Eye color is one of the most fascinating features that make us unique. But where exactly does eye color come from and why do some people have brown eyes while others have blue or green? In this article, we’ll explore the genetics behind eye color inheritance, look at the prevalence of different eye colors, and find out why brown eyes are the most common eye color worldwide.
What Determines Eye Color?
The color of your eyes is determined by the amount and type of melanin pigment in the iris. Melanin is produced by specialized cells called melanocytes and comes in two forms:
- Eumelanin: This melanin pigment is brown/black in color.
- Pheomelanin: This melanin pigment is red/yellow in color.
People with brown eyes have a large amount of eumelanin in their irises, while people with blue eyes have very little melanin. Green eye color is caused by an intermediate amount of melanin. Hazel eyes have a combination of melanin types and eye colors can also change over time due to factors like light exposure and aging.
The key genes involved in eye color determination are:
- HERC2 – Regulates OCA2 expression
- OCA2 – Major contributor to eye color, produces melanin
- SLC24A4 – Influences OCA2 function
- SLC24A5 – Dilutes melanin production
- TYR – Catalyzes melanin production
These genes provide instructions for producing and distributing melanin pigments within the iris. Variations in these genes alter their expression and function, leading to different melanin levels and types in the eyes.
How is Eye Color Inherited?
Eye color is a polygenic phenotypic trait, meaning multiple genes influence its expression. The inheritance pattern follows these rules:
- If both parents have brown eyes, the child will almost certainly have brown eyes.
- If one parent has brown eyes and the other has blue eyes, the child has around a 75% chance of having brown eyes and a 25% chance of having blue eyes.
- If both parents have blue eyes, the child will most likely have blue eyes as well.
This is because brown eye color (high melanin) is dominant over blue eye color (low melanin). However, eye color predictions are not always so straightforward due to the influence and interaction of many genes. Other factors like epigenetics can also play a role.
More on Eye Color Genetics
Here is some more information on the genetics behind common eye colors:
Brown Eyes: Require at least one dominant allele of the OCA2 gene. Over 75% of people worldwide have brown eyes. Brown is the most common eye color.
Blue Eyes: Caused by a recessive OCA2 allele, resulting in low melanin levels. Around 8% of people have blue eyes.
Green Eyes: Result from an intermediate amount of melanin. Requires a yellowish pheomelanin pigment. Only 2% of the population has green eyes.
Hazel Eyes: Represent a combination of eye colors and melanin types in the iris. Appear to shift between brown, green, and orange.
Gray Eyes: Little melanin present in the iris. The light scattering effect produces a silvery gray appearance. This is a rare eye color.
Amber and Red Eyes: Very rare. Caused by a reduced amount of melanin and the presence of pheomelanin. Often seen in people with albino traits.
Prevalence of Eye Colors Globally
The prevalence of different eye colors varies significantly across the world. Here is a breakdown of common eye colors by geographic region:
Brown Eyes
- 79% in Africa
- 73% in Asia
- 70% in South America
- 69% in North America
- 63% in Europe
Blue Eyes
- 8% in Northern Europe
- 2% in Africa and Asia
Green Eyes
- 15% in Northern Europe
- 7% in North America
- 2% in Asia and Africa
Intermediate/Mixed Colors
- Common in Central Asia and parts of South America due to gene mixing
As you can see, brown eyes dominate in all major geographic regions, but especially among people native to Africa, Asia, and the Americas. Blue and green eyes are rarest globally but most common in people of Northern European descent.
Why Are Brown Eyes So Common?
Now that we’ve explored the genetics behind eye color and seen statistics on eye color prevalence, you may be wondering – why are brown eyes so much more common globally compared to other colors? There are a few key reasons:
1. Brown eye color has a strong genetic advantage
As mentioned earlier, brown eye color is a dominant trait. This means even one copy of the main OCA2 genetic variant will result in brown eyes being expressed. Blue/green eye colors require two copies of recessive alleles, which is much rarer.
2. Originated in Africa
Scientific evidence suggests brown eye color first evolved in people indigenous to Africa around 100,000 years ago. As humans migrated to other regions, brown eyes spread across the world.
3. Better protection from sun damage
Melanin provides some UV radiation protection for eyes. Over time, brown eyes may have provided a survival advantage in sunny climates vs. lighter eye colors prone to damage.
4. Less susceptibility to disorders
Certain eye conditions and vision problems occur more frequently with reduced melanin. Brown eyes may have an adaptive health advantage compared to blue/green eyes.
5. Founder effects and population bottlenecks
In some populations, such as Northern Europeans, blue/green eye color mutations occurred and propagated due to founder effects and isolation. But globally, brown eyes persisted as the dominant trait.
Conclusion
In summary, brown eye color is determined mainly by the presence and production of the dark brown eumelanin pigment in the iris. It follows a dominant inheritance pattern, making it very common worldwide. Brown eyes originated in Africa before spreading across the globe as humans migrated. They seem to offer adaptive benefits such as UV protection and reduced disorders, which may explain their high prevalence compared to rare colors like blue and green eyes. So in essence, the high amount of melanin in my irises led to my brown eye color inheritance! I hope this breakdown gave you some insight into the fascinating genetics that determine our eye colors.