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Humans have approximately 25000 genes. Why are these genes on 46 chromosomes? Why not 40 or 50?

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marked as duplicate by AMR, rg255, AliceD, kmm, anongoodnurse Dec 27 '15 at 4:26

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  • $\begingroup$ In general, the chromosome number has to be even because each set of chromosomes comes from 1 parent, so 2 x N is always even. But different species have different chromosome numbers. No real reason why humans have to be at 46. There are plenty of people who survive with 47. $\endgroup$ – user137 Dec 23 '15 at 11:59
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    $\begingroup$ Chromosome rearrangements are just a type of mutations, hence the process is stochastic. Some chromosomes split, some merge, some duplicate etc. These events are very rare, though. Like all mutations these influence fit to some extent, since karyotype structure is known to play important role in epigenetic regulation. $\endgroup$ – Eli Korvigo Dec 23 '15 at 12:20
  • $\begingroup$ user137 and @EliKorvigo I think one of you can extend your comment into an answer. $\endgroup$ – Remi.b Dec 23 '15 at 17:18
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According to this article, no one knows; it's an "evolutionary mystery." As a general rule, closely related species generally have a similar number of chromosomes, and the number typically increases with the complexity of the species. However, the actual number varies wildly.

The author also points, out,

Each egg or sperm gets 23 chromosomes (half of each pair). Which chromosome they get in the pair is totally random. When you do the math, this comes out to 10 trillion different possible combinations. If we had only one pair of chromosomes, the number drops to 4.

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    $\begingroup$ The fact about closely related species having similar numbers likely comes from chromosomes fusing or splitting at some point during speciation, increasing or decreasing the number by 1. I would speculate that changes in chromosome number would drive speciation because it would be harder for two individuals with different chromosome numbers to produce viable fertile offspring. $\endgroup$ – user137 Dec 23 '15 at 12:11
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    $\begingroup$ In addition to @user137, closely related species generally have a similar number of chromosomes is nothing but a phylogenetic signal, that is data points are not independent. It does not suggest anything about the selection pressure encountered by related species. $\endgroup$ – Remi.b Dec 23 '15 at 17:16
  • $\begingroup$ Note also btw, that the linked article is NOT peer-reviewed and after a very quick look to it it seems to support user137 and EliKorvigo point of view rather than yours. $\endgroup$ – Remi.b Dec 23 '15 at 17:18

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