How does eye color develop?
Eye color differs from person to person.
Whether blue, green, blue or a mixture of different colors – there are numerous nuances.
You can find out how the different eye colors come about in this article.
What is the eye color?
When people talk about eye color, they are referring to the color of the iris – also known as the iris because of its coloration.
The iris can take on a wide variety of colors.
Brown eyes are the most common, while green is the least common.
Only around two percent of people have a green iris.
The color is determined by so-called melanocytes, which produce the coloring substance melanin.
Melanin – a pigment
Melanin is a pigment that gives hair, skin and eyes their color.
Eye color is determined by the amount of melanin.
A lot of melanin makes for darker colors – i.e. brown eyes.
A lower proportion of melanin can be found in gray or green eyes.
Blue eyes also contain little melanin.
How the blue color appearance ultimately comes about has not been conclusively researched.
It is assumed that two other elements play a role:
- Pigment epithelium – If the pigment epithelium becomes visible through the stroma iridis, the iris appears blue.
- Collagen content – How well the pigment epithelium can transmit light depends on the collagen content in the stroma iridis.
The collagen content determines how much light is reflected.
What influences the melanin content?
How high the proportion of melanin is in humans is said to depend on environmental factors, among other things.
Another key factor is the inheritance of melanin concentration.
Melanin content due to UV radiation
People in southern regions with high UV radiation are more likely to have brown eyes.
Melanin has a protective effect against UV rays.
It is assumed that the proportion of melanin is correspondingly higher in people in the south due to environmental adaptation.
In northern countries with little sunlight, on the other hand, there are comparatively many people with blue eyes.
A genetic mutation causes a loss of melanin storage.
This is an advantage for people from the north in that better absorption of the already low levels of sunlight is possible.
A high absorption of UV radiation in turn leads to better vitamin D production.
Genetic factors influencing eye colorIf you want to understand the inheritance of eye color, two terms are important first:
- Genotype – the genetic code that is not externally visible
- Phenotype – the external visible appearance
Do brown eyes always prevail?
The genotype is determined by the parents, among other things.
However, it is an outdated assumption that only one single gene is responsible for the final eye color.
For a long time, it was assumed according to the Davenport model that the gene for brown eyes is dominant and always prevails over the recessive gene for blue eyes.
This combination would therefore always result in the “brown eyes” phenotype.
It has since been discovered that this assumption is not entirely correct.
It is not uncommon for both parents to have a different eye color than their offspring.
Polygenic inheritance of eye color
It is now known that eye color is inherited polygenically.
Several genes are involved in coloration – but which genes are involved has not been conclusively clarified.
What we know: Everyone carries genes for two eye colors.
When inherited, one gene is passed on from each parent.
However, this does not always have to be the phenotype, i.e. the visible eye color of the parent.
In general, it can be said that some eye colors are more dominant than others.
This can at least be used to calculate probabilities for the eye color.
Here is the order of decreasing dominance:
- Brown eyes
- Green eyes
- Blue eyes
- Gray eyes
Disorders of melanin production
There are some congenital diseases that affect melanin production in the body and therefore influence eye color.
Albinism
Albinism results in a disorder of melanin production.
The coloring substance can be missing in the skin, hair or iris.
If the iris is affected, this results in very light eye colors.
When exposed to light, a reddish glow may appear.
It is not the case in humans that the iris turns completely red – as is known from some animal species.
Iris heterochromia
Iris heterochromia is a very rare disorder of melanin production.
In this case, one of the two eyes has a melanin production disorder.
This results in eyes of different colors.
There is also so-called partial heterochromia.
In partial heterochromia, one eye alone is split into different colors.
Does eye color change in the course of life?
Newborn babies very often have blue eyes.
This is because no melanin has yet been stored in the womb.
The eye color develops in the first months and years.
The final eye color is usually visible after two years at the latest.
Changes in eye color with age
Changes in eye color with age can have various causes:
- Hormone fluctuations
- Biochemical processes
- Diseases
You should consult a specialist, especially if there are sudden changes in color, to rule out serious illnesses.
Development of eye color – frequently asked questions & answers
Why do people in more northern countries have blue eyes more often?
Only around eight percent of people worldwide have blue eyes.
In Finland, the percentage of people with blue eyes is around 90%.
Melanin acts as UV protection.
Due to the low UV radiation, it is assumed that people in the north develop less melanin storage.
In more southerly regions, on the other hand, people have a high melanin content due to the high level of radiation, which ultimately protects them from the sun.
Do all newborns have blue eyes?
Many newborns have blue eyes at birth due to a lack of melanin production in the womb.
The eye color only develops in the following months when melanin is produced.
If the parents have a darker skin and eye color, melanin production may also begin in the womb.
As a result, the newborn may be born with different eye colors.
Why does eye color change with age?
If the eye color changes with age, this may be due to biochemical processes or hormonal factors.
However, diseases such as inflammation of the optic nerve, Horner’s syndrome or glaucoma can also lead to a change in color, which is why you should consult a specialist in this case.
