Carl Zimmer in his blog, The Loom:
Our genes are arrayed along 23 pairs of chromosomes. On rare occasion, a mutation can change their order. If we picture the genes on a chromosome as
ABCDEFGHIJKLMNOPQRSTUVWXYZ
a mutation might flip a segment of the chromosome, so that it now reads
ABCDEFGHISRQPONMLKJTUVWXYZ
or it might move one segment somewhere else like this:
ABCDLMNOPQRSTUEFGHIJKVWXYZ
In some cases, these changes can spread into the genome of an entire species, and be passed down to its descendant species. By comparing the genomes of other mammals to our own, scientists have discovered how the order of our genes has been shuffled over the past 100 million years. In tomorrow’s New York Times I have an article on some of the latest research on this puzzle, focusing mainly on two recent papers you can read here and here.
One of the most interesting features of our chromosomes, which I mention briefly in the article, is that we’re one pair short. In other words, we humans have 23 pairs of chromosomes, while other apes have 24. Creationists bring this discrepancy up a lot. They claim that it represents a fatal blow to evolution.
More here.