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. This is the same frequency-dependence of scattering that accounts for the blue appearance of the sky. The result is a “”Tyndall blue”” structural color that varies with external lighting conditions.
In humans, 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 suggested 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 Eiberg. 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 probably living in the northwestern part of the Black Sea region (around modern Romania) 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.”” He added:
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, eyes and skin would be melanin-less, a
condition known as albinism.
Blue eyes are most common in Ireland, the Baltic Sea area and Northern Europe, and are also found in Eastern, Central, and Southern Europe. Blue eyes can also be found in parts of Central, South, and West Asia especially among the Jewish population of Israel. Many modern Israeli Jews are of European Ashkenazi
origin, among whom this trait is common (A study taken in 1911 found
that 53.7% of Jews in Galicia in Eastern Europe had blue eyes).
Y-Chromosome DNA testing performed on ancient Scythian skeletons dating to the Bronze and Iron Ages in the Siberian Krasnoyarsk region found that 10 out of the 11 subjects carried Y-DNA R1a1 (most commonly found today in Eastern Europe and Sikh clans of North India), with blue or green eye color and light hair common, suggesting mostly European origin of that particular population.
In Estonia, 99% of people have blue eyes, stated Hans Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen. In Denmark 30 years ago, only 8% of the population had brown eyes, though through immigration, today that number is about 11%. In Germany, about 75% have blue eyes.
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. As of 2006, one out of every six people, or 16.6% of the total population, and 22.3% of the white population, has blue eyes. Blue eyes are continuing to become less common among American children
Blue eyes are rare in mammals; one example is the quite recently discovered marsupial, the Blue-eyed Spotted Cuscus (Spilocuscus wilsoni). The trait is hitherto known only from a single primate other than humans – Sclater’s Lemur (Eulemur flavifrons) of Madagascar. While some cats and dogs have blue eyes, this is usually due to another mutation that is associated with deafness. But in cats alone, there are four identified gene mutations that produce blue eyes, some of which are associated with congenital neurological disorders. The mutation found in the Siamese cats
is associated with strabismus (crossed eyes). The mutation found in
blue-eyed solid white cats (where the coat color is caused by the gene
for “”epistatic white””) is associated with deafness. However, there are
phenotypically identical, but genotypically different, blue-eyed white
cats (where the coat color is caused by the gene for white spotting)
where the coat color is not strongly associated with deafness. In the
blue-eyed Ojos Azules
breed, there may be other neurological defects. Blue-eyed non-white
cats of unknown genotype also occur at random in the cat population”