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So is the second X chromosome in women supposed to be an exact copy of the first or do they differ? That's fascinating to me all by itself, and more so that so

So is the second X chromosome in women supposed to be an exact copy of the first or do they differ? That's fascinating to me all by itself, and more so that some genes in the dupe can reinforce the first even though the rest of the chromosome is suppressed. EDIT: Ah, wikipedia explains that the two X chromosomes in a standard-issue woman come from the mother and paternal grandmother, and then ... "Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells (cells other than egg and sperm cells). This phenomenon is called X-inactivation or Lyonization, and creates a Barr body. If X-inactivation in the somatic cell meant a complete de-functionalizing of one of the X-chromosomes, it would ensure that females, like males, had only one functional copy of the X chromosome in each somatic cell. This was previously assumed to be the case. However, recent research suggests that the Barr body may be more biologically active than was previously supposed." Which must be related to the EXITS that the OP refers to. Being a male, I'm troubled by the fitness of the Y-chromosome over the generations. It seems to me that it can only accumulate errors and mutations. However, I suppose the same could be said about the mitochondria that come exclusively from females. This partial double-X scheme seems like a useful way of mixing up traits intrageneration and maybe strengthen healthful cell behaviors.

incongruity 10y ago
> It seems to me that it can only accumulate errors and mutations. Except some of those mutations can be useful – some deadly, but some could be selected for as one's environment/niche shifts. That's evolution and selection in a nutshell. Not all mutations are bad and not all bad mutations are always bad.
- coredog64 10y ago
  Concrete example: Sickle cell anemia is bad, but sickle cell carriers also have protection against malaria.
- EdSharkey 10y ago
  Yes, truly over the long haul, the fittest survive - helpful mutations account for all survival thus far. I just envy what I interpret as an intragenerational bolstering of genetic traits that females have, assuming the EXITS thing is truly what's going on. They don't have to wait a generation to perhaps select out the bad, they get to do it on the fly.
  - incongruity 10y ago
    It's a conservative strategy -change from one generation to another will likely be moderated/averaged by the influence of the second X chromosome for female offspring. Alternatively, single copy governed traits (as in males) mean greater ability to adapt (at a higher mortality rate) and those successful remaining males can propagate their single X chromosome (new and improved, thank you) to their daughters. It's a cool pattern of balances, IMHO.
    - EdSharkey 10y ago
      Hmm. Are you saying that men's imputing of adaptations to their one X chromosome is somehow more concentrated/effectual than the adaptations a woman imputes to her 2 X chromosomes? What is the mechanism and how did you draw that conclusion? Because she only passes one along? It seems to me that chiefly the non-suppressed X chromosome in females would see action by its workings and thus accumulate adaptations.
      - echelon 10y ago
        Females have both X chromosomes active, but only one copy per cell due to Barr body deactivation. Some female cells have one copy active, others utilize the other copy. This scheme can lessen certain X-linked genetic disorders in women since not all cells will be affected.
  - Godel_unicode 10y ago
    Ah, but how do you define helpful? Sickle cell (as cited up-thread) is a selected-for mutation.
- james_pm 10y ago
  As a parent of a daughter with Fragile X Syndrome, I'm glad she has two X chromosomes. The "good" one in her case helps reduce the severity of her symptoms. Males with the Syndrome face far more difficulties as their lone X chromosome is the one with the mutations.
sndean 10y ago
> Being a male, I'm troubled by the fitness of the Y-chromosome over the generations. It seems to me that it can only accumulate errors and mutations. In fact, the Y-chromosome has completely disappeared in some species [0]. [0] https://en.wikipedia.org/wiki/Y_chromosome#Future_evolution https://en.wikipedia.org/wiki/Y_chromosome#Future_evolution
- QML 10y ago
  Is it me, or does this seem like a biological version of the gamblers ruin problem? http://mathworld.wolfram.com/GamblersRuin.html http://mathworld.wolfram.com/GamblersRuin.html
- yourapostasy 10y ago
  Fascinating, did not know that "loss of Y" was possible in vivo, not just evolutionarily across spans of generations. This led me to finding out that loss of Y is tentatively tied to Alzheimer's, and wondering if chronically low testosterone levels are possibly linked to loss of Y.
godmodus 10y ago
Which is why human engage in mate selection and are great at multiplying. Plus innate abity to advance technology. We'll engineer ourselves to survive. Or nuke everything, whichever comes first. Heh. Edit: not to knock off the downvotes, if they're due, they're due. But may I enquire as to why I am being downvoted?
- adventured 10y ago
  I didn't downvote you, but I think it's pretty obvious: > Or nuke everything, whichever comes first. Heh. is the why. Casual amusement at nuclear holocaust/extinction doesn't tend to get a lot of upvotes on HN. Plus, it didn't make any sense: 1) a nuclear war wouldn't involve nuking everything under any plausible scenario; 2) you could direct every nuclear missile on earth at just the US (including the US arsenal) and still not manage to kill all the people in the US with either the initial blasts or the fallout, which is another way of saying that not only can we not nuke everything, but we couldn't even eradicate ourselves with nuclear war if we wanted to for some reason (much less wipe out the planet, much less wipe out all of humanity, that's all beyond far-fetched).
- EdSharkey 10y ago
  I didn't downvote you because you replied to me, and apparently I can't. In my opinion, you didn't add much to the conversation and then with that last crack you did a snarky mic drop where you totally blew it. I like to ask questions, perhaps with a careful dash of snark if I think I can flash out with some wit, and keep the conversation going. For example, I might reply to you, > Which is why human engage in mate selection and are great at multiplying. Are we really selecting mates based on any useful criteria? It seems random to me, rarely based chiefly on reproductive fitness. ... and then someone smart chimes in with a technocratic article on how we DO (for the most part) select based on X number of traits that increase the survivability of our offspring - and we roll on for a bit more and I learn new things.
  - godmodus 10y ago
    Thanks for the reply! I didn't mean to sound snarky, it was more of the pessimist in me. But chiefly an optimist. I trust evolution is doing the right thing. But that is indeed a weak argument, seeing as it's not supported by either more philosophy or science. I do apologise for how it came through, in was on mobile and didn't have time to put in more prose. I should have brought in more facts into my own argumet to spark a discussion. I'll take the critism to mind and adjust.
alias_ 10y ago
This is why all calico cats are female. Fur color comes from the X chromosome. The different color areas are descendant from different initial somatic cells.
- mitchty 10y ago
  Incorrect, male calicos exist. Though they are sterile due to the extra X chromosome. http://www.mnn.com/family/pets/stories/why-are-male-calico-cats-so-rare http://www.mnn.com/family/pets/stories/why-are-male-calico-c...
amyjess 10y ago
> Being a male, I'm troubled by the fitness of the Y-chromosome over the generations. It seems to me that it can only accumulate errors and mutations. One possible safety hatch that comes to mind is that it's possible for XX males to exist. There's only one gene on the Y chromosome which has anything to do with male development, the SRY gene, which causes the proto-gonads to develop into testes (all male development following that is the result of the testes producing androgens). During meiosis, it's possible for the SRY gene to break off of the Y chromosome on one sperm and make its way into the X chromosome on the other sperm. When this happens, it's called de la Chapelle syndrome [0]. So if the Y chromosome eventually becomes unusable, we might end up in a future where males are created by inducing de la Chapelle syndrome in vitro by splicing an SRY gene into the zygote. The only problem is that males with de la Chapelle syndrome are sterile, but I'm sure that if we get to the level of technology where this kind of gene splicing is commonplace, we can come up with some kind of hack to get around it. It's been speculated for a while that eventually we'll develop the technology to make one egg fertilize another in vitro, resulting in a healthy XX zygote, so that's one way (though the eggs wouldn't come from people with de la Chapelle's... which means we wouldn't need to induce it in the first place... so female-only world it is). [0] https://en.wikipedia.org/wiki/XX_male_syndrome https://en.wikipedia.org/wiki/XX_male_syndrome
tathougies 10y ago
Um the X chromosome in a women comes from the mother and the paternal grandmother and grandfather because there is crossover between X and Y chromosomes (albeit very little). There's no need to worry about the Y chromosome disappearing because it's remarkably stable. This makes sense -- males with Y mutations die out and men are extremely good at producing offspring. This only good Y chromosomes are passed on. Nevertheless, the genes on the Y chromosome are like any other and if the Y chromosome were to disappear, natural selection would still push for a 50/50 sex ratio. The Y chromosome itself isn't necessary to make a male -- its genes can be spliced onto the X chromosome and function properly. Thus, if we saw loss of Y, we would likely see a 50/50 gender split but now based on presence of the SRY gene.
- EdSharkey 10y ago
  > Thus, if we saw loss of Y, we would likely see a 50/50 gender split but now based on presence of the SRY gene. If that were to occur, we'd get into more X-SRY X-SRY situations much more often though, right? Sounds unhealthy, I'd prefer to keep the Y separate and intact for as long as possible. :)
echelon 10y ago
It's worth pointing out that since Barr body deactivation occurs later in development, some female cells have one of the X chromosomes active, while other cells have the other one active.
- EdSharkey 10y ago
  So is it wrongheaded to think all women are a bit of a chimera, then? It would seem females would benefit for the most part from this chromosomal arrangement healthwise than males, more diversity.
  - echelon 10y ago
    That term is reserved for organisms comprised of cells that derived from two or more zygotes, but you've definitely got the right line of thinking. Some of the cells of a female will express the genes from one of the X chromosomes, whereas the rest will utilize the other. It certainly can lessen the ill effects of certain X-linked diseases. Some of the other posters in the thread already mentioned calico cat fur coloration. The underlying mechanism is the textbook example of Barr body inactivation. You should check it out on Wikipedia.

incongruity 10y ago

> It seems to me that it can only accumulate errors and mutations. Except some of those mutations can be useful – some deadly, but some could be selected for as one's environment/niche shifts. That's evolution and selection in a nutshell. Not all mutations are bad and not all bad mutations are always bad.

coredog64 10y ago
Concrete example: Sickle cell anemia is bad, but sickle cell carriers also have protection against malaria.
EdSharkey 10y ago
Yes, truly over the long haul, the fittest survive - helpful mutations account for all survival thus far. I just envy what I interpret as an intragenerational bolstering of genetic traits that females have, assuming the EXITS thing is truly what's going on. They don't have to wait a generation to perhaps select out the bad, they get to do it on the fly.
- incongruity 10y ago
  It's a conservative strategy -change from one generation to another will likely be moderated/averaged by the influence of the second X chromosome for female offspring. Alternatively, single copy governed traits (as in males) mean greater ability to adapt (at a higher mortality rate) and those successful remaining males can propagate their single X chromosome (new and improved, thank you) to their daughters. It's a cool pattern of balances, IMHO.
  - EdSharkey 10y ago
    Hmm. Are you saying that men's imputing of adaptations to their one X chromosome is somehow more concentrated/effectual than the adaptations a woman imputes to her 2 X chromosomes? What is the mechanism and how did you draw that conclusion? Because she only passes one along? It seems to me that chiefly the non-suppressed X chromosome in females would see action by its workings and thus accumulate adaptations.
    - echelon 10y ago
      Females have both X chromosomes active, but only one copy per cell due to Barr body deactivation. Some female cells have one copy active, others utilize the other copy. This scheme can lessen certain X-linked genetic disorders in women since not all cells will be affected.
- Godel_unicode 10y ago
  Ah, but how do you define helpful? Sickle cell (as cited up-thread) is a selected-for mutation.
james_pm 10y ago
As a parent of a daughter with Fragile X Syndrome, I'm glad she has two X chromosomes. The "good" one in her case helps reduce the severity of her symptoms. Males with the Syndrome face far more difficulties as their lone X chromosome is the one with the mutations.

sndean 10y ago

> Being a male, I'm troubled by the fitness of the Y-chromosome over the generations. It seems to me that it can only accumulate errors and mutations. In fact, the Y-chromosome has completely disappeared in some species [0]. [0] https://en.wikipedia.org/wiki/Y_chromosome#Future_evolution https://en.wikipedia.org/wiki/Y_chromosome#Future_evolution

QML 10y ago
Is it me, or does this seem like a biological version of the gamblers ruin problem? http://mathworld.wolfram.com/GamblersRuin.html http://mathworld.wolfram.com/GamblersRuin.html
yourapostasy 10y ago
Fascinating, did not know that "loss of Y" was possible in vivo, not just evolutionarily across spans of generations. This led me to finding out that loss of Y is tentatively tied to Alzheimer's, and wondering if chronically low testosterone levels are possibly linked to loss of Y.

godmodus 10y ago

Which is why human engage in mate selection and are great at multiplying. Plus innate abity to advance technology. We'll engineer ourselves to survive. Or nuke everything, whichever comes first. Heh. Edit: not to knock off the downvotes, if they're due, they're due. But may I enquire as to why I am being downvoted?

adventured 10y ago
I didn't downvote you, but I think it's pretty obvious: > Or nuke everything, whichever comes first. Heh. is the why. Casual amusement at nuclear holocaust/extinction doesn't tend to get a lot of upvotes on HN. Plus, it didn't make any sense: 1) a nuclear war wouldn't involve nuking everything under any plausible scenario; 2) you could direct every nuclear missile on earth at just the US (including the US arsenal) and still not manage to kill all the people in the US with either the initial blasts or the fallout, which is another way of saying that not only can we not nuke everything, but we couldn't even eradicate ourselves with nuclear war if we wanted to for some reason (much less wipe out the planet, much less wipe out all of humanity, that's all beyond far-fetched).
EdSharkey 10y ago
I didn't downvote you because you replied to me, and apparently I can't. In my opinion, you didn't add much to the conversation and then with that last crack you did a snarky mic drop where you totally blew it. I like to ask questions, perhaps with a careful dash of snark if I think I can flash out with some wit, and keep the conversation going. For example, I might reply to you, > Which is why human engage in mate selection and are great at multiplying. Are we really selecting mates based on any useful criteria? It seems random to me, rarely based chiefly on reproductive fitness. ... and then someone smart chimes in with a technocratic article on how we DO (for the most part) select based on X number of traits that increase the survivability of our offspring - and we roll on for a bit more and I learn new things.
- godmodus 10y ago
  Thanks for the reply! I didn't mean to sound snarky, it was more of the pessimist in me. But chiefly an optimist. I trust evolution is doing the right thing. But that is indeed a weak argument, seeing as it's not supported by either more philosophy or science. I do apologise for how it came through, in was on mobile and didn't have time to put in more prose. I should have brought in more facts into my own argumet to spark a discussion. I'll take the critism to mind and adjust.

alias_ 10y ago

This is why all calico cats are female. Fur color comes from the X chromosome. The different color areas are descendant from different initial somatic cells.

mitchty 10y ago
Incorrect, male calicos exist. Though they are sterile due to the extra X chromosome. http://www.mnn.com/family/pets/stories/why-are-male-calico-cats-so-rare http://www.mnn.com/family/pets/stories/why-are-male-calico-c...

amyjess 10y ago

> Being a male, I'm troubled by the fitness of the Y-chromosome over the generations. It seems to me that it can only accumulate errors and mutations. One possible safety hatch that comes to mind is that it's possible for XX males to exist. There's only one gene on the Y chromosome which has anything to do with male development, the SRY gene, which causes the proto-gonads to develop into testes (all male development following that is the result of the testes producing androgens). During meiosis, it's possible for the SRY gene to break off of the Y chromosome on one sperm and make its way into the X chromosome on the other sperm. When this happens, it's called de la Chapelle syndrome [0]. So if the Y chromosome eventually becomes unusable, we might end up in a future where males are created by inducing de la Chapelle syndrome in vitro by splicing an SRY gene into the zygote. The only problem is that males with de la Chapelle syndrome are sterile, but I'm sure that if we get to the level of technology where this kind of gene splicing is commonplace, we can come up with some kind of hack to get around it. It's been speculated for a while that eventually we'll develop the technology to make one egg fertilize another in vitro, resulting in a healthy XX zygote, so that's one way (though the eggs wouldn't come from people with de la Chapelle's... which means we wouldn't need to induce it in the first place... so female-only world it is). [0] https://en.wikipedia.org/wiki/XX_male_syndrome https://en.wikipedia.org/wiki/XX_male_syndrome

tathougies 10y ago

Um the X chromosome in a women comes from the mother and the paternal grandmother and grandfather because there is crossover between X and Y chromosomes (albeit very little). There's no need to worry about the Y chromosome disappearing because it's remarkably stable. This makes sense -- males with Y mutations die out and men are extremely good at producing offspring. This only good Y chromosomes are passed on. Nevertheless, the genes on the Y chromosome are like any other and if the Y chromosome were to disappear, natural selection would still push for a 50/50 sex ratio. The Y chromosome itself isn't necessary to make a male -- its genes can be spliced onto the X chromosome and function properly. Thus, if we saw loss of Y, we would likely see a 50/50 gender split but now based on presence of the SRY gene.

EdSharkey 10y ago
> Thus, if we saw loss of Y, we would likely see a 50/50 gender split but now based on presence of the SRY gene. If that were to occur, we'd get into more X-SRY X-SRY situations much more often though, right? Sounds unhealthy, I'd prefer to keep the Y separate and intact for as long as possible. :)

echelon 10y ago

It's worth pointing out that since Barr body deactivation occurs later in development, some female cells have one of the X chromosomes active, while other cells have the other one active.

EdSharkey 10y ago
So is it wrongheaded to think all women are a bit of a chimera, then? It would seem females would benefit for the most part from this chromosomal arrangement healthwise than males, more diversity.
- echelon 10y ago
  That term is reserved for organisms comprised of cells that derived from two or more zygotes, but you've definitely got the right line of thinking. Some of the cells of a female will express the genes from one of the X chromosomes, whereas the rest will utilize the other. It certainly can lessen the ill effects of certain X-linked diseases. Some of the other posters in the thread already mentioned calico cat fur coloration. The underlying mechanism is the textbook example of Barr body inactivation. You should check it out on Wikipedia.