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, however it is a polygenic trait ( meaning that it is controlled by the interactions of several genes, not just one). 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 around the Black Sea region 6,000-10,000 years ago, except blue eyes with brown spots around the pupil who are not related to this mutation.
Blue eyes are mostly common throughout Northern and less in Eastern Europe. One survey estimated that nearly 90% of Icelanders have blue or green eyes. Blue eyes are common in Germany, Netherlands, Iceland, Austria, Sweden, Norway, Denmark, Finland, and Estonia.  They are also present in Southern Europe and rarely in India and in some population of Central Asia and Middle East especially in lebanon. They are seen as a common trait among the Pashtun and tajik community in the North, and East of Afghanistan. A 2002 study found the prevalence of blue eye color among Whites in the United States to be 33.8% for those born between 1936 and 1951 compared to 57.4% for those born between 1899 and 1905.
As melanin production generally increases during the first few years of life (especially if exposed to the sun), the blue eyes of some babies may darken as they get older.