I have added some additional text in red, to clarify what I think this post might mean.
@ Darren Garrison - Did I do this correctly?
Yes, thanks. It ain’t easy to proof-read yourself. You end up seeing what you meant to say, not what you actually said.
Ok, I apologize that actually I missed this last paragraph, where you were explaining why you were asking this. So, the basis of your question is - when a gross chromosome-level mutation such as a telomere fusion occurs in one individual, how does it proliferate in a population that initially lacks the fusion, and ultimately sometimes go to fixation (as in human Chr2)?
To address your speculation about duplication of the fusion in a diploid cell that’s heterozygous for the fusion:
There is an important DNA repair process for double-strand-breaks called HDR Homology directed repair - Wikipedia, in which the repair template may be the non-sister chromatid (the non-identical homolog) in either mitosis or meiosis. So, it’s certainly true that if damage occurs, the cell sometimes copies part of the homolog to replace it. Could a cell “copy” a telomere fusion to duplicate it within a diploid cell via HDR? At first glance it seems implausible to me, because chromosomes have these end pieces called telomeres. Telomeres are lost in the fusion, but will still both be present on the ends of the unfused chromosomes. Even if one telomere on an unfused chromosome broke off in a double-strand-break, the other one would still be there. It’s unwise to say never in biology, but I’d be skeptical that it could occur routinely. But it’s an empirical question - it seems to me that it would be possible to set up an experiment to test this in S.cerevisiae, which has telomeres and most of the same DNA repair mechanisms as mammals.
But in any event, I don’t anything like this is necessary for proliferation of a fusion mutation in a population that lacks it. Provided that meiosis can complete successfully in an individual that’s heterozygous for the fusion, and I think it almost certainly can at least sometimes, the fusion is carried into viable offspring, and evolution proceeds.
It could then go to fixation solely by genetic drift. As for natural selection: I don’t know how a fusion per se might have any selectiive advantage, but another beneficial mutation might arise on a fused chromosome in a position that’s tightly linked to the fusion point, then natural selection might be a factor through hitchhiking, perhaps offsetting any reduction in fertility in the heterozygote.
Short answer: no, among other things, the repair mechanism is generally kicked off by seeing free DNA ends (without telomeres), and the two unfused chromosomes should be fine in that regard, and furthermore there is no normal mechanism that will make a “repair” that drastic. It’s nonetheless possible to successfully transmit the fused chromosome, but only under unlikely conditions.
Looking at your OP, I think I should point out that this meiosis would be happening in the individual who first had the fused chromosome, regardless of whom their eventual partner is, and since it is wildly unlikely that both sets of chromosomes would undergo the same rearrangement at the same time, this person will have one fused chromosome, and one couple of unfused chromosomes.
Therefore, at the end of meiosis I would expect something of a mess; if the normal number of chromosomes in this species is 2N, the individual in question will have 2N-1, which means they will have an odd number of chromosomes, which is bad news. The mechanism in meiosis for correctly partitioning the chromosomes at the time of cell division depends on pairing them up; in this case, there would be two major possibilities:
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The fused chromosome has two centromeres, one from each of the two original chromosomes. In this case, even if the two unfused chromosomes manage to pair correctly with the appropriate half of the fused chromosome, when the cell tries to separate the pairs, the fused chromosome will be ripped apart, leaving a mess of naked ends that may go on to cause additional problems. Note that this would still be a problem even if the individual started with two copies of the fused chromosome and no copies of the unfused ones (two centromeres on one chromosome is always bad news).
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The fused chromosome only has one centromere, the other one having been deleted during fusion. In this case, it would be likely that only one (possibly neither) of the unfused chromosomes would be able to pair with the fused one, leaving the other one hanging (or both). In this case, the individual would act as if they were functionally aneuploid. This is also bad news for meiosis, but one possible (not terribly likely) outcome is that the gamete receives only the fused chromosome (in good condition) and neither of the unfused chromosomes. In this case mating with a normal individual could produce offspring with the same karyotype as the starting individual; i.e., one fused chromosome and the two unfused counterparts. If the individual had several such offspring, and they interbred, you could end up with people with only the fused chromosome.
Highly unlikely, as stated initially.
No. Telomeres exist, amongst other reasons, to prevent exactly this happening.
Moderating
Technically, this violates the “no alteration of attributed text within a quote box” rule. However, since you clearly indicated your additions, and they were helpful, I don’t see it as a problem. In the future though, it’s better to remove the attribution of the quote if you make changes.
Colibri
General Questions Moderator