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Color blindness in some form affects up to 4-7% of all males. It occurs most often in boys because the genes for red and green color vision are carried on the X chromosome (that is, color blindness is X-linked). It is thus rare in girls; they carry the gene for color blindness but unless they possess two defective genes, they have normal color vision.
Color vision testing is not routinely done (or necessary) in younger children, but the issue may arise when a young child seems to have difficulty learning colors but seems developmentally normal in other ways. Color vision can be tested as soon as a child can name or trace out test symbols such as X's and O's or geometric shapes.
There are several variations of color blindness. They are caused by production of abnormal red, green, or blue cone pigments in the retinal cone cells. Total red (protanopia) or green (deuteranopia) color blindness affects 2-3% of males. Total blue blindness is referred to as tritanopia and is exceedingly rare, because it is carried on a non-sex chromosome.
Achromatopsia is the total lack of color vision associated with poor visual acuity, nystagmus, and photophobia - the avoidance of bright light. It is more rare still.
Rarely, a change in color discrimination can be a sign of optic nerve or retinal disease, but this is very rare in childhood and would probably be discovered after other visual problems were noticed.