This node was originally written when slashdot posted a story on it, but lacked some of the details those above are demanding.
Tetrachromatism is the condition where genetic expression in a female leads to there being four different color receptors instead of the standard three, which are red, green, and blue. Two researchers, Dr. Neitz at the Medical College of Wisconsin and Dr. Jordan currently at the University of Newcastle, are attempting to find women with this condition. Due to the unique characteristics of this genetic expression, women with this gene likely have color-blind male children. This is because the genes for green and red photoreceptors are found on the X chromosome. Men only have one X chromosome, which means that they only have one set of genes for those photoreceptors. Occasionally, X chromosomes swap genetic material with each other while undergoing meiosis - leading to slight alterations in these photoreceptors. This crossing-over can lead to the possibility of an X chromosome that, instead of a green and a red gene, instead express two different greens or two different reds. When combined with a "normal" X chromosome that has a red and a green receptor specified, this can lead to the ability to express four different receptors instead of three.
How do they test for this? (Paraphrased from the Red Herring article)
They believe they have, in fact, found at least one tetrachromat. How did they do this? An experiment was setup in which the subjects attempted to determine whether a pair of colored lights matched. Using a joystick, they blended two wavelengths of light to produce a match. In order to eliminate the blue photoreceptor, the hues were picked to lay outside its spectrum. (The blue photoreceptor does not appear on the X chromosome). The subjects would then try to reproduce it. Because a trichromat would have only two receptors to use, they would find a large number of colors that apparently "matched". However, the tetrachromats would produce a single, precise match every time. Two subjects were capable of doing this.
Why X inactivation is important (not from the article. I am not a geneticist, but I'm married to one, and we did discuss this subject...): To express tetrachromacy, what one has is an X chromosome with two slightly different greens or two slightly different reds and an X chromosome with a green and a red. Now, what X inactivation means is that some cells will rely on one of the chromosomes and some will rely on the other -- in other words, divergent expression.
Why would the brain be able to tell? First off, the brain notices color in the first place. There are animals without the ability to distinguish color -- why should we have it? There are even different versions of color blindness. IMhO, I think it would be fairly obvious to the brain when adjacent receptors function differently. Some reading indicates, interestingly, that the brain actually does assign more space to pattern recognition than wavelength.
For more information, check out the original slashdot article at http://slashdot.org/science/00/11/28/1536204.shtml
or the Red Herring
article it points to at http://www.redherring.com/mag/issue86/mag-mutant-86.html
, or learn about color vision genetics at http://www.arbor.edu/~michaelb/chroab2.htm
. There is a lot of information out there, actually; a search on Google
for color vision genetics
will lead to many links and examination of color vision in many species.