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  1. #341
    null Jonny's Avatar
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    Quote Originally Posted by Salomé View Post
    Your conclusions attributing this to a faster evolving Y chromosome are just wrong, however.
    I recommend Matt Ridley's Genome and The Red Queen as a good layman's introduction to the subject.
    Would you please give me the gist of your reasoning for dismissing the effects of a chromosome that is prone to more frequent genetic mutations? Consider two separate populations. One population has very frequent mutations, one does not. Would it not follow that the population with more frequent mutations would, on average, be subject to more frequent phenotypical changes and, to the extent that these changes don't lead to such a population's quick demise, be more greatly variable. Applying that same logic to men and women, who differ in only the Y chromosome, this seems reasonable.

    I am by no means suggesting that it is solely attributable to this, only that it might play a roll, given the complex nature of genetics and their role in human development. The presence of only one X chromosome might also have an effect as well, as I stated previously.

    Suppose that a particular skillset is linked with the X chromosome. Further suppose that we can quantify the relative "skilledness" associated with an X chromosome.

    A man has an X chromosome with a skill level of 0.4. A woman has two X chromosomes, one with a skill level of 0.9, and one with a skill level of 0.3; we can consider the woman's skill level roughly the average 0.6.

    They have a daughter. She has a

    50% chance of getting these two: 0.4 and 0.9 ~ 0.65
    50% chance of getting these two: 0.4 and 0.3 ~ 0.35

    Thus, the variance would be 0.0225.

    They have a son. He has a

    50% chance of getting: 0.9
    50% chance of getting: 0.3

    Thus, the variance would be 0.09.

    Does this make sense?
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  2. #342
    meh Salomé's Avatar
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    No, it doesn't make sense, because that's not how heredity works. At all.
    Quote Originally Posted by Jonnyboy View Post
    Would you please give me the gist of your reasoning for dismissing the effects of a chromosome that is prone to more frequent genetic mutations?
    Genetic mutations are deleterious more often than not.
    The Y chromosome evolves only in men.
    The X chromosome evolves in both men and women - twice as many opportunities for advantageous evolution, and a built in corrective mechanism (redundancy) to weed out harmful ones.

    It has been shown that there is a kind of "arms race" between these chromosomes which the X wins, because of its universality.
    Without the ability to recombine during meiosis, the Y chromosome is unable to expose individual alleles to natural selection. Deleterious alleles are allowed to "hitchhike" with beneficial neighbors, thus propagating maladapted alleles in to the next generation. Conversely, advantageous alleles may be selected against if they are surrounded by harmful alleles (background selection). Due to this inability to sort through its gene content, the Y chromosome is particularly prone to the accumulation of "junk" DNA. Massive accumulations of retrotransposable elements are scattered throughout the Y.[10] The random insertion of DNA segments often disrupts encoded gene sequences and renders them nonfunctional. However, the Y chromosome has no way of weeding out these "jumping genes". Without the ability to isolate alleles, selection cannot effectively act upon them.
    A clear, quantitative indication of this inefficiency is the entropy rate of the Y chromosome. Whereas all other chromosomes in the human genome have entropy rates of 1.5–1.9 bits per nucleotide (compared to the theoretical maximum of exactly 2 for no redundancy), the Y chromosome's entropy rate is only 0.84.[17] This means the Y chromosome has a much lower information content relative to its overall length; it is more redundant.

    Genetic drift
    Even if a well adapted Y chromosome manages to maintain genetic activity by avoiding mutation accumulation, there is no guarantee it will be passed down to the next generation. The population size of the Y chromosome is inherently limited to 1/4 that of autosomes: diploid organisms contain two copies of autosomal chromosomes while only half the population contains 1 Y chromosome. Thus, genetic drift is an exceptionally strong force acting upon the Y chromosome. Through sheer random assortment, an adult male may never pass on his Y chromosome if he only has female offspring. Thus, although a male may have a well adapted Y chromosome free of excessive mutation, it may never make it in to the next gene pool.[10] The repeat random loss of well-adapted Y chromosomes, coupled with the tendency of the Y chromosome to evolve to have more deleterious mutations rather than less for reasons described above, contributes to the species-wide degeneration of Y chromosomes through Muller's ratchet.[18]
    That's why the Y is shrinking. It only really contains the SRY gene (plus a lot of "junk") which is basically a "switch" for maleness. Everything important happens on X.
    The SRY gene is remarkably consistent between men. There are virtually no point mutations in the human race.
    Quote Originally Posted by Ivy View Post
    Gosh, the world looks so small from up here on my high horse of menstruation.

  3. #343
    insert random title here Randomnity's Avatar
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    Quote Originally Posted by Salomé View Post
    I recommend Matt Ridley's Genome and The Red Queen as a good layman's introduction to the subject.
    Even as a biologist, the red queen was excellent.
    -end of thread-

  4. #344
    null Jonny's Avatar
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    Quote Originally Posted by Salomé View Post
    No, it doesn't make sense, because that's not how heredity works. At all.Genetic mutations are deleterious more often than not.
    The Y chromosome evolves only in men.
    The X chromosome evolves in both men and women - twice as many opportunities for advantageous evolution, and a built in corrective mechanism (redundancy) to weed out harmful ones.

    It has been shown that there is a kind of "arms race" between these chromosomes which the X wins, because of its universality.


    That's why the Y is shrinking. It only really contains the SRY gene (plus a lot of "junk") which is basically a "switch" for maleness. Everything important happens on X.
    The SRY gene is remarkably consistent between men. There are virtually no point mutations in the human race.
    http://web.mit.edu/newsoffice/2010/y...some-0114.html
    This article seems to contradict some of what you said, am I wrong? I'd also like to note that this study was done after those sourced in the Wikipedia sections you quoted.

    Are there really twice as many opportunities for advantageous evolution? I would guess there would be 3 times as many. Twice in a woman (well, actually two different chromosomes evolving at once, but the logic is the same as what you used to claim twice as many), and once in a man.

    Here's an analogy for you: a rock is given 50x the opportunities as a Human, and yet the rock achieves nothing. Opportunities, in conjunction with one's ability to seize them, determine outcomes. To the extent that the Y chromosome is experiencing a faster form of genetic mutation, it needn't have the same number of opportunities to achieve greater variation. I'm not saying it actually is, but more opportunities =/= faster evolution if all else isn't equal.

    In any event, I think the fact that the entirety of any X-chromosome evolution that does occur is recognized in a man vs. the mitigated recognition in a woman might be the primary cause of the variance. What say you about this?
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  5. #345
    null Jonny's Avatar
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    Quote Originally Posted by Salomé View Post
    No, it doesn't make sense, because that's not how heredity works. At all.Genetic mutations are deleterious more often than not.
    Rereading this I'm slightly horrified. I figured what I said might have been a bit unclear. Did you think I meant that a guy gets 50% of each of his mom's X chromosomes (and something slightly analogous for a girl)? If I'd actually meant that, there would be no variance at all. I can't believe you think me so ignorant, haha. I realize what I said was unclear though, so I've (hopefully) corrected the misunderstanding with an edit.

    Here's how I feel (sort of):

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  6. #346
    meh Salomé's Avatar
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    The level of ignorance I encounter on this board ceased to surprise me a long time ago.
    Quote Originally Posted by Ivy View Post
    Gosh, the world looks so small from up here on my high horse of menstruation.

  7. #347
    meh Salomé's Avatar
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    Quote Originally Posted by Jonnyboy View Post
    To the extent that men have a healthy X chromosome, the fact that they don't have another one in the mix to potentially offset the problems inherent in the first one is immaterial.
    Not immaterial. In fact, "fragile-X" is most common inherited cause of mental retardation and affects males much more severely (because they only have 1 X chromosome.)

    http://en.wikipedia.org/wiki/Fragile_X_syndrome
    Quote Originally Posted by Ivy View Post
    Gosh, the world looks so small from up here on my high horse of menstruation.

  8. #348
    null Jonny's Avatar
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    Quote Originally Posted by Salomé View Post
    Not immaterial. In fact, "fragile-X" is most common inherited cause of mental retardation and affects males much more severely (because they only have 1 X chromosome.)

    http://en.wikipedia.org/wiki/Fragile_X_syndrome
    Yeah, I understand that; it is, in effect, what I was saying. I said "to the extent that men have a healthy X chromosome..."
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  9. #349
    ^He pronks, too! Magic Poriferan's Avatar
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    That piece is incredibly stupid.

    To call the Y chromosome an altered X chromosome might make sense. Calling it broken is a very specific word choice with a value attachment. If you're like me, you think that a broken thing is a thing that fails to achieve a purpose, and you think natural purposes don't exist, so purpose being a subjectively attributed thing, the Y chromosome is broken only insofar as one defines the purpose of a Y chromosome as whatever a Y chromosome can't do as well as an X chromosome, but of course you could also define it as basically anything else with equal validity. Lateralus is right, this is a rhetorical choice of words used by someone looking to push an opinion in a non-insightful way. The other possibility is that it's used by someone who just wants to be provocative for the hell of it, but that doesn't appear to be the case here.

    The second thing is worse, though. It's the same problem, just done worse. It follows the logic that by whittling a stick into a spear I have "damaged" its ability to club people. Indeed, when certain saurians started developing flight, it dealt grave damage to their ability to walk. You could phrase it that way, it's just no more accurate than saying I've altered it, or that I've improved it. The choice of words does not exist for accuracy or insight, it exists to enforce a sentiment in exactly the same way as if I had said that the Y chromosome is an upgrade of the X chromosome. And why not? The writer tries to point out male related medical issues, but totally skips over this - " If it weren't for the fact that the Y chromosome also causes masculinization, and our society highly values masculinization," - like that fact of existence isn't anything worth investigating. It's amazing that a defective mutation not only biologically thrived but lead to dominance. It wouldn't be amazing for a mutation to do that, it's just amazing that a defective, "broken", "damaged" mutation did that. It must be proof that it's really an improvement!

    So we get this at the end.
    "But as with Heina’s statement, the outrage over Greg’s is directly attributable to challenging our societal notion that “male” equals “good”. Without that, we’d all already be able to see that if masculinization gains you something, it loses you something as well."

    It's basically that whole bit about how your disagreement is already invalid because it's presumed it will be driven by moral outrage rather than reason. Blah, blah, blah. More wanky rhetorical shit. But that last detail is interesting. With masculinization you gain something? Really? That already makes it sound pretty different from being broken and damaged. It already starts to suggest why those are stupid word choices and what a charade the whole thing is.

    So the last thought I leave you with, is that I believe by combining the reasons of the two analyses above, I could argue that we are all broken, damaged reptiles (which are of course just broken, damaged amphibians...)
    Last edited by Magic Poriferan; 12-29-2012 at 07:12 PM.
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  10. #350
    Mojibake sprinkles's Avatar
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    A chromosome is basically like a container or structure. It contains a sequence of genes, and not all chromosomes are created equal - not even ones of the same 'type'.

    So there's not JUST an X or Y chromosome. These are convenient labels for a structure that seems to have a specific function, but also has a lot of variance.

    For example, there's XX male syndrome where what appears to be a male has actually two full X chromosomes - and nothing else, no Y or anything - and they appear to be male because one of the X chromosomes obtained the SRY gene, which it shouldn't have normally. So the person ends up genetically female but phenotypically male.

    Edit:
    To make my point more clear because it seems from a certain rep that somebody didn't understand it -
    The Y chromosome really has little to do with maleness other than the fact that it contains SRY, and SRY itself is only pertinent to males in placental mammals.

    Yes the XY scheme generally works in humans, but it just so happens that it turned out that way. It's not some inherent law. It would be possible that the Y chromosome goes away entirely somehow one day - not likely, but possible. Genes really don't care, if they get slapped together in a way that works, then they work. With the right setup a female can self reproduce.

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