Quick Answers
There are a few key reasons why some people have very dark colored eyes:
- Genetics – The amount of melanin pigment in your irises is determined by genetics. More melanin leads to darker eye color.
- Ethnic background – Those with an African, Asian or Hispanic background are more likely to have very dark brown or black eyes.
- Lack of other pigments – Blue and green eyes get their color from the presence of other pigments like lipochromes. An absence of these leads to brown/black.
- Thick irises – Thicker irises allow more melanin to be deposited and absorbed, creating a darker effect.
So in summary, dark eye color is primarily attributed to having more deposits of melanin pigment in the iris. Genetics and ethnic background play a major role in determining how much melanin you have.
The Genetic Determinants of Eye Color
The specific color of your eyes is determined by the amount and type of pigment in your irises. The main pigment that influences eye color is called melanin. The amount of melanin present in your iris is controlled by multiple genes.
The key genes involved are:
- HERC2 – Turns on OCA2 expression
- OCA2 – Stimulates melanin production
- SLC24A5 – Reduces melanin production
- TYRP1 – Influences ratio of brown to black melanin
People with super dark eyes tend to have gene variants that increase OCA2 activity and melanin concentrations. They also often have variants of HERC2 and TYRP1 that promote higher ratios of black-brown eumelanin over other pigments.
In contrast, people with blue/green eyes have less total melanin production and higher levels of other pigments like lipochromes that mask the brown color. So in summary, dark brown or black eyes arise when genetic factors maximize melanin levels in the iris.
The OCA2 Gene
The OCA2 gene provides instructions for making a protein called the P protein. This protein is located within pigment cells called melanocytes where it controls the production of melanin.
People with significant mutations in OCA2 experience oculocutaneous albinism type 2, characterized by very pale skin, hair, and eyes due to minimal melanin production.
Those with dark eyes tend to have variants of the OCA2 gene that increase its activity leading to more melanin. The specific genetic variant most associated with brown/black eyes is called HERC2 rs12913832. This variant enhances the expression of OCA2, boosting P protein levels and melanin synthesis.
In essence, the P protein catalyzes several steps during melanin biosynthesis. More P protein equals more melanin pigment. Variants of OCA2 are by far the biggest genetic contributor to dark eye color.
The HERC2 Gene
As mentioned, there is a strong association between dark eyes and the HERC2 rs12913832 variant. But how does this variant connect to OCA2 and melanin synthesis?
It turns out that HERC2 encodes a regulatory protein that can turn on or turn off the expression of other genes. The rs12913832 variant alters the activity of the HERC2 protein so that it better stimulates expression of OCA2.
In particular, when the HERC2 protein has an adenine base at rs12913832, it binds strongly to the OCA2 promoter region and activates the OCA2 gene. This boosts melanin production and leads to dark brown/black eyes.
So while HERC2 does not directly impact melanin synthesis, variants in this gene can substantially increase OCA2 activity and melanin levels indirectly. HERC2 variants combined with OCA2 variants create a synergistic effect in promoting super dark eye colors.
The SLC24A5 Gene
SLC24A5 provides instructions for making a protein that imports calcium and potassium ions into melanocytes. By changing the ionic environment inside pigment cells, this can reduce their melanin production.
The SLC24A5 gene is more associated with light eye colors like blue/green. Those with dark eyes tend to have variants of this gene that limit its expression so that it has minimal impact on melanin synthesis.
A notable variant is called rs1426654 which inhibits SLC24A5 activity. This allows OCA2 and other pathways to maximize melanin production unimpeded, leading to very dark eye colors.
The TYRP1 Gene
The TYRP1 gene helps encode an enzyme called tyrosinase-related protein 1. This enzyme catalyzes some of the later steps of melanin synthesis within melanocytes.
Notably, TYRP1 helps influence the ratio between the brown/black eumelanin versus the red/yellow pheomelanin pigments. Variants in TYRP1 sway this ratio in favor of eumelanin over pheomelanin.
Those with dark brown or black eyes tend have a higher eumelanin content which intensifies the darker color. So TYRP1 works alongside OCA2 to enrich the melanin composition with extra black-brown pigments leading to very dark irises.
The Impact of Ethnic Background on Eye Color
There are distinct differences in eye color distribution among ethnic populations around the world. Dark brown eyes are far more prevalent among those with African, Asian, and Hispanic ancestry. In contrast, light eyes are most common in European populations. What accounts for this variation?
The answer again comes back to genetics. Certain eye color alleles have higher frequencies in different ethnic groups. Here are some key differences:
African Ethnicity
Those with African ancestry overwhelmingly have brown or black eyes due to high melanin content. Some key genetic factors include:
– 99-100% frequency of HERC2 variants that boost OCA2 expression
– Similar frequency of OCA2 variants that maximize melanin synthesis
– Common SLC24A5 variants that limit depigmentation
– High levels of TYRP1 variants favoring black-brown eumelanin
Together these lead to uniformly dark eye pigmentation.
Asian Ethnicity
Asian populations also have a high prevalence of brown/black eyes. Contributing genetic factors are:
– Almost 100% frequency of HERC2 variants enhancing OCA2
– Very high levels of OCA2 alleles boosting melanin
– High prevalence of TYRP1 variants enriching eumelanin content
This creates a genetic profile favoring very dark eyes.
Hispanic Ethnicity
Hispanic people derive ancestry from both European and Native American roots. This is reflected in their eye color distribution:
– 61-80% brown eyes
– 15-25% intermediate colors like amber
– 5-10% light eyes
The Native American heritage contributes to high melanin content. While there is more variation than in African/Asian groups, most still have genetic propensity for darker eyes.
European Ethnicity
In contrast, Europeans have a wider range of eye colors but lower rates of brown eyes:
– Around 45% intermediate eye colors like hazel/amber
– 27% blue eyes
– 18% green eyes
– Only 8-10% brown eyes
This is attributed to two key genetic factors:
1. Lower frequency of OCA2/HERC2 variants that maximize melanin
2. Higher rates of variants for genes like SLC24A5 that depigment eyes
Thus while dark eyes can and do occur in those of European descent, they are far less common due to underlying genetic differences.
The Impact of Iris Structure and Thickness
The amount of melanin present clearly plays the most significant role in determining eye darkness. But could structural factors also be involved?
It turns out iris thickness and density can also influence eye color. Some key points:
- Thicker irises have more surface area for melanin to be deposited
- More melanin can accumulate in thicker irises
- Denser iric collagen fibers may absorb more melanin
- Thicker irises could block reflection of light from the back of the eye
This indicates that thicker, denser irises paired with high melanin levels can contribute to a very dark eye color. Differences in iris structure are subtle and secondary to melanin content though.
Here is a table summarizing how iris thickness may correlate to darkness:
| Iris Thickness | Typical Eye Color |
|---|---|
| Thinner | Lighter |
| Average | Intermediate |
| Thicker | Darker |
However, more research is needed to fully establish if and how iris dimensions factor into eye color. The influence seems small compared to that of melanin levels.
Conclusion
In summary, dark brown or black eye color arises from these key factors:
– Genetic variants leading to increased melanin production, particularly in OCA2 and HERC2
– Ethnic background contributing to higher melanin levels
– Less impact from depigmenting genes like SLC24A5
– Enrichment in black-brown eumelanin pigments
– Potentially thicker irises allowing more melanin accumulation
So in essence, super dark eyes reflect an interplay of genetic, ethnic, and structural influences that maximize the amount of melanin pigment deposited in the iris. While many find dark eyes more mysterious and exotic, they simply represent a variation in human eye color controlled by defined biological factors.
