Re: [asa] chromasome fusion #2

Thanks for the fascinating reply. Unfortunately the paper about mice
only works if you have a subscription to the journal.

I think what I've learnt probably highlights the way certain
anti-evolution arguments get formulated. The simplistic view is that
mating cannot occur between species with differing numbers of
chromosomes & I guess that is what RTB are taking as the assumed truth
(and what I, with my limited knowledge as a non-biologist) took as the
default position. However, the truth is always much more subtle than
that.

Iain

On Mon, Jul 13, 2009 at 10:06 PM, Dennis Venema<Dennis.Venema@twu.ca> wrote:
> The fertility and viability of the 47-chromosome individual is no problem at
> all. All the info is there, and the two free chromosomes would pair along
> the length of the fused chromosome. Meiosis would proceed just fine; worst
> case scenario would be that only about half of the gametes would work.
>
> This is nothing like a trisomy – the problems there arise from gene number /
> gene balance issues.
>
> You can do this (and much, much worse) to the chromosomes of fruit flies
> without obvious problems.
>
> There is good evidence for major remodeling of mouse chromosomes in very
> short order – I’ll see if I can dig up a paper.
>
> Dennis
>
>
> On 13/07/09 1:56 PM, "Iain Strachan" <igd.strachan@gmail.com> wrote:
>
> On Mon, Jul 13, 2009 at 8:53 PM, Randy Isaac<randyisaac@comcast.net> wrote:
>> That's how I understand it. This is a scenario of which the probabilities
>> are in the range of plausibility, though obviously not too frequent.
>
>
> That's only true insofar as you grant that a 47-chromosome individual
> with fused and non-fused copies of the chromosome 2 material is viable
> and capable of reproduction.  All the probabilities I gave are
> conditional on that being true & I don't know how you estimate that
> probability.  As I understand it, for example Down's syndrome
> individuals have severely impaired fertility.  I know it's a different
> scenario (three copies of chromosome 21), but do we know of any other
> instances where the chromosomes don't match up in pairs and yet the
> individual remains viable and fertile?
>
> Iain
>
>
>> Evidence that this did happen is from the detailed genetic similarity
>> between the relevant chimp chromosomes and the fused human chromosome, in
>> particular the inactivated centromere in the latter.
>>
>> Randy
>>
>> ----- Original Message ----- From: "Iain Strachan"
>> <igd.strachan@gmail.com>
>> To: "Randy Isaac" <randyisaac@comcast.net>
>> Cc: <asa@calvin.edu>
>> Sent: Monday, July 13, 2009 12:55 PM
>> Subject: Re: [asa] chromasome fusion #2
>>
>>
>> OK, let's see if I've understood this right - in layman's terms!
>>
>> It would appear that my assumptions, and RTB's claim that you can't
>> have a viable embryo and one that can reproduce if it has 23
>> chromosomes from one parent and 24 from the other is incorrect.
>>
>> So the heterozygous individual in Darryl Falk's explanation would have
>> 47 chromosomes, and be viable and able to mate with 48-chromosome
>> individuals.  My superficial knowledge of the subject had assumed that
>> you had to have 1-to-1 pairing of the chromosomes.  However, as I
>> understand it, Down's syndrome individuals have 47 chromosomes via a
>> different route (an extra copy of chromosome 21), though this renders
>> them less fertile.
>>
>> So a single 47-chromosome heterozygous individual carrying the fused
>> chromosome from one parent would mate with a 48-chromosome individual
>> and produce  a 47 or 48 chromosome'd offspring with equal probability.
>> Later on down the line two 47'ers could mate, and would could produce
>> an offspring with 48 (probability 1/4),   47 (probability 1/2) or 46
>> (probability 1/4) chromosomes respectively.  Then the assumption is
>> that the 46 variant has some form of selective advantage and that it
>> increases through the population.
>>
>> Is that about right?
>>
>> Iain
>>
>> On Mon, Jul 13, 2009 at 3:52 PM, Randy Isaac<randyisaac@comcast.net>
>> wrote:
>>>
>>> About six months ago, I learned that on some RTB blogs, the claim was
>>> circulating that humans and chimps could not have had common ancestry
>>> because of the difference in chromosome count. The claim was that an egg
>>> and
>>> a sperm could not form a viable embryo if one had 23 chromosomes and the
>>> other had 24. A fusion event would therefore have to occur independently
>>> and
>>> simultaneously in both the egg and the sperm in order to achieve
>>> viability.
>>> The probability of this occurring is so incredibly small that it is
>>> essentially impossible.
>>>
>>> I then asked Darrel Falk, biology professor at Pt Loma Nazarene U about
>>> this. He gave me permission to post his response:
>>>
>>> "Regarding your question, no one knows for sure how something like this
>>> might have happened, but here's the type of thing that would have
>>> occurred.
>>> The two chromosomes would have become attached to each other in a sperm,
>>> egg, or a cell on the lineage to become a sperm or egg. That cell then
>>> passes the fused chromosome into the fertilized egg. The fertilized egg
>>> now
>>> has two copies of the chromosomal material, one fused and one set
>>> unfused.
>>> (We say it is heterozygous for the fused chromosome---one copy of the
>>> chromosomes fused, and one copy of the chromosomes unfused). There is no
>>> reason why it would need to be homozygous (i.e. that the same event would
>>> have happened twice.)
>>>
>>> The heterozygote is viable and could easily pass the new fused chromosome
>>> into half of his sperm or her eggs. If the population was small it
>>> wouldn't
>>> be too long (perhaps three or four generations) when individuals that are
>>> homozygous (two copies) for the fused chromosomes would be produced.
>>> (This
>>> would happen through matings between cousins---(first, second, third or
>>> whatever). Once homozygous in some individuals it could spread through
>>> the
>>> population.
>>>
>>> The real question for this event is not whether it would need to take
>>> place
>>> in two sets of chromosomes at the same time (it wouldn't), but whether a
>>> fused chromosome would be viable given that it has two centromeres---the
>>> part of the chromosome that attaches to the little string-fibers which
>>> pull
>>> the two identical duplicate parts of the chromosomes to opposite cells
>>> during the process of cell division. Having 2 centromeres rather than
>>> one---would likely result in the chromosome being ripped apart during
>>> cell
>>> division. So two events must happen at about the same time---the fusion,
>>> but also the inactivation of one of the centromeres. We know this type of
>>> thing happens in cancer cells and thereby doesn't always make the cells
>>> less
>>> viable in the process so we assume that it does not seem like a stretch
>>> to
>>> think that it could happen in evolutionary history as well.
>>>
>>>
>>>
>>> I'll let the biologists comment further.
>>> Randy
>>>
>>> ----- Original Message ----- From: "Iain Strachan"
>>> <igd.strachan@gmail.com>
>>> To: "Bill Powers" <wjp@swcp.com>
>>> Cc: <asa@calvin.edu>
>>> Sent: Monday, July 13, 2009 9:17 AM
>>> Subject: Re: [asa] chromasome fusion #2
>>>
>>>
>>>> Bill,
>>>>
>>>> I don't think my second post really contradicts the first post. It is
>>>> still a small change to the overall genetic structure that has been
>>>> made - all the information is still there.
>>>>
>>>> If, on the other hand, we had NOT found the chromosome fusion event -
>>>> then this indeed would be a huge problem - I'm guessing, for example
>>>> deleting a whole chromosome would lead to a non-viable offspring.
>>>> That's what I meant by a non-gradual change.
>>>>
>>>> The best explanation as to how the 46-chromosome individual managed to
>>>> produce offspring that I can come up with is to suggest that maybe
>>>> there is some genetic abnormality in one or both parents that causes
>>>> the fusion event to be more likely, and that it could have then
>>>> occurred more than once in different pregnancies (or in non-identical
>>>> twins in the same pregnancy). Then there would be two 46-chromosome
>>>> individuals that could mate with their siblings. ( or if the
>>>> abnormality had been propagated through generations, it might have
>>>> been cousins etc).
>>>>
>>>> However, this is all completely speculative on my part, and I'd like
>>>> to see what a real biologist has to say about it.
>>>>
>>>> Iain
>>>>
>>>> On Mon, Jul 13, 2009 at 1:53 PM, Bill Powers<wjp@swcp.com> wrote:
>>>>>
>>>>> Iain:
>>>>>
>>>>> I was going to reply immediately to your prior post in which you assert
>>>>> that
>>>>> the fusion indicates a "gradual" evolution, but this post makes it
>>>>> unnecessary.
>>>>>
>>>>> The problem you present here is one that has puzzled me about all
>>>>> aspects
>>>>> of
>>>>> evolutionary theory.
>>>>>
>>>>> In the case of asexual reproduction the problem does not appear to be
>>>>> great
>>>>> (although I even wonder at that in specifics), but the case of sexual
>>>>> reproduction it appears to be a mystery how such "freaks" can be
>>>>> replicated.
>>>>> I've never understood how this is suppose to happen with all
>>>>> speciation,
>>>>> since the genetic code is discrete and not continuous.
>>>>>
>>>>> bill
>>>>>
>>>>> On Mon, 13 Jul 2009, Iain Strachan wrote:
>>>>>
>>>>>> Actually a couple of puzzling questions about the chromosome fusion
>>>>>> event occur to me and I wonder if a biologist or geneticist on the
>>>>>> list could give me an answer.
>>>>>>
>>>>>> The chromosome fusion is going to be an exceedingly rare event, so
>>>>>> when it first happened, it happened in one individual. There are a
>>>>>> couple of questions that occur to me.
>>>>>>
>>>>>> Since an embryo is made by the fusion of a sperm and an ovum cell,
>>>>>> each of which has 23, rather than 46 chromosomes, one drawn at random
>>>>>> from each pair, is it the case that a double fusion would have
>>>>>> occurred (one from the sperm cell and one from the ovum cell)?
>>>>>>
>>>>>> Secondly, since there is now one "freak" individual that has 46
>>>>>> chromosomes and everyone else has 48, how does this individual mate
>>>>>> and produce offspring that also have 46 chromosomes?
>>>>>>
>>>>>> If anyone can enlighten me (I realise this is a science rather than
>>>>>> science/faith question) I'd be fascinated to know.
>>>>>>
>>>>>> Iain
>>>>>>
>>>>>> On Mon, Jul 13, 2009 at 7:49 AM, Iain Strachan<igd.strachan@gmail.com>
>>>>>> wrote:
>>>>>>>
>>>>>>> Surely the whole point about it being potentially a problem for the
>>>>>>> theory of evolution is this;
>>>>>>>
>>>>>>> Suppose we had NOT found evidence of the fusion of two chimp
>>>>>>> chromosomes & our chromosome 2 was nothing like any of the ape
>>>>>>> chromosomes (or suppose the extra chromosome pair was just missing).
>>>>>>> Both these instances show a massive change to the genome that
>>>>>>> occurred
>>>>>>> somewhere along the line, which is not consistent with a gradualistic
>>>>>>> process. However, a fusion of two is a single event, consistent with
>>>>>>> gradualism. So the finding or not finding of the fusion evidence is
>>>>>>> potentially a falsifier of a gradualist theory of evolution.
>>>>>>>
>>>>>>> Iain
>>>>>>>
>>>>>>> On Mon, Jul 13, 2009 at 2:55 AM, wjp<wjp@swcp.com> wrote:
>>>>>>>>
>>>>>>>> Apparently chimpanzees, and other primates, have 48 chromosomes
>>>>>>>> while humans only have 46.
>>>>>>>>
>>>>>>>> From an evolutionary standpoint this is suppose to be a problem.
>>>>>>>> Why is that?
>>>>>>>>
>>>>>>>> It is presumed that chimps and humans have a common ancestor.
>>>>>>>> So I suppose the reasoning is that if one ancestor of the
>>>>>>>> common ancestor has 48 chromosomes and another has 46 there
>>>>>>>> is a problem in believing they had the same ancestor.
>>>>>>>>
>>>>>>>> The reasoning might be that since chromosome number is
>>>>>>>> directly related to inherited traits that it might be
>>>>>>>> difficult to see how an ancestor with 48 chromosomes could
>>>>>>>> produce (in no matter the number of steps) an offspring
>>>>>>>> with only 46.
>>>>>>>>
>>>>>>>> Now I, being naive, don't see why this is suppose to be so
>>>>>>>> great, or insurmountable a problem.
>>>>>>>> After all, if evoultion is correct, something like this must
>>>>>>>> be commonplace. Presumably the earliest of creatures had fewer
>>>>>>>> chromosomes than later species. So somehow chromosomes must be
>>>>>>>> added and I'm not certain why it should any more mysterious how
>>>>>>>> chromosomes can be added than that they can be taken away.
>>>>>>>>
>>>>>>>> In any case, Ken Miller asserts that this is so great a problem that
>>>>>>>> unless it were resolved evolution must be wrong.
>>>>>>>> I am astonished by this statement and can hardly believe that he
>>>>>>>> really
>>>>>>>> means it. In fact, it seems far more obvious that the reason he
>>>>>>>> says this is because he believed at the time of the statement that a
>>>>>>>> resolution was already at hand.
>>>>>>>>
>>>>>>>> In any case, the resolution supposedly is that the second chromosome
>>>>>>>> fused
>>>>>>>> with another chromosome, and since chromosomes come half from each
>>>>>>>> parent,
>>>>>>>> this would result in 46 chromosomes instead of 48.
>>>>>>>>
>>>>>>>> All I want to say about the supposed evidence that a chromosome had
>>>>>>>> fused
>>>>>>>> is that it does not entail that evolution occurred, rather it is
>>>>>>>> merely
>>>>>>>> consistent with an evolutionary development.
>>>>>>>>
>>>>>>>> The story, I suppose, would be something like that the ancestor of
>>>>>>>> both
>>>>>>>> man and chimp has 48 chromosomes, but somehow one chromosome in man
>>>>>>>> became fused to another, while that of the chimp and other primates
>>>>>>>> did
>>>>>>>> not.
>>>>>>>>
>>>>>>>> The notion of fused chromosomes is not necessarily associated with
>>>>>>>> an
>>>>>>>> evolutionary process, unless one means by evolution that something
>>>>>>>> that existed previously was used in the creation of something new.
>>>>>>>> Such a view of evolution could as well be the work of an intelligent
>>>>>>>> designer, which is why I am confused by Ken Miller's apparent
>>>>>>>> confidence that evolution is clearly a superior explanation.
>>>>>>>>
>>>>>>>> The very notion of fusion appears to entail a process whereby
>>>>>>>> something
>>>>>>>> changed from not being fused to being fused. The notion appears to
>>>>>>>> entail that there was a time when they were not fused and somehow
>>>>>>>> became
>>>>>>>> fused. It is true that if we presume that such processes must take
>>>>>>>> place,
>>>>>>>> then fusion would be consistent with that presumption. But does the
>>>>>>>> evidence for fusion really entail that a fusion has taken place?
>>>>>>>> In order for fusion to have taken place we must have a time when
>>>>>>>> they were not fused. But the mere fact that they appear to be fused
>>>>>>>> does not entail that they were ever not fused.
>>>>>>>>
>>>>>>>> It seems then that here, as in all of science, we proceed
>>>>>>>> abductively,
>>>>>>>> from theory to evidence and then back again to theory.
>>>>>>>> But in all cases the science finds a theory that is consistent with
>>>>>>>> the
>>>>>>>> evidence. There is no way to argue from the evidence to a unique
>>>>>>>> theory. The supposed discovery of the fusion of chromosome #2 is
>>>>>>>> consistent with an evolutionary story, but it could just as well be
>>>>>>>> consistent with other theories and explanations. This is, of course,
>>>>>>>> true of all our knowledge of the physical world.
>>>>>>>>
>>>>>>>> What is surprising to me is that some think that this discovery is
>>>>>>>> of great importance. Yet it seems to me that the result is more
>>>>>>>> or less assured by the supposed discovery that chimp DNA and
>>>>>>>> human DNA are so very similar.
>>>>>>>>
>>>>>>>> I do not understand, I confess, why chromosomes are so important.
>>>>>>>> It seems to me that they are mostly an artifact from an earlier
>>>>>>>> state of biological science. Hence, I don't understand why fusion
>>>>>>>> would
>>>>>>>> seem so important. But, then again, I probably don't understand why
>>>>>>>> the bunching of DNA that can be observed under a microscope should
>>>>>>>> be
>>>>>>>> so
>>>>>>>> important.
>>>>>>>>
>>>>>>>> bill
>>>>>>>>
>>>>>>>>
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>>>>>>>> "unsubscribe asa" (no quotes) as the body of the message.
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> -----------
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>>>>>>> (\__/)
>>>>>>> (='.'=)
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>>>>>>
>>>>>>
>>>>>>
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>>>>>> -----------
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>>>>>> (='.'=)
>>>>>> (")_(") This is a bunny copy him into your signature so he can gain
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>>>>>
>>>>
>>>>
>>>>
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>>>
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>>
>>
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>
>
>
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