The X Chromosome, Barr Bodies and Color Blindness

Discussion in 'Biology & Genetics' started by invert_nexus, Jan 29, 2008.

  1. invert_nexus Ze do caixao Valued Senior Member

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    So.
    I have a question.

    Males are more prone to color blindness because the gene that, when defective, causes color blindness lies on the X Chromosome of which men have only one copy. Therefore, if the man's one X chromosome has that defective gene, then he's fucked.

    However, here's the issue. Women do have two X chromosomes, but only one of them is active. The other is wrapped up in heterochromatin and tucked away. The inactive X isn't completely inert. There are some genes that are still active, but very few. And, for the most part, these genes are pseudoautosomal genes, meaning that these are genes which are also expressed on the Y chromosome. So, the color blindness gene can't be one of these.

    Therefore, women basically also have only one X chromosome, just like men. But they are statistically less likely to be color blind?

    Is there a biological reason for this? Is the color blindness gene expressed on the inactive chromosome? Or is it a statistical reason? I.e. 3/4 of all X/Y genes in population are X because women have two while men have one. Despite the fact that only one is active, does this statistically confer an advantage on women in the color blindness issue?

    I'm kinda confused here.
     
  2. S.A.M. uniquely dreadful Valued Senior Member

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    72,822
    Suppose

    X=color
    x= color blind

    In females, each cell contains two X chromosomes. The X chromosome that is inactivated is determined randomly (the dots represent Barr bodies)

    [​IMG]

    A heterozygous woman with Xx will have some cells with the X active and some with the x active. The X will enable her to see color.
     
    Last edited: Jan 29, 2008
  3. invert_nexus Ze do caixao Valued Senior Member

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    9,686
    So, you're saying it's statistical? Because, in the end, they still only have one functioning X chromosome. Just like a man.
     
  4. invert_nexus Ze do caixao Valued Senior Member

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    I suppose you could say that women get two coin tosses where men only get one?
     
  5. S.A.M. uniquely dreadful Valued Senior Member

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    In the daughter cells that are formed, the pattern is fixed. ie all daughter cells of a pink inactivated will have a pink inactivated.

    Since inactivation occurs at the embryo stage there are quite a few randomisations.

    Be interesting to know how many functioning X you'd need to see a color.

    Or if different numbers of functioning X changed color perception.
     
  6. invert_nexus Ze do caixao Valued Senior Member

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    9,686
    Oh.
    Wait.

    So. The decision on which X to inactivate is made on a cell by cell basis? It's not made at an early enough stage of embryo formation that all subsequent daughter cells have the same X inactivated?

    That makes things even more random....

    Definitely statistical then? Men have only one X to choose from so they will always have the defective gene active while women have two to choose from and thus it's a numbers game.

    That makes sense.


    Wait:

    No. Now you're saying that the decision is made early in the process and thus the same X is inactivated over and over again...

    So. A question to ask is at what stage of embryo development does this occur? Do the different cell lines (I forget the categories... hemopoetic cells, etc...) have different x's active?
     
  7. S.A.M. uniquely dreadful Valued Senior Member

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    72,822
    The inactivation takes place randomly in the cells present between 10-15 weeks of gestation. I assume there are a few million cells around, and one would assume there is a 50% probability of getting an X over an x.:)

    Once the X (or x) is inactivated, subsequent daughter cells will carry on the chromosome from the initially inactivated cell.
     
    Last edited: Jan 29, 2008
  8. invert_nexus Ze do caixao Valued Senior Member

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    9,686
    Hmm.
    Tricky.

    Still, it's definitely a numbers racket. Just the variables are tricky.

    By the way, it's interesting to note that the embryo initially has the paternal X inactivated always. It's then reactivated at some point and then the random inactivation occurs.

    A point to ponder on this would be the time span between reactivation and reinactivation. How many generations of cells are created during this time with two functioning X's.
     
  9. Hercules Rockefeller voltage gated ion channel Moderator

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    2,716
    The concept of X-inactivation frequently causes problems because people assume that the same X chromosome is inactivated in all the cells of a female. This is not the case.

    During embryonic development, the decision to inactivate an X chromosome does not occur at the single cell stage. Rather it occurs at a later stage (I forget precisely at what stage) when the embryo contains a number of cells. The choice of X chromosome for inactivation is random from cell to cell. Thus, an adult female contains clonal patches of cells that have different X chromosomes inactivated.

    So, females generally manage to avoid the effects of recessive X-linked alleles despite the phenomenon of X inactivation because any given tissue/organ is likely to contain cells with varying X chromosome inactivation.

    However, females do occasionally suffer from X-linked disorders because due to the vagaries of cell division occasionally the cells of a given organ/tissue will have derived from early embryonic cells with (by chance) the same X chromosome inactivated. In this way, occasionally a female might have eyes (for example) that contain cells with mostly the same X chromosome inactivated.
     
  10. S.A.M. uniquely dreadful Valued Senior Member

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    I think the paternal X is inactivated always only in marsupials, but Hercules may know better?


    Also, I have to correct the timing.

    According to some studies, inactivation in a majority of tissues is seen by 5 weeks from conception
    http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1762755
     
  11. invert_nexus Ze do caixao Valued Senior Member

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    http://en.wikipedia.org/wiki/X-inactivation
     
  12. Hercules Rockefeller voltage gated ion channel Moderator

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    Ahhh, “mosaicism” was the term I was trying to think of!

    Case closed! :D
     
  13. invert_nexus Ze do caixao Valued Senior Member

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    Thanks, guys. Once the hurdle of thinking that the same X is inactivated throughout the body is over, it's a relatively simple concept.
     
  14. Roman Banned Banned

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    11,562
    Precisely. There's and age-old genetics question that asks how you would know the gender of a calico cat (the answer isn't looking at genitals).

    It's calico due to random pigment gene inactivation on the X chromosome in skin cells. Males are either black or orange, since they only get one pigment gene.
     
  15. iceaura Valued Senior Member

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    16,870
    This mosaic structure of women extends to the brain.

    There's two different genetic people in there, doing the thinking. Seems to explain a lot, no?
     

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