Eye color is determined by the abundance, density and distribution of melanin in the iris, being higher in dark eyes than in light ones. In addition, there are other extracellular factors that determine the absorption and scattering of light, which also contribute to give the iris its color. This is a genetically very complex human trait in which numerous genes are involved.
The iris is the colored ring that surrounds the pupil. It is a contractile membrane that opens or closes depending on the external light, thus regulating the amount of light entering the eyeball. The color of this iris is determined mainly by the abundance of melanin in the pigment epithelium, which is greater in brown eyes than in blue eyes, and by the density and distribution of melanocytic stromal cells. The proportions of the two forms of melanin, eumelanin and pheomelanin, within the iris, as well as the absorption and scattering of light by extracellular components are additional factors that give the iris its color. In brown eyes, the ratio of eumelanin/pheomelanin is higher, while in light eyes there is less of both and pheomelanin is relatively higher.
The possibilities of eye color are manifold, although in the world there is relative uniformity, with brown being the most common. However, in Europe the diversity is greater, with a large proportion of light eyes.
In some situations, eye color may vary. Often, newborns have a small amount of melanin in the anterior part of the iris which, as it is exposed to light, may increase and the eye color changes progressively towards its adult coloration. In some elderly people, pigmentation may reduce over the years and iris coloring may change slightly. In addition, some diseases or drugs may also cause variations in eye pigmentation.
Several studies have published the link between eye color and some diseases. For example, the relationship between eye color and intraocular pressure has been described as being higher in darker eyes. Also, the association of age-related macular degeneration with a lower degree of iris pigmentation has been proven.
13.5 million variants
Simcoe M, et al. Genome-wide association study in almost 195,000 individuals identifies 50 previously unidentified genetic loci for eye color. Sci Adv. 2021 Mar 10;7(11):eabd1239.
Mitchell R, Rochtchina E, Lee A, Wang JJ, Mitchell P; Blue Mountains Eye Study. Iris color and intraocular pressure: the Blue Mountains Eye Study. Am J Ophthalmol. 2003 Mar;135(3):384-6.
Frank RN, Puklin JE, Stock C, Canter LA. Race, iris color, and age-related macular degeneration. Trans Am Ophthalmol Soc. 2000;98:109-117.