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Brown irises contain more or less melanin. Some eyes have a dark ring around the iris, called a limbal ring. Eye color in non-human animals is regulated differently. For example, instead of blue as in humans, autosomal recessive eye color in the skink species Corucia zebrata is black, and the autosomal dominant color is yellow-green. [101]
Approximately, only six in every 10,000 people in the United States have two different colored eyes—but it's certainly possible. Heterochromia iridum is the name of this rare phenomenon.
Although infrequently seen in humans, complete heterochromia is more frequently observed in species of domesticated mammals. The blue eye occurs within a white spot, where melanin is absent from the skin and hair (see Leucism). These species include the cat, particularly breeds such as Turkish Van, Khao Manee and (rarely) Japanese Bobtail.
Waardenburg syndrome is a group of rare genetic conditions characterised by at least some degree of congenital hearing loss and pigmentation deficiencies, which can include bright blue eyes (or one blue eye and one brown eye), a white forelock or patches of light skin.
According to CNN, Dr. Gary Heiting, a licensed optometrist and senior editor of All About Vision explained why all human eyes are actually brown, no matter if they look blue or greenish.
Hazel eyes tend to change colors due to Rayleigh scattering—the same factor that makes the sky appear blue. This optical effect occurs in the stroma, which is a thin layer of tissue in front of ...
The cooperative eye hypothesis is not the only one that has been proposed to explain the appearance of the human eye. Other hypotheses include the proposal that white sclerae are a sign of good health, useful in mate selection , or that eye visibility promotes altruistic behaviour by letting people know they are being watched.
The eye develops in utero during the first three months of pregnancy, and a gap called the choroidal fissure appears at the bottom of the stalks that eventually form the eye, the AAO explains.