In 2008, geneticists tied most modern blue eyes to a single, shared DNA change a tiny edit that still sits like a fingerprint in millions of irises today.
Looking at blue eyes it seems to the unobservant observer as though it were some color choice: nature choosing a new paint. Biology tells it another way. Most of the human history, the default in terms of color in the eyes was brown and this default was the result of very high concentrations of the melanin in the iris. Then some mutation changed the amount of melanin deposited in front of the iris not by creating a new color but by switching off an old system.
That system revolves around a genetic relationship that is well researched based on chromosome 15. The OCA2 gene aids in regulating the production of melanin, and similar to OCA2 is the gene HERC2 which has a regulatory element that can affect the intensity of OCA2 expression. One of the most common ones, which is commonly discussed through SNP rs12913832 of HERC2, decreases the regulatory “push” that would otherwise maintain OCA2 active in the iris. A reduced level of OCA2 activity results in a reduced amount of melanin being deposited in the iris stroma and the eye acquires a light-scattering structure as opposed to developing a dyed appearance.
Physics, and that is blue. No blue pigment can be found in the iris as there is in a bird feather or a paint chip. Rather, in the presence of low melanin in the stroma the incoming light passes into the tissue and interacts with the microstructure. The shorter wavelengths are more easily scattered, producing a color effect of structure, often referred to as Tyndall scattering in the iris stroma, of the same principle as the appearance of the sky being blue. The layer behind the iris is still darker, and as the light still comes through it, it is scattered, rather than stained. This is also the reason as to why blue eyes may appear different with varying light and why blue is a spectrum between icy gray-“blue” to darker ones.
The most interesting aspect is the frequency with which that same genetic alteration is repeated together with the same neighborhood of DNA. On closer inspection of the wider haplotype of the important variant, researchers discovered that the majority of blue-eyed people bear slightly different flanking sequences signs that there is probably a single ancestral route on which the trait has been passing, rather than numerous independent reinventions. However, it is not completely unique: work in population genetics has also explained cases of exceptions, in which blue eyes are formed as other variants, that is, not all blue-eyed individuals have the same “blue-eye” founder at that locus.
Nevertheless, such a common signature can justify why blue eyes tend to be very clustered in some areas. In Europe and some of Western Asia, migrations, founder effects and long-term mixing assisted the low-melanin iris arrangement to propagate. Brown is the most widespread, at approximately 79% of the world, with blue in the minority, and affected by ancestry and the polygenic and complicated structure of iris pigmentation.
That complication is also the reason the old concept of classroom that blue is merely recessive, brown dominant cannot work in actual families. The color of the eyes is a manifestation of the interaction between numerous genes, regulatory switches, and tissue optics, the results of which can be unexpected even in cases where the colors of parents are considered “predictable.”
Finally, blue eyes do not represent an innovation but some dimmer switch of antiquity the switch that reduced the melanin in the iris and left the rest of the work to light.
