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Mitosis is a complex mechanism that, through mutations and crossover, determines how a chromosome is composed. But at that point, the number of chromosomes is already determined: how can this number evolve through generations? What mechanism is involved?

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A mutation is a general term referring to any alteration of a nucleotidic sequence. It includes small mutations like indels (insertion or deletion of a single nucleotide) and substitutions, or bigger mutations such as e.g. chromosome fusions, whole chromosome duplications, Robertsonian translocation, whole genome duplications and gene deletions. Some of these bigger mutations can affect the number of chromosomes (and ploidy number).

Such of these mutations can typically be caused by the non-disjunction of homologous chromosomes during meiosis.

Most of these mutations are deleterious but some aren't and might eventually fix (fixation = reach a frequency of 1.0) in the population.

A very common example of such mutations in humans is the mutation that is causing the Down syndrome.

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  • $\begingroup$ Note that I mainly recycled this answer of mine but as the question is phrased in a much more pleasant way, I did not want to mark it as a duplicate. $\endgroup$ – Remi.b May 22 '18 at 15:15
  • $\begingroup$ your answer is interesting, thanks. Do you have some source you can suggest to read? $\endgroup$ – HAL9000 May 22 '18 at 19:27
  • $\begingroup$ It appears to me that very introductory source of information tend to neglect those types of "big mutations" (e.g. evo101) and after that you're only left with slightly more advanced source such as a good textbook or directly in the scientific literature.... $\endgroup$ – Remi.b May 22 '18 at 19:54
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    $\begingroup$ ... I am not sure what to point to like that. I would advice you to just have a look at the wikipedia entries of gene duplication, Robertsionian translocation and others and go into the scientific literature (you can use GoogleScholar to search in the scientific literature) if you have a more accurate, clearly defined question about it. $\endgroup$ – Remi.b May 22 '18 at 19:54
  • $\begingroup$ Sure, but when a mutation reduces - in one individual only - the number of chromosomes like it happened some time ago for humans (chr. 2 fusion), how can it pass into the germ line with a population having one mutant only and all other indiviuals with a different karyotype. Pathenogenesis being ruled out. $\endgroup$ – Alain Pannetier May 23 '18 at 4:14

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