05/04/2008 07:41 PM
Los Angeles Times: DNA matches aren't always a lock
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From the Los Angeles Times
DNA matches aren't always a lock
Genetic evidence is widely viewed as ironclad. In 'cold hit' cases, however, the truth is often elusive.
By Jason Felch and Maura Dolan
Los Angeles Times Staff Writers
May 4, 2008
Police found the naked body of Diana Sylvester near her Christmas tree.
The 22-year-old San Francisco nurse had been sexually assaulted and stabbed in the heart. She lay on her back, her neck laced with scratches and
her mouth open as if frozen in a scream.
For more than three decades, Sylvester's slaying went unsolved. Then, in 2004, a search of California's DNA database of criminal offenders
yielded an apparent breakthrough: Badly deteriorated DNA from the assailant's sperm was linked to John Puckett, an obese, wheelchair-bound
70-year-old with a history of rape.
The DNA "match" was based on fewer than half of the genetic markers typically used to connect someone to a crime, and there was no other
Puckett insisted he was innocent, saying that although DNA at the crime scene happened to match his, it belonged to someone else.
At Puckett's trial earlier this year, the prosecutor told the jury that the chance of such a coincidence was 1 in 1.1 million.
Jurors were not told, however, the statistic that leading scientists consider the most significant: the probability that the database search had hit
upon an innocent person.
In Puckett's case, it was 1 in 3.
The case is emblematic of a national problem, The Times has found. Prosecutors and crime labs across the country routinely use numbers that
exaggerate the significance of DNA matches in "cold hit" cases, in which a suspect is identified through a database search.
Jurors are often told that the odds of a coincidental match are hundreds of thousands of times more remote than they actually are, according to
dozens of interviews with leading scientific and legal authorities, a comprehensive review of scientific and academic papers in the field and court
documents in cold hit cases.
Two national scientific committees, including the FBI's DNA advisory board, have recommended portraying the odds more conservatively. But
interviews with expert witnesses and DNA analysts from crime labs across the country show that few if any have adopted that approach.
The FBI lab, which oversees the nation's offender databases, has disregarded the recommendation of its own advisory board, bureau officials
So far, the courts have ruled in law enforcement's favor on this issue.