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Color blindness is generally a congenital genetic disorder where an individual cannot distinguish between two colors under normal lighting conditions. It can rarely be caused by damage to the pigments of the eye.
The eye depends on pigment cells called "cones" to distinguish between different frequencies of light. The three types of cones are sensitive to one of three general wavelengths - red, green and blue. If these are not coded correctly on one of the mother's X chromosomes, there is no corresponding correct copy on the father's Y chromosome. As such, color blindness is overwhelmingly more common in males.
True color blindness where a person essentially sees in black and white is rare. Most common is the inability to properly distinguish between green and red, although other combinations are possible. As a rule, the retina has far more blue sensitive cells than any other kind (the red and green tend to be concentrated at the center of the retina), and inability to distinguish blue is rather more rare.
Color blindness is very common, affecting about 5% of males. Persons who study evolutionary theory believe the reason why this deficiency has persisted (it is one of the most common genetic disorders of any kind) is because the different balance of cone cells with their light sensitive counterparts, "rods", which are color blind, give human beings a wide variation of vision. For example, most animals with very poor color vision, such as most carnivores, see much better in low light because cones require a great deal of light compared to rods. One of the studies that gives credence to this theory pitted color blind persons against people with normal vision in spotting things under camouflage - color blind persons were far better at this task.