Clockwise from top left: images courtesy of Affymetrix, Illumina, Sequenom and Illumina

Take any two individuals, sequence and compare their genomic DNA, and you'll find that the vast majority (about 99.9%) of the sequences are identical. In the remaining 0.1% lie differences in disease susceptibility, environmental response, and drug metabolism. Researchers are understandably keen to dissect these variations, most of which take the form of single-nucleotide polymorphisms (SNPs).

A SNP (pronounced "snip") is a substitution of one base pair at a given location on the genome. At position 11,294,479 on human chromosome 7, for instance, some people have an A, while others have a G. On average, SNPs are spaced every 300 bases throughout the human genome and are estimated at nearly 10 million. Each is a genomic landmark, a surveyor's marker that researchers can use to chart the location of disease genes and heritable traits, for instance.

Most SNPs...

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