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What allows sequencers to conclude that Denisovans had 46 chromosomes rather than merely knowing Denisovans had the crossover material arranged in 48 or say 44 chromosomes?

See http://genetics.thetech.org/ask-a-geneticist/denisovan-chromosome-2

What I am asking its what allows sequencers in this case to make an inference to an actual number of chromosomes (packaging) rather than the mere presents of corresponding DNA?

In the article above, interbreeding was rejected as a sufficient condition for inferring equality to humans in chromosome number and the author points to a man in China known to have the same cross over (fusion) known in humans and only 44 chromosomes indicating having the cross over that reduced 48 to 46 is also not a sufficient condition for concluding 46.

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  • $\begingroup$ We know Denisovans interbred with other members of the genus Homo, therefore they must have had the same number of chromosomes. Individuals with different numbers of chromosomes cannot interbreed. $\endgroup$ – MattDMo Aug 31 '15 at 0:55
  • $\begingroup$ The referenced link says "As of now we don’t know have any hard DNA evidence for how many chromosomes Neanderthals had. We are guessing that since humans and Neanderthals probably had babies together that they shared the same number of chromosomes. But this doesn’t necessarily have to be the case." so this is not the reason for the inference of 46 for Denisovans. $\endgroup$ – John Perkins Aug 31 '15 at 1:05
  • $\begingroup$ @MattDMo People with Trisomy 21 are fertile, although at a much lower rate than in humans with the standard set of 46 chromosomes. $\endgroup$ – AMR Aug 31 '15 at 2:21
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    $\begingroup$ John, I think the answer is that without having a living cell that we can perform a karyotype on, we cannot know for sure. That being said, telomeres and centromeres are highly conserved, so if you're sequencing a female specimen, and it gives you 23 sets of centromeres and 46 sets of telomeres, then I would say that it would be safe to make the educated guess that the individual had 2n = 46. $\endgroup$ – AMR Aug 31 '15 at 2:26
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    $\begingroup$ @MattDMo Now if you wanted to make the case that Denisovins and modern humans couldn't produce offspring because the cell surface receptors that mediate gamete fusion were incompatible, that I would go along with. I think that the lethality with additional chromosomes comes from "dosage" problems and whether or not enhancer/promoter regions were preserved and TFs responded to the same chemical signaling and could bind to the DNA to drive transcription. $\endgroup$ – AMR Aug 31 '15 at 16:43

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