Genes contain the coding instructions for the pigments present in the cones, and if the coding instructions are wrong, the cones will be sensitive to different wavelengths of light (resulting in a color deficiency). There is also a very rare blue-yellow deficiency or weakness, which I myself experience, the result of a mid-life cerebral infarction (stroke). The eye with its multitude of rods and cones can be perfectly normal and healthy; the altered color perception is purely a result of neurological damage. A person who is red-weak sees less “red tone� in any color in terms of its depth of color and its brightness. For example, red, orange, yellow, and yellow-green, all appear more green and paler than when seen by a normal observer. The redness component that a normal observer sees in a violet, lavender, or purple color is so weakened that the color may look like a simple shade of blue. If the differences are slight these individuals may not be aware that their color perception is abnormal. Many go through life with very little difficulty doing tasks that require normal color vision. However, all three color sensitive cones are defective in Achromatopsia. This total color blindness occurs when two copies of the mutated genes that code for the disease are present. Congenital Achromatopsia is an extremely rare hereditary vision disorder that affects 1 person out of 33,000 in the United States.
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