The Existence of Blue Eyes

By Mritika Senthil

Image Credit: Flickr @ brubsbastos

In popular culture, it is stated many times that all individuals with blue eyes have a common ancestor. However, how is such a phenomenon possible? After all, currently 8% of our global population has blue eyes. Therefore, how can such a large portion of people be impacted by a single individual? The key lies in a genetic mutation.

The idea of blue-eyed individuals having a common ancestor began with a study on the demographics of eye color. Scientists interestingly found that blue-eyed participants in this research mostly belonged to nationalities originating near the Baltic sea and northern Europe. Considering such close proximity in location among these individuals, researchers began to suspect that blue-eyed populations are descended from a common European ethnic group that has presently diverged. Based on this presumption, genetic tests began to be conducted.

Image Credit: Flickr @ National Center for Advancing Translational Sciences

Research began to be conducted on populations with various eye colors in 1996. Scientists initially began observing the OCA2 gene, which determines the value of brown pigment in our eyes by catalyzing the maturation of melanosomes, or cellular structures that produce and store melanin. Blue eyes essentially have no pigment so it was speculated that the factor most significantly influencing pigment value would have to be mutated for no pigment to be present. However, after several years, this study concluded that individuals with blue eyes do not have mutations on the OCA2 gene collectively but contain an abnormal HERC2 gene. In general, it is a large E3 ubiquitin protein ligase (a protein that uses an E2 ubiquitin-conjugating enzyme to transfer ubiquitin from the E2 to the protein substrate) involved in DNA repair regulation, neurological disorders, and, most importantly, pigmentation.

The HERC2 gene has a section of DNA that operates the expression of the OCA2 gene. Under normal circumstances, the HERC2 gene would have allowed the OCA2 gene to increase the melanin within the eye, hence resulting in iris colors with darker pigmentation. However, there is currently at least one mutated polymorphism of the HERC2 gene that hinders the ability for the OCA2 gene to produce pigmented cells within the iris, therefore resulting in blue eyes.

It was initially hypothesized that blue eyes resulted from a common ethnic group. After extensive studies, this notion was altered to determine that such a mutation is too unlikely to simultaneously occur within a population. This means that the only potential scenario would have been for a European individual to have a HERC2 mutation. Eventually, their descendants would inherit this mutation, ultimately constituting 8% percent of the global population. For such a significant blue-eyed population to be present, the initially mutated individual would have lived between 6,000 and 10,000 years ago.

Based on the results of this research, a common question that arises is based off of a prevalent phenomenon today: how do two brown-eyed parents have a child with a blue eye color? This is the result of multiple other genes that play a minor role in the production of eye color, including the ASIP, IRF4, SLC24A4, TYR, and TYRP1 genes. Basically, many genes have the ability to change the value of pigment within the eyes but HERC2 and OCA2 are the most influential. Furthermore, the blue-eyed allele is characterized as being recessive. This means that browned-eyed parents could carry the HERC2 mutation but due to the inheritance of more dominant genes influencing a darker eye color, their phenotype would result in brown eyes. But by carrying the blue-eyed allele, they have the ability to pass only this recessive genetic mutation resulting in a child being highly likely to inherit blue eyes. Based on these factors, it is clear that there are various aspects influencing a child’s blue eye color that are not completely influenced by the previously stated common ancestor.

Considering the unlikely nature of the occurrence of blue irises, it is not a surprise that the number of blue-eyed individuals are globally decreasing. For example, various studies have shown that only 6% of the American population has blue eyes today compared to 50% 100 years ago. While this phenomenon is immediately attributed to the movement of darker-eyed groups into the United States and adding to the population count, a more significant factor is intermarriage. Previously, those with blue eyes were more likely to marry others with blue eyes, meaning that their children contributed to a growing group of blue-eyed individuals. However, today this is not a significant factor determining marriage. With the darker-eyed genes being more dominant, children of blue and darker-eyed people most likely contribute to a collective population without blue eyes.

Interestingly though, blue eyes are unlikely to “disappear.” Due to intermarriage, the HERC2 mutation will be present in the genetic makeup of a larger population. As a result, if a child inherits the correct alleles from both parents, it is likely that they will have blue eyes. Basically, despite the HERC2 mutation facing many genetic impediments, a single individual from 6,000-10,000 years ago will have an everlasting impact on the global phenotype and genotype.

What Did You Learn?

Q: Why is the blue eye color unlikely to disappear after multiple generations despite reduction of its prevalence in phenotypes?

A: The reason that the blue-eyed population is decreasing is due to intermarriage between blue-eyed and darker-eyed individuals, with the dominant trait for dark eyes overpowering the recessive trait of lighter ones. That being said, the gene continues to be present in the genotypes of descendants of individuals with blue eyes. If they have children with someone carrying the trait as well, the child may possibly have blue eyes.

Q: How does the HERC2 mutation impact the OCA2 gene in individuals with blue eyes?

A: The HERC2 mutation causes the OCA2 gene to halt maturation of melanosomes, structures that carry melanin within the irises. As a result, no pigm