Defendant John Puckett, right, in a San Francisco courtroom with his attorney, Kwixuan H. Maloof. Puckett insisted he didn't kill Diana Sylvester, saying that although DNA at the crime scene happened to match his, it belonged to someone else. The jury found him guilty. Genetic evidence is widely viewed as ironclad. In 'cold hit' cases, however, the truth is often elusive.
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.
Victim From cell to DNA profileThe odds of a match 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 physical evidence.
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 a review of scientific literature and interviews with leading authorities in the field.
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 acknowledged. So far, the courts have ruled in law enforcement's favor on this issue.
As a result, some experts fear, a technology best known for freeing the innocent could be causing its own miscarriages of justice.
"It is only a matter of time until someone is wrongfully convicted because of this," said Keith Devlin, a Stanford mathematician who has studied the problem.
DNA profiles are widely perceived as a unique genetic fingerprint. In fact, they are slivers of the human genome -- up to 13 markers that contain about a millionth of the information on all the chromosomes. Relatives often share many markers, and even unrelated people on average share two or three.
So DNA "matches" by themselves can never definitively link someone to a crime.
The best science can do is to estimate the likelihood that a match has occurred by sheer chance. These statistics are easily distorted or misunderstood by lawyers, judges, juries and even expert witnesses.
This potential for distortion is compounded in cold hit cases.
For years, police used DNA only to compare a crime-scene sample to a single person they had other reasons to suspect. In court, prosecutors could legitimately cite the remote odds that someone selected at random off the street would share the same genetic profile.Posts: 15202 | Registered: Jun 2004
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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 physical evidence.
DNA profiles are widely perceived as a unique genetic fingerprint. In fact, they are slivers of the human genome -- up to 13 markers that contain about a millionth of the information on all the chromosomes. Relatives often share many markers, and even unrelated people on average share two or three.
So DNA "matches" by themselves can never definitively link someone to a crime.
Yes.
On another note, and may well be a given,
When I hear for instance, that this president or vice-president, or some celebrity is related to one presidential candidate or the other—several generations removed, short of research into the matter, I have to just assume that this would be based on not only autosomal DNA "informative" markers and/or patrilineal-specific and matrilineal-specific marker, but also extensive family records of the family tree of the subjects at hand. As many of us—who are adequately-read on molecular genetics— probably know, the major designated paternal-specific or maternal-specific markers are likely to reappear in samples relatively more frequently than say designated atDNA markers, while on the other hand, atDNA 'informative markers' can only take one so far through generations before outliving its informative usefulness. So, for one to skim through literally thousands of records in the sample databases, and then single out just two or a few more individuals—particularly if the samples were initially treated as "unknowns"—as unequivocally closely-related people, one has to assume that additional information must have come in handy to buttress the finding; information like that of an adequately-kept record, detailing the family trees of the selected candidates.
Posts: 233 | Registered: Apr 2007
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