Color blindness, also called color vision deficiency, is a condition caused by a defect in the eye that makes an individual unable to distinguish various colors and shades.
Red means stop. Green means go. It is one of the earliest lessons a child learns. For millions of people, however, telling the difference between these two colors is not as simple as it sounds. These people usually are called color blind, although it is more accurate to say that they have poor color vision, since only a few people are unable to see any color at all.
There are three kinds of color blindness. The most common is the inability to distinguish red from green. This is called red-green color blindness. Many fewer people have blue-yellow color blindness, and even fewer see no color at all.
People with color blindness are most often born with the disorder. The condition usually is inherited, although in older people, color blindness can be a side effect of taking certain prescription drugs. The gene * that causes color blindness is almost always inherited from the mother.
The gene that causes most color blindness is on the X chromosome. Girls have two X chromosomes (XX), one from each parent. Boys have one X chromosome from the mother and one Y chromosome from the father (XY). In order for a person to be color blind, all the X chromosomes must carry the gene that causes the disorder. This means a girl must have two X chromosomes with the color-blind gene, while a boy needs only one—the X chromosome he got from his mother. Because of this, color blindness affects boys more often than girls. About 1 in 12 boys have some form of color blindness, but only about 1 in 200 girls have the disorder. People of Northern European descent have the highest rate of color blindness. Most people with color blindness have no other eye defects.
As light passes through the eye an image is focused on the retina. The retina contains layers of cells at the inside rear of the eyeball and contains millions of receptors called rods and cones. Rods detect the intensity of light. Although humans have about seven million cones, the cones come in only three types, each of which is particularly sensitive to a different wavelength of light corresponding to the colors blue, green, or red. When cones respond, they release light-sensitive chemicals that trigger electrical signals that travel along nerves called optic tracts to the part of the brain known as the visual cortex. Rather like the way an artist mixes different paint colors to create a vast array of hues, the visual cortex combines the information it receives from all the cones and rods to produce the large range of colors—about eight million of them—that humans can see.
People with poor color vision have cones that do not function properly because they do not release some of the light-sensitive chemicals when light strikes them. Alternately, they may have too few cones that detect a certain color. As a result, these people see only certain colors and shades. The most common form of color blindness is a difficulty in distinguishing red from green, but people with red-green color blindness can distinguish other colors, such as blue and yellow. Color blindness is not an all or nothing phenomenon. It can be mild, moderate, or severe. Some people can differentiate colors in strong light that they cannot tell apart in dim light. Some people with red-green color deficit are able to tell red from green but only because green appears to be a little lighter than red rather than an entirely different color.
The first signs of poor color vision often appear when a child enters school and begins to learn to identify colors. A simple vision test can determine if the problem is color blindness. This test involves showing a child an image made up of dots, for instance, a number made up of green dots on a background of yellow and orange dots. If the child cannot see the green number distinctly because it appears to blend in with the background, he or she may have color blindness.
There is no treatment or cure for color blindness. People often learn to adapt and cope with their color blindness in various ways. They might note that traffic lights usually have the red light on top and green on the bottom or they might code their clothing in some way so that they can tell which pieces go harmoniously together.
See also Blindness
Genetics Home Reference. “Color Vision Deficiency.” U.S. National Library of Medicine. https://ghr.nlm.nih.gov/condition/color-visiondeficiency (accessed March 20, 2016)
MedlinePlus. “Color Blindness.” U.S. National Library of Medicine. https://www.nlm.nih.gov/medlineplus/colorblindness.html (accessed March 20, 2016).
American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120-7424. Telephone: 415-561-8500. Website: http://www.aao.org (accessed June 29, 2016).
American Optometric Association. 243 N. Lindbergh Blvd., Floor 1, St. Louis. MO 63141. Toll-free: 800-365-2219. Website: http://www.aoa.org (accessed June 29, 2016).
National Eye Institute Information Office. 31 Center Drive MSC 2510, Bethesda, MD 20892-2510. Telephone: 301-496-5248. Website: http://www.nei.nih.gov (accessed June 29, 2016).
* gene (JEEN) is a chemical structure composed of deoxyribonucleic acid (DNA) that helps determine a person's body structure and physical characteristics. Inherited from a person's parents, genes are contained in the chromosomes found in the body's cells.