R/G colorblindness and barr bodies

The gene for R/G colorblindness ® is X-linked recessive. Females are much less likely than males to be R/G colorblind because they are more likely to be heterozygous than homozygous for the trait (R/r rather than r/r). Males only have one X chromosome, and so if they inherit the deleterious allele, they don’t have a second allele to counteract it (r/O).

Females, however, undergo X-inactivation, where one of their X chromosomes becomes wrapped in heterochromatin and tucked away as a Barr body. Genes on this chromosome (for the most part) are no longer expressed. Calico and tortoiseshell cats are a consequence of this phenomenon. Cats carry a gene for coat color on the X chromosome that encodes for orange or black pigment. X-inactivation causes half of the cat melanocytes to produce orange, and half to produce black. Since X-inactivation occurs very early in the development of the cat, each single cell will multiply to produce a splotch of cells with the same pigmentation, which is why calico and tortoiseshell cats have patchy rather than pointillist coloration. I believe a similar dispersal happens in human females with hypohidrotic ectodermal dysplasia, which results in (among other things) the absence of sweat glands on large patches of the skin.

Why does R/G colorblindness differ? It seems that carriers for the trait should have regions in their visual field which can distinguish between the two colors and regions that can’t. Or maybe one eye that can and one eye that can’t. Does X-inactivation occur after the structure of the eye is mostly in place, but before the skin has yet to form?

There are multiple genes that encode for proteins called opsins that detect color. All opsin genes are on the X chromosome. Colorblindness differs among individuals because some opsins are more integral to color than others. Colorblindness refers deletions or mutations in any of those genes. R/G differences is likely to be gender-specific as males don’t undergo X-inactivation. Your scenario of having overlapping visual fields may work in females since half of their X genes are inactivated but I don’t know for sure.

As for barr bodies, I remember them occuring after meiosis in females but I don’t think barr bodies occur in males. (I wish there was a head-scratching emoticon).

  • Honesty

There have, indeed, been examples of human women having one eye colorblind and one not, different parts of the visual field affected differently, and so forth. Testing for it can be a bit tricky, though.

Yeah, this isn’t really my field, but as a WAG, I’d say that it does happen as you suggest, but the brain is able to adapt to where you wouldn’t really notice it. If part of your eye is able to tell your brain that “hey, that shirt is green,” I bet the brain could fill in for the part that can’t tell the difference.

Nearly all the red-sensitive and green-sensitive cone cells are densely packed together in, or close to, the fovea, which corresponds to only about 2degrees of visual angle at the center of your visual field. The whole visual field is about 200 degrees, but in fact there is very little capacity for color vision over most of it. Although the blue-sensitive cones are distributed more widely, the vast majority of the of the receptors in the periphery are rods, which are not involved in color vision.

So, really you can only see things in full color when you look pretty much straight at them. I know it does not seem like that, but color vision in the periphery is largely an illusion. Part of the reason for the illusion is that normally we move our eyes about very easily and freely (and mostly unconsciously) which means that as soon as you think to check whether any part of the scene in front of you is colored, you automatically turn your eyes to look at it, focusing it onto the fovea, and so see its colors. I do not think that is the whole story, though, because I (and, I think, most people) find that if I deliberately hold my eyes as steady as I can, staring at the same spot, I still have the impression that the periphery of my visual field is colored. However, this is an illusion. We may “know” what the colors in the rest of the periphery are because we have previously looked, but we do not truly see them. You can demonstrate this lack of color and detail vision in the periphery to yourself by staring straight ahead, keeping your eyes as still as possible (it is impossible to hold your eyes totally still, but you can get near enough for this), and bringing an object, whose color you do not know (this is important, because otherwise you may “imagine” you can sense its color long before you really can), gradually into range of your peripheral vision, and then slowly further in towards the center of your gaze. You should be able to see that something is there long before you can say either what it is (unless you already know) or what color it is, and even when you can first detect the color, it will probably not seem nearly as vibrant as it does when it is right in front of you, fully foveated.

As far as the foveal area itself goes, in normally sighted people the red-sensitive and green-sensitive cones there are distributed very irregularly and unevenly (regardless of sex) and quite differently in different individuals. Although the blue sensitive cones are spaced fairly regularly, in some regions of the fovea there may be relatively large patches containing red cones but no green ones, and vice versa, or in other parts they may be more intermingled. It seems to be fairly random (but with a tendency for the same type to clump together). Furthermore, some people seem to have a noticeably higher proportion of the red-sensitive or the green sensitive type. But we are talking about people with perfectly normal color vision here. These differences between people’s retinas, and across an individual’s retina, do not seem to have any effect on a person’s ability to discriminate colors. This is probably because the eyes are in constant rapid motion (even when you think you are holding them still) so that any point in the visual field will have been focused on many different cone cells before the brain (to put things a bit crudely) has got around to figuring out the color at that point.

So, all in all, yes you are right that women will have their red-sensitive and green-sensitive cone cells unevenly distributed in their retinas, but men do too, and it does not matter a damn.

On the distribution of cones see:
Conway, B.R. (2009). Color Vision, Cones, and Color-Coding in the Cortex. The Neuroscientist, 15 #3, 247-290.
Roorda, A. & Williams, D.R. (1999). The Arrangement of the Three Cone Classes in the Living Human Eye. Nature, 397 #6719, 520-522.
[Both paywalled I am afraid, but the second has some amazing pictures of people’s retinas, showing the arrangement of the different classes of cone.]

On eye movements in general, and the, basic structure of the retina see:
Richardson, D.C. & Spivey, M.J. (2004). Eye Tracking. In G.E. Wnek and G.L. Bowlin (Eds.), Encyclopedia of Biomaterials and Biomedical Engineering (pp. 568-582). New York: Marcel Dekker. [PDF]

On the importance of the constant small movements the eyes make even when we are trying to hold them still (but not speciffically concerned with color vision) see:
Martinez-Conde, S. & Macknik, S.L. (2007). Windows on the Mind. Scientific American, August 2007, 56-63. [large and colorful PDF]
Martinez-Conde, S., Macknik, S.L., & Hubel, D.H. (2004). The Role of Fixational Eye Movements in Visual Perception. Nature Reviews: Neuroscience, 5, 229-240. [PDF]
Rucci, M. & Desbordes, G. (2003). Contributions of Fixational Eye Movements to the Discrimination of Briefly Presented Stimuli. Journal of Vision, 3 #11, Article 18, 852-864.

For some work dealing more specifically with the role of eye movements in color vision, see:
Bompas, A., & O’Regan, J.K. (2006). More evidence for sensorimotor adaptation in color perception. Journal of Vision, 6 #2, 145-153.

Wow, thanks for the responses. I definitely did not expect to get journal citations. And don’t worry about the paywall. I still mooch journal access from my alma mater.