Why all great apes have 24 pairs of chromosomes, but we only have 23

This is very cool. Once you find out about chromosome pairs (high school biology in USA's public schools) a lot of things about Mendelian inheritance fall into place. But if you've got any curiosity, you start to wonder about the 23-pair vs 24-pair thing.

One question this article doesn't answer is "How does this work in a breeding population?"

Most chromosomal abnormalities cause spontaneous abortions, and the ones that don't (Down's Syndrome, etc) would seem to cause some pretty non-adaptive problems.

So how would a merger of 2 pairs of chromosomes start to breed true in a population?