Editing Iris (anatomy) (section)

===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.