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Color blindness is an inability to distinguish certain colors. By far the most common type is inherited red-green color blindness, which affects 8 percent of men and boys but only 0.5 percent of women and girls.
Total color blindness, which is very rare, and pastel-shade color blindness are believed to be inherited. Other types, including blue-yellow color blindness and red-green color blindness, can be either inherited or acquired. Disease or injury affecting the retina may also be the cause of color blindness.
Description
Color blindness results from a defect within the cone-shaped light-sensitive cells of the fovea (tiny depression in the macula area of the retina that is the center for perceiving color). The 7 million cone cells differ from the 130 million rod-shaped cells in the rest of the retina. While the rod cells register only black and white, the cone cells contain pigments for red, green, and blue -- the colors that can combine to produce all the colors of the spectrum.
The pigments become more vivid or fade in response to colors that the eye sees. The changes in pigmentation produce tiny flashes of electricity, which are carried by means of the optic nerve to the visual center of the brain. There, the electrical signals are combined into a full-color picture. In the color-blind person, the cones are missing or defective. No cure is known.
Inheriting Red-Green Color Blindness
Why is red-green color blindness inherited by boys more often than by girls? The reason is that the defective gene that directs production of the defective pigment is carried on the same pair of chromosomes that determines the sex of the child. In a female child (who has two X sex chromosomes), a defective gene on one X chromosome is almost always counteracted by a normal gene on the other X chromosome; as a result, the girl is born with normal color vision.
In a male child (who has an X and a Y sex chromosome), there is no matching normal gene to block the defect on the only X chromosome the boy has; the boy is, therefore, born color-blind.
Red-green color blindness cannot be passed from a father to his sons, nor will his daughters be color-blind, unless the mother carries the defective gene as well. However, his daughters will all be carriers of the defective gene, and the daughters' sons will have a 50 percent chance of being color-blind.
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Color Blindness
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Please copy/paste the following text to properly cite this How Stuff Works article:
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