MadSci Network: Evolution

The color of the iris

Area: Evolution
Posted By: Stephen Schaeffer, Faculty Biology
Date: Tue Sep 10 13:57:58 1996
Message ID: 841498459.Ev

MadSci Network: Evolution

The iris of the eye is an extension of the choroid, a layer of cells that surrounds the eyeball. The retinal cells that are responsible for sensing light from the environment are located on top of the choroid cell layer. The choroid/iris derives its color from a pigment known as melanin. The pigment in the choroid layer prevents light reflection throughout the globe of the eyeball, much like the black paint found on bellows within a camera (pp 615 Guyton, A. C. 1977. Basic Human Physiology: Normal Function and Mechanisms of Disease. W. B. Saunders Co. Philadelphia). Albinos have visual difficulties because they carry a genetic mutation that prevents the synthesis of melanin pigments throughout the body including the iris and choroid. When an albino individual is exposed to a single discrete light source light, much more of the retina responds to the light stimulation than in a normal individual because light is reflected off of the unpigmented choroid layer resulting in overstimulation of the retinal cells and vision problems.

As you know, iris pigments vary among humans from highly pigmented or brown eyes to less pigmented or blue eyes. Eye color is likely to be a quantitative trait controlled by several genes responsible for the production and deposition of pigments to the iris and choroid. See a recent question to the Genetics section for more information on iris pigments. In addition, the Online Mendelian Inheritance in Man explains the latest thoughts on the genetics of eye color. Brown eyed people would be expected to have less problems with increased sensitivity to light than blue eyed people.

Lower light intensities in northern latitudes may explain why blue eyes are more prevalent. The overstimulation of the retina due to reduced melanin may be offset by the reduced light intensity in northern latitudes. On the other hand, higher light intensities in more equatorial latitudes may favor brown eyes because of the reduced reflectivity of the choroid layer. Melanin production in skin cells also protects cells from the damaging effects of the sun's rays. The production of melanin in skin cells is strongly associated with eye and hair color, i.e., individuals tend to have dark skin, brown hair, and brown eyes or light skin, blonde hair, and blue eyes.

Current Queue | Current Queue for Evolution | Evolution archives

Return to the MadSci Network

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci
MadSci Network
© Copyright 1996, Washington University. All rights reserved.