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What is Color Blindness?
An inability to distinguish between certain colors is termed color blindness. The colors seen by normal eyes are the result of combinations of three primary colors: red, green and blue. Each of these colors is detected by a particular type of nerve ending or receptor in the retina of the eye. Color blindness is caused by the absence of one of these nerve endings, and the sufferer is therefore unable to distinguish certain colors from others.
The most common form of color blindness is an absence of the receptors sensitive to red. People with this defect cannot distinguish between red and green. When red light enters the eye, it can stimulate only the green-sensitive receptors. However, If the nerve endings sensitive to green are missing, light of any color in the range from green to red can stimulate only the red-sensitive receptors, and again the person sees only one color within this range. When a person lacks either the red or the green receptors he is said to be 'red-green' color blind.
Color blindness is an inherited defect. It is much more common in men than in women because it is a 'sex-linked' characteristic; this means that the gene for this defect appears on the X chromosome.
As a male has only one X chromosome, all of the necessary information for color vision must appear on this one chromosome if he is not to be color blind.
Because the female has two X chromosomes, she can be a 'carrier' for the defect without herself being color blind. For a woman to be color blind, both X chromosomes must carry the defect; therefore, she must inherit the trait from both parents. The disability caused by color blindness can be so insignificant that many men are not aware that they have the defect until they are examined. Color blindness is diagnosed by presenting a patient with a series of charts in which colored spots have been arranged in the form of numbers and shapes, which will be misinterpreted by the color blind person.