Y chromosome evolving faster

A new study shows the Y chromosome is evolving faster than the rest of the genetic code.

Could it be that those genes which make people men are just working harder to catch up with the second X chromosome  which makes other people women?

9 Responses to Y chromosome evolving faster

  1. PaulL says:

    It could be, but by that logic since men have both an X and a Y, then whatever skills an X chromosome confers men have already……

    My guess would be it is evolving faster since it is only ever inherited from your father. That is to say, if a particular person has a mutation in their X chromosome, then their child has only a 50% chance of inheriting it.

    Actually, I’m not sure if the two X chromosomes in a woman are interchangeable – is it the same X twice, or is it one X from the father and one from the mother. Depending on how you do the conditional probability, I’d say that the chance of your X chromosome coming from one parent is somewhere between 25% and 100%:
    – if all three Xs are interchangeable, and you’re a boy, then a one in three that you got that X from one particular X in your mother, or your father’s one X
    – if the two X’s that your mother have aren’t interchangeable, then presumably if you’re a boy, then 50/50 as to whether you got your X from either parent
    – if the two X’s aren’t interchangeable, and you’re a girl, then one of your two Xs must have come from your mother

    And so on and so forth.

    No doubt somewhere on the internet there is a detailed study that explains all of this.

  2. PaulL says:

    Ah. Once you say “it’s probably somewhere on the internet” you have to go and look. Wikipedia tells us:

    The sex chromosomes X X are one of the 23 homologous pairs of chromosomes in a female. The X chromosome spans more than 153 million base pairs (the building material of DNA) and represents about 5% of the total DNA in women’s cells, 2.5% in men’s.

    Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, whereas males have one X and one Y chromosome. Both males and females retain one of their mother’s X chromosomes, and females retain their second X chromosome from their father. Since the father retains his X chromosome from his mother, a human female has one X chromosome from her paternal grandmother, and one X chromosome from her mother.

    Presumably this quite specific descent path impacts on likelihood of mutation. A Y chromosome always comes from your father, and it it had a mutation, that mutation will always be passed to you.

    Your X chromosome if you’re a boy always comes from your mother, but it isn’t clear whether it is randomly picking one of the two that your mother has – so a 50% chance of passing on a mutation.

    If you’re a girl, one of your X’s comes from your father, so any mutation will definitely be passed on (but you might not pass it on to your children). The other comes from your mother, and 50% chance of getting the one with the mutation.

    In other words, it makes sense that Y chromosomes are mutating faster.

  3. PaulL says:

    And, even further (you can’t stop me now), on the Y-Chromosome page we learn:

    High Mutation Rate

    The human Y chromosome is particularly exposed to high mutation rates due to the environment that it is housed in. The Y chromosome is passed exclusively through sperm, which undergo multiple cell divisions during gametogenesis. Each cellular division provides further opportunity to accumulate base pair mutations. Additionally, sperm are stored in the highly oxidative environment of the testis, which encourages further mutation. These two conditions combined put the Y chromosome at a risk of mutation 4.8 times greater than the rest of the genome

  4. david w says:

    Paul,

    The mode of inheritance isn’t actually all that important here. It’s true you and I know where our y-chromosomes come from but that doesn’t mean the probability of a new Y-chromosome mutation being passed on is greater (after all, your Dad might have had girl)

    I haven’t actually read the paper (still getting out of holiday mode…) so take this with a few grains of salt but I suspect one of the reasons for the increased rate is actually something along the lines of what you’ve said. The y-chromosome is unique in that (for the most part) it doesn’t have a partner to swap genes with. So, if a new mutation arises it’s the whole chromosome (and not just the gene affected) that is selected for or against. So called “selective sweeps” in which a whole raft of mutations are dragged along with the one under selection can make large scale changes quickly

    Also, most of the genes that are left on the Y are prime targets for selection – genes with direct effects on sperm production and fertility. This is likely even more important for the chimpanzee half of this equation since chimps are very polygynous and sperm competition is an important level of selection.

    Add that to the evidence you’ve provided about a high mutation rate and you have new mutations coming up more often that elsewhere followed by greater selective pressure once they turn up.

    But perhaps I should actually read it before say too much 🙂

  5. david w says:

    Oh, and at risk of sounding even more didactic: I forgot to say it can’t be the Y-chromosome working twice as hard to keep up with the XX crew. Not least because woman don’t have two active X-chromosomes in their cells!

  6. homepaddock says:

    Thank you both for all that information: science 1 – sexism 0.

  7. PaulL says:

    David: reading the info, it just seems quite wondrous. If someone has a mutation on their Y chromosome, it will only be propagated to those of their direct descendents that have a direct partrilineal line to them.

    But then, when I think about it, that is also true of sorts for any other mutation being propagated – it will only be propagated to those of your direct descendants for whom your copy of that gene (v’s the other side of the family tree’s copy) was the one that turned up in the child.

    I guess it’s a bit like the day my brother told me I was an idiot for picking all the prime numbers in Lotto because “what’s the chance of all the prime numbers coming up.” Makes you realise how fragile the natural selection process really is.

  8. david w says:

    If anyone is sufficiently interested I’ve now actually read the paper and put a couple of thoughts together here – much the same as my comments above.

    Paul,

    You might be surprised to learn how quickly all Y-chromosomes coalesce to a common ancestor. I’ve hinted at in the post linked above but the small effective population size of the Y-chromosome means all modern Y-chromosomes descend from an ancestor (dubbed Y-chromosomal Adam) who lived as recently as 60 000 years ago.

    And playing the first 6 (7?) primes in lotto is a great idea because not only are they as likely to come up as any other combination but if you did win you’d be the guy that won lotto while being a maths geek!

  9. Dork says:

    “Thank you both for all that information: science 1 – sexism 0.”

    Everyone knows idiocy travels with the Y-Chromosome! I really do think men are idiots though I know nothing about science.

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