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Color vision occurs because the retina of the human eye has both rod cells, which distinguish light in general, and cone cells, which are highly sensitive only to specific wavelengths. While there are three groups of peak wavelength sensitivity, any given cone cell will only respond to a single peak. This occurs because the cone cell is rich in a pigment which is sensitive to decomposition at a single wavelength.
Different genes code for the different pigments, but all the genes are located on the X chromosome. There are several common mutations known, each of which causes one of these genes to be unable to code for proteins which produce a functional pigment. These genes are recessive, so a person carrying a defective gene can, if they have a second, correct copy of the gene, still make the pigment. This is true in females, who can thus be carriers. However in males, who have one X and one Y chromosome, there is only one version of each of the pigment encoding genes present. If it is defective, the person is color blind.
Consequently, color blindness is not a disease, and cannot be cured. At present no way has been found to induce an affected person's retinal cells to produce correctly formed pigment molecules. Since the pigments must be produced constantly and are only found in the cones of the retina there is no practical method of providing these molecules as supplements. Perhaps at some point in the future gene therapy will be available to remedy this common genetic problem.
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