P.Z. Myers (Yes, the one who was expelled) responds to an e-mail asking how chromosome numbers can increase in sexually reproducing species. For good measure, he explains how they can decrease.
In essence, you start with a chromosome with genes and one centromere.
Here's something fairly common. An error in copying the DNA can lead to the loss of a piece of DNA. This happens with a low frequency, but it does happen — if we sequenced your DNA, we might well find a few bits missing here and there. We can get situations like this, where a whole gene gets lost.
Another kind of error that can happen with a low frequency is a duplication, where the machinery of the cell accidentally repeats itself when copying, and you get an extra copy of a piece of a chromosome...
You may have noticed that nothing has changed the chromosome numbers yet. Here's a situation that can lead to the formation of a new chromosome: what if there is a duplication of the centromere, rather than a gene?...
One last thing: what about reducing chromosome numbers? That's easy, too. Here's an organism with an AB chromosome, and a different chromosome with the genes MN on it. They can simply fuse in the region of the centromere.
Read the rest for the full explanation, and pictures.
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