This is where HapMap comes in. If the sequenced genome is a massive
single-volume encyclopedia, HapMap breaks it up into hundreds of easy-to-read books. A strand of DNA comprises roughly three billion chemical units, known as A, T, C and G. Generally speaking, we all have the same letters in the same slots along that strand, which is why we resemble one another. Variations called single-nucleotide polymorphisms, or SNPs, however, pop up every 1,200 slots or so. Though tiny, these genetic disparities confer traits like hair, eye color and susceptibility to disease. HapMap has examined SNPs from the genomes of 270 individuals descended from four groups-Western Europeans, Japanese, the Yoruba of Africa, and the Han Chinese.