Editing Iris (anatomy) (section)

==Eye color==
{{main|Eye color}}
{{human-centric}}
[[File:Eye colors map of Europe.png|thumb|Human eye pigmentation in Europe]]
[[Image:Ageev iris.jpg|thumb|right|Among human phenotypes, blue-green-gray eyes are a relatively rare{{Citation needed|date=May 2023}} eye color and the exact color is often perceived to vary according to its surroundings.]]
The iris is usually strongly [[pigment]]ed, with the [[color]] typically ranging between brown, hazel, green, gray, and blue. Occasionally, the color of the iris is due to a lack of pigmentation, as in the pinkish-white of oculocutaneous [[albinism]],<ref name="iris" /> or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, the only pigment that contributes substantially to normal iris color is the dark pigment [[melanin]]. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of an organism. Structurally, this huge molecule is only slightly different from its equivalent found in [[skin]] and [[hair]]. Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue- and green-eyed people. The [[limbal ring]] appears as a dark ring encircling the iris on some individuals, but is a result of the optical properties of the region between the [[cornea]] and [[sclera]], not of pigments in the iris.

===Genetic and physical factors determining iris color===
[[File:Human eye with limbal ring, anterior view.jpg|thumb|A light brown iris with prominent limbal ring. Light brown irises contain pheomelanin.]]
Iris color is a highly complex phenomenon consisting of the combined effects of texture, pigmentation, fibrous tissue, and blood vessels within the iris [[stroma of iris|stroma]], which together make up an individual's [[epigenetic]] constitution in this context.<ref name="iris1"/> An organism's "eye color" is actually the color of one's iris, the [[cornea]] being transparent and the white [[sclera]] entirely outside the area of interest.

Melanin is yellowish to dark hazel in the stromal pigment cells, and black in the [[iris pigment epithelium]], which lies in a thin but very opaque layer across the back of the iris. Most human irises also show a condensation of the brownish stromal melanin in the thin anterior border layer, which by its position has an overt influence on the overall color.<ref name="iris1"/> The degree of dispersion of the melanin, which is in subcellular bundles called [[melanosomes]], has some influence on the observed color, but melanosomes in the iris of humans and other vertebrates are not mobile, and the degree of pigment dispersion cannot be reversed. Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color (see [[heterochromia]], below). Colors other than brown or black are due to selective reflection and absorption from the other stromal components. Sometimes, [[lipofuscin]], a yellow "wear and tear" pigment, also enters into the visible eye color, especially in aged or diseased green eyes.{{citation needed|date=November 2022}}

The optical mechanisms by which the nonpigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature. Simple selective absorption and reflection by biological molecules ([[hemoglobin]] in the blood vessels, [[collagen]] in the vessel and stroma) is the most important element. [[Rayleigh scattering]] and [[Tyndall effect|Tyndall scattering]], (which also happen in the sky) and [[diffraction]] also occur. [[Raman scattering]], and constructive [[Interference (wave propagation)|interference]], as in the feathers of birds, do not contribute to the color of the eye, but interference phenomena are important in the brilliantly colored iris pigment cells ([[iridophore]]s) in many animals. Interference effects can occur at both molecular and light-microscopic scales, and are often associated (in melanin-bearing cells) with quasicrystalline formations, which enhance the optical effects. Interference is recognised by characteristic dependence of color on the angle of view, as seen in [[eyespot (mimicry)|eyespot]]s of some [[butterfly]] [[wing]]s, although the chemical components remain the same.
White babies are usually born blue-eyed since  no pigment is in the stroma, and their eyes appear blue due to scattering and selective absorption from the posterior epithelium. If melanin is deposited substantially,  brown or black color is seen; if not, they will remain blue or gray.<ref name="eb">"Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 175 Encyclopædia Britannica, 1987</ref>

All the contributing factors towards eye color and its variation are not fully understood. Autosomal recessive/dominant traits in iris color are inherent in other species, but coloration can follow a different pattern.

===Different colors in the two eyes===
{{main|Heterochromia}}
[[Image:Heterochromia.jpg|thumb|right|240px|Example of [[heterochromia]] – one eye of the subject is brown, the other hazel.]]
Heterochromia (also known as a heterochromia iridis or heterochromia iridum) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). Uncommon in humans, it is often an indicator of ocular disease, such as chronic iritis or diffuse iris melanoma, but may also occur as a normal variant. Sectors or patches of strikingly different colors in the same iris are less common. [[Anastasius I of the Byzantine Empire|Anastasius the First]] was dubbed ''dikoros'' (having two irises) for his patent heterochromia since his right iris had a darker color than the left one.<ref>{{Cite journal|last=Baldwin|first=Barry|title=Physical Descriptions of Byzantine Emperors|date=1981|journal=Byzantion|volume=51|issue=1|pages=8–21|jstor=44170668|issn=0378-2506}}</ref><ref>{{Cite journal|last1=Fronimopoulos|first1=John|last2=Lascaratos|first2=John|date=1992-03-01|title=Some Byzantine chroniclers and historians on ophthalmological topics|journal=Documenta Ophthalmologica|language=en|volume=81|issue=1|pages=121–132|doi=10.1007/BF00155022|pmid=1473460|s2cid=26240821|issn=1573-2622}}</ref>

In contrast, heterochromia and variegated iris patterns are common in veterinary practice. [[Siberian Husky]] dogs show heterochromia,<ref name="hc">{{Cite web |url=http://www.environmentalgraffiti.com/featured/heterochromia-animals/18455 |title = Heterochromia in Animals | website = Environmental Graffiti | last = Fabricius | first = Karl |access-date=2010-10-27 |archive-url=https://web.archive.org/web/20100923005934/http://www.environmentalgraffiti.com/featured/heterochromia-animals/18455 |archive-date=2010-09-23 |url-status=dead }}</ref>{{better source needed|reason=Original source lists three Wikipedia pages as sources, thus a possible source of [[citogenesis]]|date=March 2021}} possibly analogous to the genetically determined [[Waardenburg syndrome]] of humans. Some white cat fancies (e.g., white [[Turkish Angora]] or white [[Turkish Van]] cats) may show striking heterochromia, with the most common pattern being one uniformly blue, the other copper, orange, yellow, or green.<ref name="hc"/> Striking variation within the same iris is also common in some animals, and is the norm in some species. Several herding breeds, particularly those with a [[Merle (coat color in dogs)|blue merle]] coat color (such as [[Australian Shepherd]]s and [[Border Collie]]s) may show well-defined blue areas within a brown iris, as well as separate blue and darker eyes.{{Citation needed|date=November 2010}} Some horses (usually within the white, spotted, palomino, or cremello groups of breeds) may show amber, brown, white and blue all within the same eye, without any sign of eye disease.{{Citation needed|date=November 2010}}

One eye with a white or bluish-white iris is also known as a "walleye".<ref>"walleye", def. 1a, Merriam-Webster Dictionary</ref>