How does eye color come about?

Anatomy & Physiology

The colored ring of our eye / our eye color is called the iris (rainbow skin). Histologically, the iris consists of several layers. The layer that is decisive for the eye color is called Stroma iridis where stroma means as much as connective tissue.

This layer consists primarily of collagen fibers and fibroblasts, i.e. cells that produce connective tissue components.

In addition, the two muscles that are responsible for the size of the pupil are located in this layer. On the one hand - Sphincter pupillae muscle, which constricts the pupil, on the other hand - Dilator pupillae muscle, who is responsible for dilating the pupil).

Eye color - what's behind it?

Another cell population is decisive for the color of the eyes: the melanocytes. They produce the dye melanin, which is also of crucial importance for the color of skin and hair, among other things. People with few melanocytes in their irises have a lighter eye color than those with many melanocytes.

So people who have very few or no melanocytes stored in their iris have blue eyes. How exactly the blue color comes about is still much discussed. There are two main components responsible:

1. the pigment epithelium located directly behind the stroma iridis (Myoepithelium pigmentosum, Warning, this should not be confused with the pigment epithelium of the retina (retina), this has a different task). If this shimmers through the iris almost unhindered, it appears blue.

2. How unhindered the pigment epithelium can shine through depends in turn on how much collagen is stored in the stroma iridis, because the collagen content determines how much light is scattered and reflected and that in turn is decisive for the eye color that is impressive in the end.

But what about the eyes that are not blue?
If there are isolated deposits of melanocytes, the iris appears green or gray. If there are numerous melanocytes in the connective tissue layer, the iris appears brown. How the innumerable color facets and shades arise from each of these colors, still remains a little mystery to which there are many hypotheses.

Inheritance of eye color

For a long time, the Davenport model was regarded as authoritative here. It assumed only one gene for the inheritance of eye color. However, it is now clear that the mode of inheritance of eye color is polygenic. This means that more than one gene is responsible for the transmission of eye color from parents to child.

Some eye colors are more dominant than others. Brown is the most dominant of all eye colors, followed in descending order by green, blue and gray. Theoretically, if the father has brown eyes and the mother has blue, brown will prevail against blue and the child of the two will have brown eyes.

However, it is not that simple after all, since there are two expressions (alleles) of each gene. For example, the father who has brown eyes (phenotype) can have one allele for brown eyes and one for blue eyes in his genetic material (genotype).

He only passes one of the two alleles on to his child. So a child born to a brown-eyed father does not necessarily have to have brown eyes. But that's far from enough. Other genes complicate the genetics around eye color many times over.

When is eye color final in a baby?

Most European babies are born with blue eyes. The reason for this is that the iris - the iris of the newborns - largely does not contain any pigments.

The iris is only colored by melanin, an endogenous dye that reacts to light. Melanin is hardly present after birth.

The eye color is determined by a person's genes and can change depending on this in the first year of life. As a rule, a person's final eye color shows up 3 to 6 months after birth.

A simple examination of the newborn's iris can give an indication of what is most likely the basic color of the eye: if you look at the iris from the side under simple fluoroscopy, you can see high or low levels of melanin.

If the iris continues to appear light blue with this method, it means that there is no melanin present. In this case, the eye color is most likely to remain blue. If the iris shimmers golden, however, this indicates a certain melanin content and the iris probably takes on a brown or green color in this case.

In newborns from Asia, Africa or Latin America, the eye color is often mostly brown at birth.

If you want to dig deeper into this topic, check out our new article: Baby's eye color - when is it final?

Can eye color change with age?

Sometimes it happens that even after the first year of life, a person's eye color changes. It has been found that hormonal fluctuations or biochemical processes in the body can have a changing effect on the iris.

For example, hormonal influences can very rarely lead to changes in the color of the eyes, for example during puberty or pregnancy.

A study of twins found that around 10% of fair-skinned people change the color of their irises over the course of their lives. However, if the eye color changes quickly, you should consult an ophthalmologist to rule out an illness as the cause.

For example, it could be an inflammation in the eye. In addition, an injury to the optic nerve can also cause the iris to change color.

Interesting facts about eye color

  • Around 90% of the world's population have brown eyes.
  • With Europeans in particular, most newborns are born with blue eyes. The formation of melanin by melanocytes only begins in the first few weeks of life, so that the final eye color only shows up after a few months to years.
  • In albinism, the iris pigment epithelium is completely absent. The eyes look very light blue to almost pink.
  • It is rare for a person's two eyes to be of completely different colors. This is then called Iris heterochromia (heteros - unequal and chroma - color).
    If only one eye has different colors, this is called partial heterochromia. Heterochromia can be congenital or acquired, for example through trauma.

What are the frequencies of eye color?

The variety of eye color varies between the different shades of color from brown to blue. The color of the iris (Iris) is inherited on several genes and is a combination of many factors.

  • Brown colour

At around 55%, brown is the most common eye color in the world population. One of the reasons for this is that the trait is dominant in inheritance (protruding) is compared to the other eye colors. Especially in Asia, Australia and Africa, the basic color of the iris is brown in the majority of people.

  • Green-brown color

About 5% of people have greenish brown eyes.

  • Blue colour

If there is hardly any melanin in a person's genetic make-up, this leads to blue eyes. 8% of the world's population have blue eyes. Estonia is the country with the largest proportion of blue-eyed people, with 99% of the population.

As a genetic characteristic, blue is recessive compared to brown, i.e. it is receding. Researchers therefore suspect that the eye color blue will generally become less common in the future.

The incidence of people with brown eyes, however, will increase.

  • Green colour

The eye color green makes up only 2% of the world population, although green prevails over blue in inheritance.

Can you change the color of your eyes with the laser?

With a certain form of laser treatment, keratopigmentation, you can now change the color of your eyes. This procedure involves applying color pigments under the cornea with a laser. This is done under local anesthesia in 1-2 procedures, each lasting about 30 minutes.

The reasons for the procedure can be very different:

  • therapeutic for corneal opacity
  • functional in anatomical pathologies
  • purely cosmetic

It is significant that neither a foreign body is introduced into the eye nor the internal structure of the eye is changed. In some cases, keratopigmentation can prevent the need for a prosthetic eye.

Alternatively, there is also the iris implant method to permanently change the color of the eyes. A colored lens is inserted into the posterior chamber of the eye.

Both methods, laser eye color and iris implantation, like all surgical interventions, involve certain risks. Therefore, they should only be carried out in specialized centers and after careful consideration.

If you don't want to change your eye color so radically then contact lenses are a perfect option for you. You can read more about this in our next topic: Contact lenses

Can you also tattoo eye color?

Even in ancient times, attempts were made to change the color of the eyes by means of tattoos. Especially in cases of stigma due to diseases of the eye.

A relatively new method is keratography, in which colored pigments are introduced into the cornea with a needle. This is done under local anesthesia. Medical reasons for the procedure are, for example, albinism, missing or tearing off the iris, coloboma or keratoconus.

Keratography can also be used for cosmetic therapy in the case of corneal opacity or after accidents with damage to the eye.

In most cases, corneal tattooing turns out to be a straightforward and quick procedure from which patients recover quickly.

In principle, however, the procedure is only recommended for patients who have previously lost vision, as the risks are relatively high. In the worst case, complications can lead to blindness.

Different eye color between the eyes

A different eye color between two eyes of a person is medically referred to as iris heterochromia. This can be innate through genetic predispositions or gene mutations.

If someone is born with heterochromia, it should be clarified whether a syndrome may also include hearing loss.

Furthermore, a different eye color between the right and left iris can be acquired due to trauma to the eye, inflammation of the eye or injuries to the optic nerve. Here, too, clarification by an ophthalmologist is required.

Overall, heterochromia is very rare.

If you are further interested in this topic, read more about it under: Iris heterochromia

Different eye color within one eye

About 1% of the world's population has different eye colors between the eyes. Sub-forms of this are sectoral or central heterochromia. A person has different colors within one eye.

In the sectoral form, only a small part of the iris is colored differently in the form of spots. In the central form, the color of the iris is circular around the pupil like a ring of a different color.

If a person has different eye colors within one eye, it does not always have to have a disease value, but can simply be innate. However, if a new one occurs, an examination should always be carried out by an ophthalmologist.

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