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The Case of the Bloody Brick
In March 2003, a drunk in southern England threw a brick off a bridge late at night, striking and killing a truck driver traveling along the freeway below. Armed with DNA from the blood on the brick, the British police searched the United Kingdom's national DNA database, which includes convicted felons and people who have been arrested, but failed to get a direct match. They then conducted a DNA dragnet, asking hundreds of young men in the area to donate a sample voluntarily, but still came up short. Without any other leads, the police decided to conduct what's called a "familial search" of the national DNA database. They were looking not for perfect matches to convicted offenders but for near matches, in the hope of using them to identify a relative who might have committed the crime.

Ordinarily, when searching for actual offenders, the British police look for a perfect match to a DNA profile that contains 10 pairs of peaks, or "alleles," with one number in each pair provided by the father and...