21/02/2023
There is no blue pigmentation either in the iris or in the vitreous body. Dissection reveals that the iris pigment epithelium is brownish black due to the presence of melanin.[53] Unlike brown eyes, blue eyes have low concentrations of melanin in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma.[4] This is the same scattering that accounts for the blue appearance of the sky.[3]: 9 [6] The result is a "Tyndall blue" structural color that varies with external lighting conditions.
The inheritance pattern followed by blue eyes was previously assumed to be a mendelian recessive trait, however, eye color inheritance is now recognized as a polygenic trait, meaning that it is controlled by the interactions of several genes.[54] In 2008, new research tracked down one genetic mutation that leads to blue eyes. "Originally, we all had brown eyes" said Eiberg.[55] Eiberg and colleagues suggested in a study published in Human Genetics that a mutation in the 86th intron of the HERC2 gene, which is hypothesized to interact with the OCA2 gene promoter, reduced expression of OCA2 with subsequent reduction in melanin production.[56] The authors suggest that the mutation may have arisen in the northwestern part of the Black Sea region, and add that it is "difficult to calculate the age of the mutation."[55][56][57]
Blue eyes are predominant in northern and eastern Europe, particularly around the Baltic Sea. Blue eyes are also found in southern Europe, Central Asia, South Asia, North Africa and West Asia.[58][59]
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