The evolution of chromosome numbers. How would it happen?

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The evolution of chromosome numbers. How would it happen?

Post by WiBarcaMa » Wed Dec 16, 2009 10:48 pm

I'm curious about chromosomal number. Humans have 46 chromosomes and (most/all?) other ape species have 48 chromosomes. Lots of evidence has been gathered, so we're relatively sure that this happened through chromosome fusion. (by the way, this isn't my main question, but it'll be interesting to see if you have an answer. Has anyone figured out what the specific benefit was to humans having 46 chromosomes versus the 48 that apes have?)

So I am wondering how exactly something like this could come about. I realize that people often have a variation in their number of chromosomes. For instance, I believe people with Down Syndrome have 47 chromosomes instead of 46. However, this seems to have a uniformly negative affect. So I think I'm safe in assuming that a beneficial change in chromosomal number must be extremely rare.

But let's assume someone ends up having two of his chromosomes fuse and this actually is a beneficial mutation. I assume, like everything else, chromosome number is determed by some gene or a combination of genes (another side question - does anyone know what genes determine chromosomal number?). Therefore, would this be passed on like any other trait? And if it is passed on like any other trait and that therefore the gene that determined this new beneficial chromosome fusion would be a dominant allele would this allele start to become dominant in the human population (assuming the guy mated of course)?

Also, why hasn't something like this happened yet? I don't know if this is a correct assumtion, but I think by now their have possibly been more humans being throughout history, than there have been apes. Therefore, of the billions upon billions of people that have lived, why hasn't one person had a new beneficial chromosome number and passed that on through reproduction?

Also, say someone did have a new beneficial number of chromosomes that was passed on. Would this lead to a rapid new stage in human evolution or not?

I appreciate any answers. Thanks

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Post by Darby » Fri Dec 18, 2009 4:53 pm

There was an article in Science a couple of years ago that showed how a fusion mutation would stabilize over time in a population. It was one of those explanations that make sense when you read it but fade with time, so there's no way I could explain it myself.

There are benefits and costs to both higher and lower chromosome numbers...

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Post by JackBean » Sun Dec 20, 2009 11:36 pm

The number of chromosomes is not given by some gene or anything. It just is. Everytime, when you divide the cell, you just copy, what you have and divide into to new packs ;) And that is, how you determine the number of chromosomes.
But sometimes may happen, that either two chromosomes fuse or one chromosome breaks into two and is not repaired...

Cis or trans? That's what matters.

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