Green eyes are one of the rarest eye colors in the world, found in only about 2% of the global population. They are most common among those of Northern and Central European descent. The inheritance of eye color, including green eyes, is a complex process that involves multiple genes. While no single gene alone determines eye color, two main ones have been identified that play a major role.
Genetics of Eye Color
Human eye color is largely determined by the amount of melanin pigment in the iris. Melanin is produced by melanocytes, specialized cells in the iris. The more melanin present, the darker the eye color. People with green eyes have low to moderate amounts of melanin.
Two key genes influence eye color:
HERC2 – This gene regulates expression of the OCA2 gene through a region known as intron 86. Variants of HERC2 reduce the activity of the OCA2 promoter, leading to less melanin production.
OCA2 – The main gene that determines the amount of melanin produced. It codes for the P protein, which is essential for melanin synthesis. Certain variants substantially reduce melanin levels, resulting in green eyes.
In addition to these major genes, other genetic factors also contribute to eye color, such as ASIP, TYR, TYRP1, and SLC24A4. However, HERC2 and OCA2 play the most significant role.
Inheritance Patterns
The inheritance of green eyes follows complex autosomal recessive patterns. This means:
– Multiple genes are involved, not just one.
– The genes are located on autosomal chromosomes, not sex chromosomes.
– Two copies of the relevant gene variants (one from each parent) are needed to express the trait.
For green eyes to occur, a person must inherit particular variants of both the HERC2 and OCA2 genes. If someone inherits two green-eye variants of both genes, they will have green eyes. The parents can also each pass on one green-eye variant and one brown-eye variant. In this case, the person will end up with one green-eye gene and one brown-eye gene at the HERC2 and OCA2 positions, resulting in green eyes.
Potential Genotype Combinations
| Parent 1 | Parent 2 | Child |
|---|---|---|
| HERC2(Brown) + OCA2(Brown) | HERC2(Brown) + OCA2(Brown) | HERC2(Brown) + OCA2(Brown) = Brown eyes |
| HERC2(Green) + OCA2(Green) | HERC2(Green) + OCA2(Green) | HERC2(Green) + OCA2(Green) = Green eyes |
| HERC2(Brown) + OCA2(Brown) | HERC2(Green) + OCA2(Green) | HERC2(Brown) + OCA2(Green) = Green eyes |
| HERC2(Green) + OCA2(Brown) | HERC2(Brown) + OCA2(Green) | HERC2(Green) + OCA2(Brown) = Green eyes |
As shown, the combinations of green-eye and brown-eye variants determine the final eye color of the child. Both parents can pass on either variant at each gene. If one green-eye variant of HERC2 and OCA2 is inherited, green eyes will result.
Population Genetics of Green Eyes
Green eye color is most prevalent among those of European ancestry. The variants associated with green eyes likely arose in Europe, becoming established due to a founder effect. They occur at the following frequencies:
– Over 30% in Iceland
– 16% in Ireland
– About 10% in Scotland, Wales, England, and parts of Western Europe
– Around 7% in Germany
– 5% or less in countries like Spain, Portugal, and Italy
The OCA2 variant rs1800407, strongly associated with green eyes, reaches its highest frequency in Europe. It is very rare outside the continent. The HERC2 variant rs12913832, also linked to green eyes, originated in Northwest Europe.
Within Europe, geographical clines exist. Green and blue eye colors become less common as one moves southward, with brown eyes dominating in the Mediterranean region. Natural selection likely played a role in this geographical distribution, as eye color is linked to sunlight intensity. Melanin provides some protection against UV radiation, which is greater nearer the equator.
Overall, the inheritance patterns of green eyes involve complex interactions between multiple genes and their variants. The autosomal recessive nature, requiring two copies of green-eye variants, restricts this rare phenotype primarily to European and European-derived populations.
Development of Green Eyes
Green eyes develop in the following way:
– Eye color is determined during early gestation. Melanocytes migrate into the iris by the 6th week of embryonic development.
– Melanin production ramps up by the 8th week. Eye color is already recognizable by the 13th week.
– Infants are usually born with blue or bluish-grey eyes due to the absence of melanin deposits in the stromal layer of the iris.
– Over the first three years of life, increasing melanin is deposited. Green eyes emerge when moderate melanin levels are reached.
– HERC2 and OCA2 gene variants limit the activity of melanocytes, restricting melanin synthesis and resulting in green irises.
– Green eyes are thus a feature present from birth, taking 2-3 years to fully manifest due to the time required for melanin accumulation in the iris. The genotype is established at conception.
The Genetic Basis of Other Eye Colors
In addition to green, other eye colors have a genetic basis as well:
Brown Eyes
– Most common eye color at over 75% globally.
– Requires at least one dominant allele of the OCA2 gene along with active HERC2. Leads to extensive melanin production.
Blue Eyes
– Caused by an OCA2 recessive allele paired with HERC2 recessive alleles.
– Results in minimal melanin in the iris.
Hazel Eyes
– Involves a combination of variants producing moderate pigment.
– Often includes a variant in the SLC24A4 gene.
Amber and Reddish-Brown Eyes
– Linked to rare variants in OCA2 and other melanin-related genes.
– Causes a shift toward reddish melanin production.
Genetic Testing
Commercially available genetic tests can be used to identify eye color genotypes. These exams PCR amplify the key regions of HERC2 and OCA2 to detect variants predictive of eye color. However, limitations exist.
– Tests cannot yet definitively identify green versus blue eyes. The genotype-phenotype correlation needs improvement.
– Other genetic factors beyond HERC2 and OCA2 also contribute to eye color. Test panels are still limited.
– Environmental and epigenetic factors can influence eye color, not just genetics alone.
Overall, genetic testing for eye color prediction is still in its early stages. While useful, results may not always correlate perfectly with the actual observed eye color of a person. Research is ongoing to refine this technology.
Conclusion
In summary, green eye color arises from low to moderate melanin production, influenced heavily by HERC2 and OCA2 gene variants. It follows a complex autosomal recessive inheritance pattern. Green eyes are most prevalent in Northern and Central Europe, likely due to natural selection related to UV radiation. The development of green eyes begins prenatally but takes up to 3 years after birth for the final pigmentation to fully manifest. Genetic testing can help predict eye color phenotypes but has limitations currently. Understanding the genetics behind green eyes illustrates the intricate process of inheritance and biology that gives rise to this rare and striking human trait.
