Color
blindness is the inability to recognize the
colors red, green, blue-violet, or some combination of these. Humans can
identify up to 8 million shades and tints. Inherited color
blindness is almost always caused by a genetic defect on the long arm of the X chromosome. It occurs primarily in men, who have only one X chromosome. About 8 percent of all men have color-deficient
vision. Women are often protected from the flawed gene because they have two X chromosomes. However, studies have shown small defects in the color vision of women who carry the gene.
Degenerative diseases or injury to the optic nerve or retina may also result in color blindness, usually to the colors yellow and blue. Color-vision testing cards and disks can determine the type and severity of the defect.
Color-blind people must take extra precautions, such as paying special attention at traffic signals. Many branches of the armed forces do not recruit color-blind people, and occupations such as airline piloting or fashion designing are not practical for them.