Mutational direction through cross-species comparison
One way to tell which of two nucleotides is the original and which is the mutation is to compare that region to the DNA of a relative who diverged from us before the mutation took place. Chimpanzees are our closest relatives, differing from us in only about 1.2% of our DNA.

Your strategy is to find regions in the chimp genome that matches each of the SNPs in your collection. As it happens, chimp chromosome 22 is very close to human chromosome 21, so you start looking there and find tens of thousands of matches, enough to keep you busy for a long time.

Too many! Which can you focus on that promise to give clues as to a major source of human genetic variation?

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