Eyes are the colored sensory organs that play an important role in the vision. The eye color is determined by the variation of genes in an individual. The color of the eyes is due to the presence of melanin in the iris. The amount of the melanin in the iris along with the presence of white collagen fibers produces green, blue or hazel color. Melanin is produced by the specialized cells called melanocytes and is stored within the compartments called melanosomes. The number of melanocytes is generally the same in all the individuals however the number of melanosomes and the quantity of melanin stored within melanosomes varies. The total quantity of melanin present determines the color of the eye. Many individuals have a lighter eye color that is due to the scattering of light rays while passing through the stroma of the iris.
The production, storage, and transport of melanin are under the control of certain genes. The color of the eyes is directly associated with the quantity of melanin present in the layers of iris. Brown color eyes are associated with large quantities of melanin in iris while blue color is due to a lesser amount of melanin in the iris. There are a total of eight genes that influence eye color. These include:
OCA2 gene: It is also known as melanosome transmembrane protein or as P gene. This gene is located on chromosome 15 and controls the blue/brown color spectrum. OCA2 gene produces a protein (P protein) that controls the formation and processing of melanin. Mutations in the OCA2 gene that lead to decreased production of P protein results in eye albinism. The individuals suffering from such condition will have lighter eye color and their capability of producing and storing melanin is lost. Sometimes non-disease causing alleles are also present that control the production of P proteins levels. The alleles that are responsible for increased P protein production results in brown color of eyes while the allele associated with the blue color of eyes decreases the P protein production.
HERC2 gene: This gene is present near the region of ORA2 gene and is also known as intron 86. This gene mainly controls the functioning of the ORA2 gene turning it on or off as needed.
Several other genes that control the melanin production and transport are ASIP, IRF4, TYR, TYRP1 genes. The functioning of OCA2 and HERC2 gene along with the other genes may increase the melanin content resulting in brown color or may reduce the melanin content resulting in the blue color of the eyes.
Some individuals have one eye color that partially or completely differs from another and this condition is often referred to as heterochromia. This condition results from the unequal distribution of melanin due to the mutation of a few cells in the iris. In this condition the color of eyes changes from blue to brown or blue to green.
Research is still being carried out to find out the genes responsible for gray and hazel color of eyes.