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The gene for the “short” wavelength cone† is located on chromosome 7, whereas the genes for the others are located on the X chromosome. Since females have two X chromosomes, it is possible for them to have 2 variant copies of the long or medium cone genes. (Since males only have one X chromosome, color vision deficiencies caused by a missing or abnormal copy of one of the medium/long cone pigment genes are much more common than for females.)

It would certainly be possible for a genetic mutation to result in an extra mutated copy of the short cone pigment gene, but I don’t think I’ve heard of variants of this gene with slightly different light sensitivity than the usual type (they might exist though?). By contrast “non-standard” version of the medium cone pigment gene is relatively common.

See: http://ghr.nlm.nih.gov/gene/OPN1SW http://ghr.nlm.nih.gov/gene/OPN1MW http://ghr.nlm.nih.gov/gene/OPN1MW2 http://ghr.nlm.nih.gov/gene/OPN1LW

† We should properly refer to the cones as long, medium, and short rather than red, green, and blue, since color is computed from differences in cone responses.

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