have completely identical iris patterns, and iris scanners can distinguish between them.