Eye color

Eye color is the visible coloration of the iris, produced mainly by melanin content and light scattering through iris tissue. It ranges from very light blue to dark brown, with green, hazel, and other appearances between.

Iris: The ring-shaped tissue surrounding the pupil that controls its size and contributes to eye color. Its layered tissue and pigment create the visible surface whose coloration defines eye color.
Human iris anatomy: The structure of the human iris, including its layers, muscles, and pigment-bearing cells. Iris layers determine where pigment and light scattering contribute to visible color.
OCA2: A gene that influences melanin production and is associated with variation in human pigmentation. Variants near OCA2 have a major influence on iris pigmentation, especially in European populations.
Ophthalmology: The medical specialty concerned with the diagnosis and treatment of eye disorders. Changes in iris color can be relevant signs in eye examinations and disease assessment.
Blue eyes: An eye-color appearance usually associated with low melanin in the iris and scattering of shorter-wavelength light. Blue is a common light-eye appearance produced largely without blue iris pigment.
Melanin: A group of pigments that absorb light and contribute to coloration in skin, hair, and eyes. The amount and distribution of melanin in the iris strongly influence how dark the eye appears.
Ocular melanocytes: Pigment-producing cells found in the eye, including in the iris and other ocular tissues. Their melanin production supplies much of the iris’s light-absorbing pigment.
HERC2: A gene containing regulatory DNA that affects expression of the nearby OCA2 gene. A well-studied HERC2 variant can alter OCA2 activity and strongly affect eye color.
Horner syndrome: A disorder caused by disruption of sympathetic nerves to the eye and face, often producing a smaller pupil and eyelid droop. In children, it can be associated with lighter iris pigmentation on the affected side.
Brown eyes: An eye-color appearance associated with relatively high melanin levels in the iris. Brown illustrates how greater iris melanin absorbs more light and produces darker coloration.

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