DNA profiling

DNA profiling compares a few DNA regions that vary greatly between people to judge whether a biological sample, such as blood, saliva or skin cells left on an object, could have come from a particular person. The British geneticist Alec Jeffreys devised the first method in the mid-1980s, using long repeated sequences called minisatellites.

Today's standard method examines short tandem repeats (STRs): sites where a unit of two to six base pairs is repeated many times in a row, with the number of repeats differing between people. At each site a person carries two alleles, one from each parent. The STR sites are copied by PCR with fluorescently labeled primers, many in one reaction, and the copies are sorted by length with capillary electrophoresis. The profile lists the repeat numbers at a standard panel of sites, plus a sex marker. Because the sites are inherited independently, agreement at a dozen or more makes a chance match between unrelated people extremely unlikely, and a match is reported as a statistic, not a certainty.

Profiles are stored in national databases and also used for paternity testing, missing persons and identifying disaster victims. Mixtures from several people, tiny or degraded samples and DNA transferred by touch make interpretation harder, and sequencing methods now add markers such as SNPs.