# Question on chromosome fusion and fertility **URL:** \ **Category:** Factual Questions\ **Created:** [November 10, 2016, 5:42pm UTC](https://boards.straightdope.com/t/question-on-chromosome-fusion-and-fertility/771377 "2016-11-10T17:42:01Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 11, 2016, 5:27pm UTC](https://boards.straightdope.com/t/question-on-chromosome-fusion-and-fertility/771377/21 "2016-11-11T17:27:36Z")
> [@Darren\_Garrison](#): > > Scenario 1–cell has one fused chromosome, two unfused chromosomes and nothing changes. At the end of meiosis, the result is one gamete with 1 fused chromosome and **one gamete with** 1 **or 2** unfused chromosome\*\*(s)\*\*. > > Scenario 2–cell has one fused chromosome, two unfused chromosomes, and some repair mechanism–triggered by seeing the fused chromosome as “correct” and the two unfused chromosomes as “broken”, patches together the two unfused chromosomes to produce a fused chromosome matching the first one. At the end of meiosis, the result is two gametes with 1 fused chromosome **each**. I have added some additional text in red, to clarify what I think this post might mean. @ **Darren Garrison** - Did I do this correctly? ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [November 11, 2016, 5:44pm UTC](https://boards.straightdope.com/t/question-on-chromosome-fusion-and-fertility/771377/22 "2016-11-11T17:44:44Z")
> [@septimus](#): > > 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. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [November 11, 2016, 5:48pm UTC](https://boards.straightdope.com/t/question-on-chromosome-fusion-and-fertility/771377/23 "2016-11-11T17:48:54Z")
> [@Darren\_Garrison](#): > > All I’m asking is if there is a mechanism where this is possible (and thus would speed the spread of the fused chromosome in a population.) You have not addressed that question (even to just say “no.”) 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](https://en.wikipedia.org/wiki/Homology_directed_repair), 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](https://en.wikipedia.org/wiki/Genetic_hitchhiking), perhaps offsetting any reduction in fertility in the heterozygote. ---
**Author:** ![JR\_Brown](https://avatars.discourse-cdn.com/v4/letter/j/d6d6ee/32.png) [@JR\_Brown](https://boards.straightdope.com/u/JR_Brown)\ **Post date:** [November 11, 2016, 6:25pm UTC](https://boards.straightdope.com/t/question-on-chromosome-fusion-and-fertility/771377/24 "2016-11-11T18:25:12Z")
Short answer: no, among other things, the repair mechanism is generally kicked off by seeing free DNA ends (without [telomeres)](https://en.wikipedia.org/wiki/Telomere), 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. > [@Darren\_Garrison](#): > > I am not trying to get into an argument here, but I understand your post, you do not understand my question. I’ll try rephrasing my question yet again. > > Scenario 1–cell has one fused chromosome, two unfused chromosomes and nothing changes. At the end of meiosis, the result is one gamete with 1 fused chromosome and 1 unfused chromosome. 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: 1. The fused chromosome has two [centromeres](https://en.wikipedia.org/wiki/Centromere), 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). 2. 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](https://en.wikipedia.org/wiki/Aneuploidy). 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. > [@Darren\_Garrison](#): > > Scenario 2–cell has one fused chromosome, two unfused chromosomes, and some repair mechanism–triggered by seeing the fused chromosome as “correct” and the two unfused chromosomes as “broken”, patches together the two unfused chromosomes to produce a fused chromosome matching the first one. At the end of meiosis, the result is two gametes with 1 fused chromosome. > > All I’m asking is if there is a mechanism where this is possible (and thus would speed the spread of the fused chromosome in a population.) You have not addressed that question (even to just say “no.”) Highly unlikely, as stated initially. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [November 12, 2016, 3:25am UTC](https://boards.straightdope.com/t/question-on-chromosome-fusion-and-fertility/771377/25 "2016-11-12T03:25:50Z")
> [@Darren\_Garrison](#): > > During prophase I of meiosis (thanks for refreshing my mind on that detail) the chromosomes line up. Is there a possibility that this alignment bringing not only the 1 fused and 2 unfused chromosomes in alignment,\* but also the two unfused chromosomes aligned end to end\* lead to some mechanism “gluing together” the two unfused chromosomes so that there are now two (virtually) identical fused chromosomes and not one fused chromosome and two unfused ones? Possibly some DNA repair mechanism “noticing” the difference between the two, “assuming” a break, and “fixing” it? Sort of like (as a rough analogy) prions causing a conformation change in “normally” shaped proteins? No. Telomeres exist, amongst other reasons, to prevent exactly this happening. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 12, 2016, 4:01pm UTC](https://boards.straightdope.com/t/question-on-chromosome-fusion-and-fertility/771377/26 "2016-11-12T16:01:04Z")
> [@septimus](#): > > I have added some additional text in red, to clarify what I think this post might mean. > @ **Darren Garrison** - Did I do this correctly? 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 [Previous page](https://boards.straightdope.com/t/question-on-chromosome-fusion-and-fertility/771377.md?page=1)