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I am working on human chromosomal speciation but cannot confirm how a "head to head telomeric fusion" (not a Robertsonian translocation) such as human chromosome 2 would express itself in the first generations. About 2/3 down there is a paper with a Punnett square of the event in the first generation:
Another paper (closed access):
It would seem that a fused chromosome that still has two functional centromeres would align then divide properly with the unfused pair, but that's just my educated guess. I'm hoping there is something I missed that would make it possible to know for sure what the Punnett squares should look like.
As in the Punnett Square that is the first paper, it is the word balanced that you should take into account for meiosis. This reminds me of the X chromosome, in which to be balanced in the female, one chromosome needs to be inactivated (into a Barr body), and thus already balanced in the male which has only one.
A fused chromosomal gamete is equivalent to the two normal chromosomal gamete - BALANCED. You can have viable offspring as long as you have the BALANCED count of 4 chromosomal gametes. If you have the normal 2 chromosomal gametes and the 1 fused chromosomal gamete, then you get viable with a count of 4.
If you have the normal chromosomal gametes (2) and a unbalanced gamete combination of one fused (count of 2) and 1/2 the normal 2 count chromosomal gamete (count of 1), then you have a count of 5, which is not viable.
Hope this helps.
Thanks Kolean, the first paragraph about inactivation of the X chromosome is interesting. I didn't know that before.
You have the right idea with balanced and unbalanced. What I am most wondering is whether during meiosis a fully balanced fusion that only involves the telomeres could even result in unbalanced germ cells. It would seem that the chromosomes would still properly align along the mid-line with the fused pair always going one way and the unfused always going the other way. Where the spindles on opposite poles of the fused chromosomes are each pulling on one of the two still viable centromeres there would be a tug-of-war type situation causing an extreme pulling force related imbalance, and where there is enough motor protein force could pull the poles to the center line which would result in unsuccessful meiosis possibly cell death. Where the spindles are bringing one unfused chromosome to one side and the other fused and unfused copy to the other there is another large force imbalance in the system. It seems like the meiosis mechanisms would be able to detect this much pulling force imbalance in the system then adjust spindles accordingly.
There does not seem to be any research where a head to head telomere fusion was induced then how it divided down in meiosis documented. But I could be wrong and was simply unable to find the paper that fully explains the result of this kind of fusion.
Maybe looking at the cell cycle during mitosis and seeing all the checkpoints might help you out. It could possibly end in cell death if the cell couldn't proceed thru its cell cycle.
It is microtubules that connects the centromeres to the poles, and they begin to pull by way of letting tubulin go from the pole end: thus pulling on the centromeres as they shorten. Are they strong enough to pull a fused chromosome apart? Depends on how condensed the chromosome at the fused part in order for it to either break off and freely go with the rest of the chromosome, or stay together and stall the cell cycle.
Now in Meiosis, there are chiasmata/crossingover going on, but that is just when the chromosome is decondensed and supporting proteins help in the process. After the process is done, the proteins are released and the cross over DNA is let loose to be a single strand, and then the condensing of the DNA into chromosomes begins. I don't know what proteins are found to help support a fusing of chromosomes, and then to hold them together during metaphase. It would have to be an intricate orchestrated process that would unfold into a viable offspring.
(Though telomeres do have unique proteins that do like to bind together, especially Sir2, in yeast models so far.)
This is a very interesting topic. I would like to know how a balanced telomeric fusion takes place? My husband has been diagnosed with a balanced telomeric fusion of chromosome 16 and 22. The long arm of one of the chromosome 16 has been attached to the long arm of the chromosome 22 and only one of the centromere is active in this fused chromosome. He has all the data present on the chromosomes and does not have any problems but his karotype is regarded as 45XY. We only found out about his telomeric fusion when we were trying to conceive and failed to naturally. We were thus advised to go through IVF. Luckily our baby has been diagnosed to have 46XY karotype through amniocentesis and I am presently 28 weeks pregnant. We were also been told that the doctors have not found anyone in the listed database to have balanced telomeric fusion of 16 and 22. Even though my amnio results are normal, could it be possible that the amnio results can show wrong results or could it be that the telomeric fusion does not affect the cell division of the fetus. 15 cells of the fetus has been analysed using G-banding technique and has been specifically tested for 16 and 22 and has found to be normal. However I cannot stop worrying since in 20 weeks ultrasound they have found a two vessel umbilical cord, which our doctor has said nothing to worry about since amnio results are normal and my maternal serum also showed my risk for chromosome 21, 13 and 18 as 1 in 100000. No anomalies have been found in ultrasounds apart from single artery umbilical cord and the babies growth is in the 50th percentile so far. If you are aware of any such telomeric fusion of chromosome 16 and 22 and could give us more information, it would be useful.
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