Human beings normally see in color. We are natural trichromats-- we have three different color receptors that permit us to see a range of colors far broader than many other mammals. Even most other primates (with the exception of old-world monkeys) have only two kinds of color receptors. We are not the top of the color vision pile though. Jumping spiders are natural tetrachromats, with four kinds of receptors, and while there are no known mammalian tetrachromats, there are believed to be tetrachromats among birds, insects, reptiles, and amphibians.
That mammalian exclusion may be about to change. Since 1993 scientists in Oxford and Cambridge have been looking for a few women compared to whom, we may all be color-blind. These women would be the first known mammalian tetrachromats. In an odd twist of fate, the same genetic glitch that creates color-blind males may create females with better-than-usual color vision.
The normal human retina's color receptors are tuned to green, blue, and red. Working together, the three give us our colorful view of the world. When one or more of those color receptors is missing the result is color-blindness. The genes for our red and green color receptors are located on the X-chromosome, giving women a redundant set of receptor genes. This is why men are far more prone to color-blindness than women. In order to be functionally color-blind a woman not only has to be missing a receptor gene on both X-chromosomes, it must be the same gene on each one. The chances of this happening are so slim that only 0.4% of the US female population is affected. By contrast male color-blindness is far more prevalent with 8% of the US male population affected - 95% of them with red or green receptor problems. Color-blindness makes it difficult or impossible to distinguish some colors, depending on which receptor is affected. The term color-blindness itself is somewhat of a misnomer, since color perception is altered, not eliminated. True color-blindness, wherein a person can distinguish no color at all, requires a malfunction of all three kinds of color receptors, and affects only 0.003% of the population regardless of gender.
The original problem that leads to color-blindness occurs in the process of meiosis, the creation of the human ovum or sperm – in this case the ovum. During meiosis each pair of chromosomes is split in half in preparation for receiving a new matching half when the egg is fertilized. The splitting process is not perfectly neat, however. Genes can blend and cross, which is normal, and sometimes they do it lopsidedly, which is not. When lopsided splitting occurs the genes' coding for the color-receptors can be affected. The genes for the red and green receptors lie right next to each other, and therefore are particularly prone to mismatching. If a mistake is made in meiosis, the X-chromosome in the egg may be missing the genes for either the red or green receptors. More rarely the genes may crisscross and the resultant chromosome will have two genes for a single receptor, be it red or green.
It’s this particular variant that makes the tetrachromat possible, for not all red receptors (or all green receptors) are identical. Normal genetic variation through the generations has meant that some are sensitive to slightly different wavelengths. In most instances a person would have only one red and one green receptor gene, so the variations would not make much difference – but what happens when a chromosome with two red receptor genes ends up with two different kinds?
This is where the tetrachromat becomes possible. A man with two red receptor genes, one normal, one modified, might have broader color vision than a normal color-blind man, but he would remain color-blind. A woman, on the other hand, with her redundant set of receptor genes, would have genes coding for not three kinds of receptors, but four.
The genetics are plain, but two questions remained. Would the new receptor be prevalent enough to alter the subjects’ vision? And would the brain be able to accommodate the additional input to produce truly superior color vision?
Dr. Gabriele Jordan of Cambridge University may have answered that one. She tested the color perception of fourteen women who each had at least one son with the right kind of color-blindness. She set up a test where the subjects had to manipulate and blend two wavelengths of colored light to produce any hue they liked. They then had to match their own results a second time. With normal color vision, several different combinations would match any given hue, with a tetrachromat the possible combinations to produce a visible match would be much reduced. Dr. Jordan reported that two of the fourteen women showed exactly the results she would have expected from a tetrachromat. At least one of the two women reports having a different sense of color from the people around her, with both better color matching and better color memory. While not completely conclusive, this initial study has so far provided our best candidates for natural human tetrachromats.
But even if we find our tetrachromats, they may not all be created equal. If the modified color receptor is sensitive to wavelengths very close to a normal receptor, then the tetrachromat would merely have slightly better color-vision. The further apart the wavelength sensitivities of the receptors, the more the tetrachromat’s vision would differ from the norm. So in all probability, even among tetrachromats few have a dramatically better color sense, but for that rare exception the world may truly be a more colorful place.