Eye color is determined by the amount and type of pigment in the iris of the eye. The two main pigments that influence eye color are melanin and lipochrome. Melanin comes in two forms – eumelanin which produces brown/black pigment and pheomelanin which produces red/yellow pigment. The combination and concentration of these pigments determine an individual’s eye color. While brown is the most common eye color worldwide, green and hazel eyes are rarer. So which is more uncommon between green and hazel eyes?
The Genetics Behind Eye Color
The main gene that controls eye color is the OCA2 gene located on chromosome 15. Different versions (alleles) of this gene are associated with different eye colors:
– The allele that produces the most eumelanin is very common and results in brown eyes.
– The alleles that produce less eumelanin result in green, hazel, amber and blue eyes.
Additional genes like SLC24A4 and SLC45A2 also play a role in controlling the amount of melanin produced.
The interaction between these genes accounts for the range of eye colors seen. Green and hazel eyes occur when there is a moderate amount of melanin present.
Prevalence of Green vs Hazel Eyes
| Eye Color | Percentage of World Population |
|---|---|
| Brown | 79% |
| Blue | 8-10% |
| Green | 2-3% |
| Hazel | 5% |
| Amber/Other | 5% |
As seen in the table above, brown eyes are by far the most common eye color globally, with nearly 80% of the world’s population having brown eyes. In contrast, green eyes are found in only 2-3% of people worldwide. Hazel eyes are slightly more common at around 5% of the global population.
The prevalence of green and hazel eyes varies by geographic region as well. Countries in Northern and Western Europe have a higher percentage of lighter eye colors like green and hazel. For example, green eyes are most common in Finland and Scotland at around 14% of the population. Hazel eyes exceed 10% in countries like Iceland and England.
In contrast, Africa, East Asia and South Asia have predominately brown eyes given their ancestral genetic makeup and minimal intermixing with European lineages.
Factors Influencing Green and Hazel Eye Rarity
There are several reasons why green eyes are rarer than hazel eyes globally:
Recessive Genetics
True green eyes are controlled by a recessive allele of the OCA2 gene. This means two copies of the allele are needed to produce green eyes. Since it is recessive, the gene can be carried without expressing the green eye trait if only one copy is present. In contrast, hazel eyes can result from a single copy of an OCA2 or SLC24A4 allele. The dominance of the hazel alleles makes this eye color more common.
Limited Geographic spread
As mentioned earlier, green and hazel eye colors are most concentrated in European populations. However, hazel eyes are also relatively prevalent in West Asia and North Africa due to genetic interactions over history. The geographic spread of the hazel eye color alleles is greater, resulting in a higher worldwide prevalence compared to pure green eyes.
Defining hazel eyes
Hazel eyes have a broader, less well-defined range compared to green eyes. Hazel can refer to eyes that are varying mixes of brown, green and orange. This wider definition of hazel increases the number of people classified as having hazel eyes. In contrast, green eyes are more distinctly defined as having a solid green hue with minimal brown.
Multiple causal genes
As mentioned earlier, hazel eyes arise from a wider range of genetic interactions not limited to just the OCA2 gene. The involvement of SLC24A4 and SLC45A2 can also produce hazel eye colors. More possibilities from these additional genes again make hazel eyes more prevalent than green.
Prediction of Future Prevalence
It is likely that hazel eyes will remain more common than pure green eyes in the future. Some reasons are:
Continued intermixing of lineages
Populations are becoming increasingly mixed in terms of geographic ancestry. This will likely maintain or even increase the prevalence of hazel eyes over green eyes in offspring.
Persistent recessive genetics
The recessive nature of green eye alleles means they are unlikely to increase dramatically in frequency. Even with mixing, a double copy is needed to express the trait.
Climate impact theories
Some theories speculate that lighter eye colors like green arose in northern latitudes to help with Vitamin D absorption. As populations migrate away from cloudier northern climates, natural selection could potentially act to reduce green eye prevalence. However, such changes would occur slowly over many generations.
Conclusion
In conclusion, hazel eyes are definitively more common than true green eyes globally. Hazel eye color results from a wider range of possible genetic interactions compared to the rare recessive genetics required for green eyes. While the prevalence of green and hazel eyes varies regionally, hazel eyes maintain higher frequencies in most world populations. This trend is likely to persist into the future given the dominant nature of hazel eye genetics and continued mixing of human populations over time. So if you have true olive green eyes, know you have a very rare treasure!
