52 ms·

Does anyone know how the X-inactivation thing works with the popular explanation for why women are less likely to be color-blind than men (ie: They have two co

Does anyone know how the X-inactivation thing works with the popular explanation for why women are less likely to be color-blind than men (ie: They have two copies of the gene for sensitivity to light of that wavelength, so both copies must be mutated in order to be insensitive)?

0xcde4c3db 10y ago
https://en.wikipedia.org/wiki/Sex_linkage https://en.wikipedia.org/wiki/Sex_linkage > X-linked recessive traits are expressed in all heterogametics, but are only expressed in those homogametics that are homozygous for the recessive allele. For example, an X-linked recessive allele in humans causes haemophilia, which is much more common in males than females because they are hemizygous (see zygosity) and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a less frequent event since the mutant allele is rare in the population.
- jessriedel 10y ago
  You've just restated the popular explanation, right? You haven't addressed aetherson's question about how to square this explanation with X-inactivation in women.
  - 0xcde4c3db 10y ago
    Yes, I think I misunderstood the question.
anonymfus 10y ago
They have 2 kinds of affected light sensitive cells, one with activated normal gene and one with mutated.
- phkahler 10y ago
  But the gene for the pigment is on the X-chromosome which is why women can have both variants and thus be tetrachormatic. But this would require both X's to be active unless this gene is also in the non-inactivated region.
azakai 10y ago
I believe it is exactly the mechanism described in the article: X-inactivation is incomplete. Some of the genes on the "inactivated" chromosome remain active. What is new in this article is that they connected specific genes known to be active, to genes relevant for cancer. In other words, the theory was that the remaining-active genes helped avoid cancer, color-blindness, etc., and the article provides strong evidence supporting that in the area of cancer, the genes are the right ones. Wikipedia suggests there are other theories too, like inactivating picking the "best" of the two options non-randomly, and the mosaic effect - it isn't the same X-chromosome inactivated everywhere, so e.g. some retinal cells might have one X and others the other. https://en.wikipedia.org/wiki/X-inactivation#Selection_of_one_active_X_chromosome https://en.wikipedia.org/wiki/X-inactivation#Selection_of_on...
jessriedel 10y ago
According to this (and contrary to the article?), X-inactivation is randomized between cells, so women carriers of the color blindness mutation have half the number of correctly functioning cone cells. http://biology.stackexchange.com/questions/41447/colorblindness-in-females-and-random-x-chromosome-inactivation http://biology.stackexchange.com/questions/41447/colorblindn... This is apparently sufficient for basically normal color vision.
skosuri 10y ago
I'll take a hack: 1. Imagine the retina as a field of neurons that are receptive to light. Imagine different types of neurons are sensitive to different types of light (RGB). Each person has two copies (or alleles) of the R, G, and B genes. The R & G genes are on the X-chromosome. 2. In a color-blind person, the R & G genes merge to form a R-G allele that is sensitive to some red-green combination. Thus, men who carry this R-G gene have cells that express B and R-G genes and thus are red-green colorblind (they can't distiguish between the two because they only have one type of receptor for the two). Remember they only have one copy of these R and G genes. 3. Women might carry the R-G allele, but in those cases their retina as a whole display R, G, B, and R-G. These people are known as heterochromats can can distinguish colors better than normal people. Note that X-inactivation leads to cells even having R-G allele, or R or G gene. 4. It's rare for a woman to have two R-G alleles. When they do, they are red-green colorblind.
- tbihl 10y ago
  In (3), are you describing what I've sometimes heard called tetrachromat? Is that now confidently considered extant? Last I'd heard, it was an exciting, but only slightly substantiated, theory, but I'd love to see more about it!
  - skosuri 10y ago
    Yes, I think you are right. I thought I remember it being heterochromat, but looking online all references seem to be to tetrachromat. In terms of evidence, I think prevalence is there, but there were some papers where they actually tested color perception differences (at least from wikipedia).
    - aetherson 10y ago
      Heterochromia iridum is the condition of having two irises of different colors, you may have blurred the name of that condition with tetrachromacy.

0xcde4c3db 10y ago

https://en.wikipedia.org/wiki/Sex_linkage https://en.wikipedia.org/wiki/Sex_linkage > X-linked recessive traits are expressed in all heterogametics, but are only expressed in those homogametics that are homozygous for the recessive allele. For example, an X-linked recessive allele in humans causes haemophilia, which is much more common in males than females because they are hemizygous (see zygosity) and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a less frequent event since the mutant allele is rare in the population.

jessriedel 10y ago
You've just restated the popular explanation, right? You haven't addressed aetherson's question about how to square this explanation with X-inactivation in women.
- 0xcde4c3db 10y ago
  Yes, I think I misunderstood the question.

anonymfus 10y ago

They have 2 kinds of affected light sensitive cells, one with activated normal gene and one with mutated.

phkahler 10y ago
But the gene for the pigment is on the X-chromosome which is why women can have both variants and thus be tetrachormatic. But this would require both X's to be active unless this gene is also in the non-inactivated region.

azakai 10y ago

I believe it is exactly the mechanism described in the article: X-inactivation is incomplete. Some of the genes on the "inactivated" chromosome remain active. What is new in this article is that they connected specific genes known to be active, to genes relevant for cancer. In other words, the theory was that the remaining-active genes helped avoid cancer, color-blindness, etc., and the article provides strong evidence supporting that in the area of cancer, the genes are the right ones. Wikipedia suggests there are other theories too, like inactivating picking the "best" of the two options non-randomly, and the mosaic effect - it isn't the same X-chromosome inactivated everywhere, so e.g. some retinal cells might have one X and others the other. https://en.wikipedia.org/wiki/X-inactivation#Selection_of_one_active_X_chromosome https://en.wikipedia.org/wiki/X-inactivation#Selection_of_on...

jessriedel 10y ago

According to this (and contrary to the article?), X-inactivation is randomized between cells, so women carriers of the color blindness mutation have half the number of correctly functioning cone cells. http://biology.stackexchange.com/questions/41447/colorblindness-in-females-and-random-x-chromosome-inactivation http://biology.stackexchange.com/questions/41447/colorblindn... This is apparently sufficient for basically normal color vision.

skosuri 10y ago

I'll take a hack: 1. Imagine the retina as a field of neurons that are receptive to light. Imagine different types of neurons are sensitive to different types of light (RGB). Each person has two copies (or alleles) of the R, G, and B genes. The R & G genes are on the X-chromosome. 2. In a color-blind person, the R & G genes merge to form a R-G allele that is sensitive to some red-green combination. Thus, men who carry this R-G gene have cells that express B and R-G genes and thus are red-green colorblind (they can't distiguish between the two because they only have one type of receptor for the two). Remember they only have one copy of these R and G genes. 3. Women might carry the R-G allele, but in those cases their retina as a whole display R, G, B, and R-G. These people are known as heterochromats can can distinguish colors better than normal people. Note that X-inactivation leads to cells even having R-G allele, or R or G gene. 4. It's rare for a woman to have two R-G alleles. When they do, they are red-green colorblind.

tbihl 10y ago
In (3), are you describing what I've sometimes heard called tetrachromat? Is that now confidently considered extant? Last I'd heard, it was an exciting, but only slightly substantiated, theory, but I'd love to see more about it!
- skosuri 10y ago
  Yes, I think you are right. I thought I remember it being heterochromat, but looking online all references seem to be to tetrachromat. In terms of evidence, I think prevalence is there, but there were some papers where they actually tested color perception differences (at least from wikipedia).
  - aetherson 10y ago
    Heterochromia iridum is the condition of having two irises of different colors, you may have blurred the name of that condition with tetrachromacy.