Amman – Recent genetic research has revealed that all people with blue eyes around the world share a single genetic fingerprint, dating back to an ancient mutation that occurred in a single human ancestor thousands of years ago.

The study explained that this fingerprint is represented by a genetic variant known as rs12913832, located within the HERC2 gene, which acts as a precise regulator similar to a “dimmer switch” that controls the level of melanin pigment production within the iris.

Scientists confirm that this discovery links hundreds of millions of people to a single change in the human genome, serving as a clear example of the effect of a simple genetic difference in a prominent phenotypic trait such as eye color.

Basic pigmentation gene

The story begins with the OCA2 gene, which is responsible for producing a protein that plays a key role in manufacturing melanin, the pigment that determines eye, skin and hair color.

Previous studies on twins and families have shown that a specific region within this gene explains most of the differences between blue and brown eyes, leading scientists to believe that this location is the primary key to iris pigmentation.

But the mystery remained: How does a regulatory region outside the gene itself control whether it turns it on or off?

Subsequent studies showed that the critical variant is not located within OCA2, but rather in the neighboring gene HERC2, which acts as a regulator of the activity of the first gene.

How does a mutation produce the color blue?

Molecular analyzes showed that the mutation does not completely stop melanin production, but only reduces it. When the concentration of pigment in the iris decreases, no actual blue pigment appears, but rather the light is scattered within the tissue in a manner similar to the phenomenon of light scattering in the atmosphere, which is the same mechanism that makes the sky appear blue.

Scientists explain that the chromosome folds itself so that this regulatory element touches the OCA2 gene. The version associated with brown eyes enhances this connection, while the version associated with blue eyes weakens it, leading to decreased gene expression and reduced melanin production.

One founder event

Extensive genetic studies indicate that all blue-eyed people today descend from a single ancestor who first carried this regulatory mutation. Before its appearance, the default color for human eyes was brown, as a result of high levels of melanin.

Although eye color is a polygenic trait affected by several factors that determine color shades, this variable remains the primary factor that determines the transition from dark to light irises in a large percentage of people, especially in Europe.

Scientific and applied importance

This discovery is not limited to understanding the origin of eye color only, but extends to practical applications, most notably in forensic medicine, where this mutation is used within DNA analysis tools to predict eye color from biological samples.

It also provides an important example in modern genetics, showing that changes in non-coding regulatory regions of DNA can profoundly influence human traits without changing the structure of the proteins themselves.

This discovery highlights a remarkable fact, which is that what appears to be a unique personality trait among millions of people is only the result of a very small genetic change, but it has left a continuous imprint linking distant generations to a single ancestor in human history.

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