> The crux of Jordan’s argument lay in the fact that the gene for our red and green cone types lies on the X chromosome. Since women have two X chromosomes, they could potentially carry two different versions of the gene, each encoding for a cone that is sensitive to slightly different parts of the spectrum. In addition to the other two, unaffected cones, they would therefore have four in total – making them a “tetrachromat”.
Why couln't this happen for either of the other cones?
The male equivalent is "Anomalous trichromacy" http://en.wikipedia.org/wiki/Color_blindness#Anomalous_trich.... You have one of the color receptors tuned to a slightly different frequency, so you see the word in a different way. (Which way is the correct way?) This condition is easy to spot with the standard Ishihara color test, so it's easy to measure the % of the population in these cases.
--- Green
The most common case of male anomalous trichromacy is the "wrong/unusual" green receptor case, so the most common case of woman tetracromat is one that has 2 copies of the usual blue receptor, 2 copies of the usual red receptor and 1 copy of the usual green receptor 1 copy of the unusual green receptor. This is the case discussed in the article.
To reduce the text size I'll denote this kind of tetracromat with BBGgRR, where the capital letter denote the usual color receptor and the noncapital letter denote the unusual color receptor. (This is unrelated to recessive and dominant genes that are usually denoted by capital and noncapital letters.)
--- Red
The second most common case of male anomalous trichromacy is the "wrong/unusual" red receptor case, so the seccond most common case of woman tetracromat is BBGGRr (1 copy of the usual red receptor an 1 copy of the unusual red receptor).
Obviously, you can have both mixed copies, so a woman can have BBGgRr and be a pentacromat. At least have the genes to "see" in a 5-dimensional color space. If she can use this ability is less clear.
--- Blue
The least common case of male anomalous trichromacy is the "wrong/unusual" blue receptor case, so the least common case of woman tetracromat is BbGGRR.
It can be mixed with the other cases, so a woman can be pentacromat or hexacromat.
The interesting part is that the blue gene is not in the X chromosome, so both male and females have two copies. So a male can be tetracromat BbG_R_ . But this chromosome is not inactivated as the 50% of the X chromosomes in females. So I'm not sure if it's possible to have a different kind of vision in this case.
--- It's more complicated
Actually you can have more than 1 copy of the color genes in each chromosome, and there are more than two variations of each gene, so it's more complicated.
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.
† 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.
Why couln't this happen for either of the other cones?