The eyes in humans are one of the most developed organs in the entire body, with a complexity second only to the brain—which ranks first—and the entire nervous system. Both are the first to develop in the embryo during gestation.
This time, we'll focus on a specific aspect of the eyes: their color. Melanin is the pigment responsible for eye color and is located in the iris . Its natural hue is brown, but we know there's a wide spectrum of eye colors. Why is this? In this article, we'll explain it all, and you'll have the opportunity to learn some interesting and curious facts about eye colors.
Eye color is genetically determined
Eye color is determined by the genes we inherit from our parents . However, our eye color is unique, and this is due to the phenomenon of genetic recombination, which makes our eye color unrepeatable on the entire planet. Yes, there is no one else in the world with the same eye color as you. Genetic recombination allows for a kind of "random mixing" of your parents' eye colors, resulting in a unique and unrepeatable pair.
The gene that determines eye color is designated OCA2 and is located on chromosome 15. It is responsible for synthesizing the pigment melanin —which is brown —in melanocytes (melanin-producing cells) and determines the color of the skin, hair, and eyes. To learn more about this process, you can consult information on changing eye color.
How is it possible the existence of so many eye colors when melanin is brown?
It depends on its presence or absence, as well as its distribution in the iris. Blue or green eyes, for example, lack melanin, and brown eyes, in all their variety, have different amounts of melanin as well as a different distribution in the iris.
There is a factor that explains this phenomenon, the ICR: it is a factor that indicates the distribution of melanin in the iris and gives us information about eye color.
This distribution also influences the iris pattern , which—like color— is unique to each person. So much so, that technology has developed iris pattern recognition as a means of digital identification, since it is a unique fingerprint, just like a fingerprint.
Types of eye colors: causes and characteristics
Brown
It is the most common color, found in approximately 50% of the population. It is more prevalent than blue or green. This is due to the presence of melanin , the brown pigment that gives the iris its color.
Black
It's important to note that there is no such thing as black melanin ; it's a brown pigment, and only brown. What gives the eyes a "black" appearance is the high density of pigment in the iris , which causes it to blend in with the pupil, making it indistinguishable. This is common in the African population , where the high concentration of pigment proved to be an advantage in their ancestors against the continent's high levels of radiation. Only less than 1% of the African population actually has "black" eyes; the majority have very dark brown eyes.
Sometimes, black eyes are due to a condition called aniridia , characterized by the total or partial absence of the iris, leaving only the pupil visible, which is indeed black. This can be caused by a genetic abnormality, trauma, or surgical complications.
Blue
It is present in 8% of the population and is believed to have originated from a common African ancestor who, upon migrating to Europe, underwent a mutation in the OCA2 gene that would have impaired their body's ability to synthesize melanin . In regions with less radiation, the change proved beneficial, which would explain the trait's persistence to this day.
Therefore, it is estimated that all people with blue eyes in the world share this common ancestor, and we're talking about a figure of 150 million. If you want to know more about eyes of this color, check out our article on green eyes.
Blue eyes are technically colorless , and it seems that this color is due to the incidence of light on the layers of the iris that is reflected outwards, similar to the effect produced by the blue color of the sea and the sky.
Some peculiarities of blue eyes
- The low proportion of melanin has its consequences: since blue eyes contain less melanin than brown, green or hazel eyes, people with blue eyes are more sensitive to light and are at increased risk of retinal damage. This is so because the receptors are saturated by not having the filter that would provide melanin, which in sufficient quantities, protects the eye from radiation by absorbing electromagnetic waves.
- However, people with blue eyes have an advantage over the rest, and that is that see better in the dark. As they have a low amount of melanin, they reflect light very little and consequently all the incident light source that reaches them is usable, so in low irradiance conditions they maximize the efficiency of light reception.
As you can see, there are many curious and interesting facts about eye colors that will continue to surprise you in the following lines.
Green
It is the least common color, only 2% of the world's population has it, so it would be one of the rarest eye colors.
The most curious thing of all is its origin: you might be surprised to learn that green eyes originated in Asia , which would explain why there are so many people with green eyes in countries like Afghanistan. In Europe, they are found mainly in countries like Hungary and Iceland, and are more common in women.
amber or hazelnut
While green is a rare color , amber or hazel is even rarer, represented by only 1% of the world's population. It's one of the colors with the most nuances and is especially visible when sunlight hits it directly (which doesn't mean the light changes their color; that's a common myth). You can read more about gray eyes in our dedicated article.
They can contain a mixture of green and brown, and sometimes even appear yellow, like those of some wolves. Without a doubt, it's one of the most fascinating eye colors in existence, giving the face a very distinctive look.
Grey
Another rarity of nature. Gray eyes are present in less than 1% of the world's population. It's a mixture of red tones with blue highlights and is usually seen in people with albinism.
Albinism is a condition characterized by the complete absence of melanin throughout the body. The eyes of albinos lack melanin, and consequently, the red color of the capillaries is reflected against a bluish background projected by the iris in the absence of pigment. The result is a violet or gray color.
But gray tones of eyes have also been seen in people who are not albinos and it is also explained by a combination of low melanin and bluish reflection of the fundus. An example of this case is found in the eyes of the famous Elisabeth Taylor.
Red
As mentioned earlier, this color is primarily found in people with albinism and results from the complete absence of melanin in the iris, which exposes the capillaries of the retina . These capillaries can be seen with the naked eye because the light that passes through them reflects the red of the blood.
The reddish color, therefore, is due to hemoglobin, the pigment that gives blood its red color. We're talking about a reddish color; it's not the bright red of laboratory mice, for example, which would appear almost ghostly in humans.
More curious and interesting facts about the colors of the eyes
Some babies are born with blue eyes that later darken, why?
On many occasions, babies are born with bluish or grayish eyes and after a few months they surprise us with a change to brown. The explanation is found once again in the melanin pigment.
Newborn babies have many immature structures , including their eyes, both in appearance and function. They don't have a permanent eye color, and their vision is often blurry and colorblind; they only perceive their immediate surroundings, which include their mother and other family members.
These babies with seemingly blue eyes have a very low concentration of melanin in their irises. Just as with blue-eyed adults, light reflects off the back of the eye in a way similar to how it reflects off the sea, resulting in that bluish appearance. As they approach six months of age, they will begin producing more melanin, and their eye color will reach its final shade.
Do you know what heteronomy is?
Heterochromia defines the condition in which a person (or animal, where it also occurs) has one eye of each color ( total heterochromia ) or the same eye of two colors in equal parts or asymmetrically ( partial heterochromia ).
This is due to a genetic anomaly or an injury sustained after birth. This was the case with British singer David Bowie, who unfortunately suffered an injury to one of his eyes.