Jan 25 2011

Tetrachromacy In Humans

Are you a tetrachromat? Probably not, but it is possible that the rare person is, with the super mutant power of enhanced color vision. OK – I would rather have Wolverine’s regeneration, but enhanced color vision would be cool.

Color vision in vertebrates is a result of the cones in the retina. Vertebrate retinas have two types of light-sensing neurons: rods see in black and white but have good light sensitivity, and so are specialized for low-light (night) vision. Cones are less sensitive than rods, but they respond to a specific range of wavelengths of light – i.e. color. By combining the color information from different cones with different wavelength sensitivities the brain is able to perceive a wide range of colors.

Different groups of vertebrates have different numbers of cones, and therefore a different range and ability to discriminate colors. Birds, for example, are tetrachromats – they have four different cones and can see farther into the ultraviolet than humans. In fact the common ancestor of tetrapod vertebrates was likely a tetrachromat. Most mammals are dichromats with only two cones. It is thought this reduction occurred during the early years of mammal evolution when our mammal ancestors were nocturnal and burrowing animals, and so needed night vision more than color vision.

Many primates, however, (including humans and our close relatives) are trichromats with three cones, and therefore have rich color vision, but not as good as birds. In fact our understanding of the genetics of cones and color vision provided yet another compelling line of evidence for evolution. Trichromatic primates do not have the same cones as their vertebrate ancestors. They did not regain one of the two cones that were previously lost. Mammals have two cones – an autosomal S-cone (a short wavelength sensitive cone), and an X-linked L/M cone (sensitive to median and long wavelength visible light and located on the X-chromosome).

Sometime after the divergence of new-world and old-world monkey, an old-world monkey ancestor underwent gene duplication of the X-linked cone gene. At first these genes would have been identical, but over time they diverged to become distinct cones with separated wavelength sensitivity. In humans these cone genes are 98% identical. The cones added sensitivity to red wavelengths and resulted in trichromacy.

The research into the evolution of color vision has also led to some interesting findings about human color vision specifically. It seems that humans have a significant degree of variability in the sensitivity of the cones. You have probably heard that some people are partially color blind, because it is standard (at least in the US) to test all school children for color blindness. But you may not have known that there is variability in the other direction as well, and that there are cases of tetrachromacy in humans.

One possible mechanism for this is that women may inherit two different versions of an X-linked gene for color vision. Women have two X-chromosomes, and in each cell one X-chromosome is inactivated essentially at random. So the retina would have a mixture (a mosaic) of cones from the two versions on the two different X-chromosomes, functionally producing four different cones in the retina.

In one study they found that most women with this condition did not demonstrate tetrachromacy on color vision tests – they still functionally were trichromats. This is likely due to the fact that the cones were not different enough. Although some hypothesize that the optic nerve or perhaps the brain combines the information from these distinct cones and treats them as one stream of color information. However, going against this hypothesis is the fact that 1 in 24 such women (according to one study) demonstrated four-dimensional (or tetrachromatic) color vision. This means that the optic nerve is capable of carrying tetrachromatic vision and the brain is capable of interpreting it.

There may be other mechanisms as well that could result in true tetrachromatic vision in humans. These cases demonstrate the plasticity of biology and the brain in particular. It also demonstrates that spontaneous mutations can result in the addition of function – in this case expanded color vision. Not only has this almost certainly happened in our evolutionary past, but it is happening today in living humans. This is not likely to result in the evolution of tetrachromacy in humans in general for two reasons. The first is that, in our modern society, there likely isn’t any selective advantage to tetrachromacy. Our primate ancestors probably benefited from trichromacy – the speculation being that it enabled them to forage for fruit and vegetables better. But unless we lived in a world dominated by fashion designers and painters, it’s hard to see how tetrachomacy would provide a significant survival advantages.

Second, humans are a large out-bred population. This does not mean that we are not evolving, but it makes it very unlikely that such a mutation will significantly spread throughout the population. It could by chance become prominent in an isolated population – the so-called founder effect. This has been demonstrated for inherited diseases, but can also occur with favorable mutations like tetrachromacy.

For now tetrachromacy remains in isolated individuals who are lucky enough to have their own mutant power.

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