basis of systems for identifying populations or individuals. Only some 3% of the DNA in the genome is devoted to genes; the rest of the DNA is mostly yards of filler material. The short tandem repeats are part of the filler material so do not affect a person's physical makeup. But some repeats lie close to genes, some of which have evolved in different ways in the various races. By selecting the right repeats, geneticists can find ones that are quite diagnostic of race, even though at present they have little idea which genes it is that give people of different races their different appearance.

Risch calculated that if the sites with short tandem repeats were chosen entirely at random, analysis of about 100 sites should suffice to say which of the five major races a person comes from. But as few as 30 sites would be enough if the sites were specially chosen so as to be diagnostic of race. Many hundreds of markers would be needed to distinguish, within a race, between two populations or ethnicities, Risch estimated.

Sets of these sites, known as Ancestry Informative Markers, can be used to identify not just an individual's race but the racial origin of individual sections of a person's genome. A company called DNAPrint Genomics has already started offering a test to assess people's continent of origin and, if of mixed race, the

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