"Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of light tend to be absorbed by the underlying iris pigment epithelium, and shorter wavelengths are reflected and undergo Rayleigh scattering. The type of melanin present is eumelanin. The inheritance pattern followed by blue eyes is considered similar to that of a recessive trait (in general, eye color inheritance is considered a polygenic trait, meaning that it is controlled by the interactions of several genes, not just one). In 2008, new research revealed that people with blue eyes have a single common ancestor. Scientists tracked down a genetic mutation that leads to blue eyes. "Originally, we all had brown eyes," said Hans Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen. Eiberg and colleagues showed 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. The authors concluded that the mutation may have arisen in a single individual in the Near East or around the Black Sea region 6,000–10,000 years ago during the Neolithic revolution. Eiberg stated, "A genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a 'switch,' which literally 'turned off' the ability to produce brown eyes."
The genetic switch is located in the gene adjacent to OCA2 and rather than completely turning off the gene, the switch limits its action, which reduces the production of melanin in the iris. In effect, the turned-down switch diluted brown eyes to blue. If the OCA2 gene had been completely shut down, our hair, pubes, eyes and skin would be melanin-less, a condition known as albinism.
Blue eyes are most common in Northern Europe and Central Europe and to a lesser degree in Southern Europe and southern Central Asia; Afghanistan is a notable example. They are also found in parts of North Africa, West Asia, and South Asia, in particular the northern areas of India and Pakistan. It rarely occurs as far south as Sri Lanka.
A 2002 study found that the prevalence of blue eye color among Caucasians in the United States to be 33.8 percent for those born from 1936 through 1951 compared with 57.4 percent for those born from 1899 through 1905. Blue eyes have become increasingly rare among American children, with only one out of every six or 16.6%, which is 49.8 million out of 300 million (22.4% of white Americans) of the total United States population having blue eyes. The plunge in the past few decades has taken place at a remarkable rate. A century ago, 80 percent of people married within their ethnic group. Blue eyes were routinely passed down, especially among people of Western and Northern European ancestry.
The outer surface of the iris of a blue-eyed person is actually clear, lacking the outer layer of pigmentation that is found in brown eyes. Their color is caused by the inner layer of pigmentation and the semi-opaque fibrous tissues that lie between the two layers."
