quote:Originally posted by Tarrsk: [QB] Actually, Rabbit, it's not so clear-cut as "men have one X chromosome while women have two," because of dosage compensation. Many genes have deleterious effects when expressed at higher levels than "normal." In the case of the X chromosome, "normal" (at least in humans) happens to equal 1 active copy of the chromosome. So each cell in a woman's body actually only has one actively working X chromosome, just like in males. The second X chromosome in women is silenced of gene expression via a process called X-inactivation.
Gene inactivation doesn't happen with all genes. There are a wide range of ways that the body regulated the level of gene expression and there are many gene products that get made from both gene copies in diploid organisms. For example the gene for sickle cell anemia is also found on the X chromosome. Women who are carriers for sickle cell disease have both normal hemoglobin and sickle cell hemoglobin. The sickle cell hemoglobin renders the women less susceptible to malaria but because it is only ~half the total hemoglobin the hemoglobin the "polymerization" process that happens in sickle cell crisis doesn't happen in women who carry the gene.
There is evidence that something similar happens with colorvision. Women who carry both the red/green color blind gene and the normal gene express both types of cones. That's not speculation, its been confirmed.
Anecdotally, my interaction with men who test colorblind indicates that colorblindness isn't a simple good or bad issue, there is a lot of variability in how well those who test colorblind can see color. That suggests that there aren't just two types of genes for the green cones but rather a wide spectrum of variability. If this is true, most women will carry and express two at least slightly different green cone genes where as men will carry and express only one type of green cone gene. The same arguments apply to red cone genes as well.
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quote:Originally posted by scifibum: You said that women might have more types of cones, but I don't see how this would necessarily change perception. If they have normal red cones and colorblind red cones, what would the colorblind cones add to the perception? It seems likely that adding less red-sensitive cones to more red-sensitive cones wouldn't improve the overall perception; in fact allowing for the same density of cones it seems like it would reduce it on average. (If the science has been done on the differences in perception, rather than on genetic cone differences, you didn't mention it.).
As I understand it, the color-blind green cones aren't "less sensitive", their response band is shifted toward the yellow. This means that for colors between red and green there is less difference between the response of the red cones and the green cones in a anomalous trichromate colorblind individual than in a normal individual. This makes it harder for the individual to discern difference in the EM spectra that range.
On the other hand, a person who has all four types of cones, has an extra spectral channel and this will allow them to detect difference in the EM spectra that can not be detected with only three channels. Since the adsorption bands for the cones are quite broad, the advantage of a forth cone in between the red and green cones is probably not large, but it would certainly be detectable mathmatically at a reasonable signal to noise ratio.
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I just checked and I was wrong about sickle cell disease being sex linked, its autosomal recessive. I was however correct about expression of two types of hemoglobin in heterozygosis carriers of sickle cell disease. What I said about sickle does in fact hold true for other sex linked genetic disorders including anomalous trichromatic colorblindness.
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The argument is reasonable at the level of eyes and chemical responses, but it might fall down at the level of image processing in the brain. (The famous 'qualia', as it were.) It is true that in principle four sensors will give you a better fix than three, but it is not obvious that the brain pathways develop in such a way that this is true. I would want to see some experimental data.
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Here is a scientific report on the question. Tetrachromacy
In at least some women, tetrachromacy does in fact lead to verifiable advantages in color perception.
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That is interesting. I note that the what's true in physics seems true in biology as well, "the common language of science is Broken English". Posts: 10645 | Registered: Jul 2004
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