In 2001, an analyst in the DNA unit of Arizona’s state crime laboratory noticed something interesting. Two seemingly unrelated individuals—one white and one black—shared the same two markers at nine of the 13 places in the standard DNA profile. Yet that particular genetic profile should have been exceedingly rare.
According to the standard method of computing how often one might expect to encounter a particular DNA profile in the population at large—what is known as the “random match probability”—if you plucked a non-Hispanic white person at random from the population, there would be only a 1 in 754 million chance of finding that profile. For African Americans, the number was 1 in 561 billion. And yet here, in a database of less than 100,000 people, it was appearing twice—and in people of different races.
The DNA-unit analyst wrote up a quick summary of her findings and submitted the results to a major international forensic-DNA conference. Her observations came to the attention of a public defender in San Francisco, who held a master’s degree in genetics and was in midst of defending a California man, John Puckett, accused of a rape and murder from decades earlier. Police had collected forensic evidence in 1972, when a nurse was found sexually assaulted and fatally stabbed, but DNA typing was still decades away. The case sat open until, more than 30 years later, investigators dusted off the badly degraded DNA samples, tested them, and ran the results through the state database. A partial match linked then 70-year-old, wheelchair-bound John Puckett to the only testable evidence—sperm found on the body. On the basis of this match, prosecutors charged Puckett with murder.