Is DNA really the gold standard of forensic evidence?

Is DNA really the gold standard of forensic evidence?

16 August 2008

When the deal finally went down, the guilty verdict in the trial of Bradley John Murdoch for the July 2001 murder of Peter Falconio was founded on just one bit of evidence, and that was a DNA match.

The police investigation turned up several bits of DNA evidence but only the tiny, weak, watery, bloodstain on the back of Joanne Lees’ T-shirt was apparently immune from any doubt because it was profiled ten months before Murdoch became a suspect.

When a match with Murdoch was established it was said that the chances of the tiny stain not being from the big drug-runner was billions to one. DNA was the gold standard of forensics – the odds against two people sharing the same genetic fingerprint were vanishingly small.

In the trial, the prosecutor, Rex Wild QC, hammered this point again and again.

“The difficulty for Murdoch”, he insisted was: “how did his DNA get onto Miss Lees’ T-shirt? That DNA was damning evidence. It really is the Lynchpin in this case” he told the jury.

But is DNA really as good as it’s been cracked up to be? Research in the US has now thrown doubt on its utter reliability.

According to the Los Angeles Times, it all began in 2001 when an Arizona crime lab analyst, Kathryn Troyer, was running tests on the state’s DNA database and turned up two crims with remarkably similar genetic profiles. Astoundingly, one was black and the other white.

The men’s DNA matched at nine of the 13 locations on chromosomes, or loci, commonly used to distinguish people.

Supposedly, the odds of unrelated people sharing those genetic markers were as remote as 1 in 113 billion. But the mug shots of the two men suggested they were not related.

Intrigued, Troyer ran comparisons across the database and found dozens of similar matches – each seeming to defy impossible odds.

When word of the discovery leaked out, defence lawyers began raising the possibility of false positives in DNA profiles and the FBI began what the LA Times described as “an aggressive behind-the-scenes campaign to block similar searches elsewhere, even those ordered by courts”.

The technical debate is ongoing and complex, but we can at least say that the odds of a false positive, while not being of the sort you’d bet the farm on, are at least comprehensible to those of us who don’t have an astrophysics degree. And the number of loci compared makes a huge statistical difference. These days, labs usually compare 13 loci, but in the past, nine or sometimes fewer, was the standard. Obviously, the fewer the loci compared, the greater the chance of a false positive.

Which brings us back to the trial and conviction of Brad Murdoch. It’s little wonder that the prosecution stressed that one piece of DNA: evidence of motive was risibly weak; the timelines were unconvincing; Joanne Lees’ account of the incident at Barrow Creek was wildly contradictory and had changed greatly over time.

There was other DNA evidence – swabs taken from the gearstick and the steering wheel of the Kombi. This certainly was sampled before Murdoch was a suspect but it proved rather inconclusive, showing traces of Falconio, Lees and a third person who may or may not have been Murdoch.

So the police faced a big problem: apart from the DNA on Lees T-shirt, every other piece of evidence was less than compelling and a jury might well believe that the tiny stain could have been accidentally acquired by Lees at any one of three places where she, Falconio and Murdoch crossed paths: a SA caravan park, the camel races, and the Alice Springs Red Rooster. I certainly wouldn’t like to go into a prosecution with evidence as slender as that, but the T-shirt sample had the virtue of having been tested before the cops settled on Murdoch as their suspect.

It is from this fact that the ugly suspicion originated that the crucial third piece of corroborating DNA evidence – a match from inside the black tape of the makeshift handcuffs used to restrain Lees – was planted by the police, after Murdoch became the chief suspect, to firm up their case. As the defence, and various other commentators pointed out, they had ample opportunity to do so. And then there was the nature of the particular form of DNA test used to make the match and to firm-up those from inside the Kombi. This was the dodgy ‘low copy’ method not recognised by the FBI as valid, and the tiny sample was destroyed in the process of the test so there could never be a re-run.

Unreliable this third piece of DNA evidence might have been, but in the context of the apparently unchallengeable match from the T-shirt it gained significance. It sharpened the likelihood of Murdoch’s guilt and made the third piece of DNA, the just maybe, possibly, perhaps, match from the Kombi’s steering wheel and gearstick, look very suggestive indeed.

But what if that original DNA match from Lees’ T-shirt was no match at all? What if it was a false positive? If I was Murdoch’s lawyer, I’d be wanting to know just how many loci the NT crime lab (and the English ‘low copy’ lab) used in the profile that linked Murdoch to the crime.

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We've been online since 1997. Check out the archives or … powered by FreeFind © Nick Possum/ Brushtail Graphics	Is DNA really the gold standard of forensic evidence? 16 August 2008 When the deal finally went down, the guilty verdict in the trial of Bradley John Murdoch for the July 2001 murder of Peter Falconio was founded on just one bit of evidence, and that was a DNA match. The police investigation turned up several bits of DNA evidence but only the tiny, weak, watery, bloodstain on the back of Joanne Lees’ T-shirt was apparently immune from any doubt because it was profiled ten months before Murdoch became a suspect. When a match with Murdoch was established it was said that the chances of the tiny stain not being from the big drug-runner was billions to one. DNA was the gold standard of forensics – the odds against two people sharing the same genetic fingerprint were vanishingly small. In the trial, the prosecutor, Rex Wild QC, hammered this point again and again. “The difficulty for Murdoch”, he insisted was: “how did his DNA get onto Miss Lees’ T-shirt? That DNA was damning evidence. It really is the Lynchpin in this case” he told the jury. But is DNA really as good as it’s been cracked up to be? Research in the US has now thrown doubt on its utter reliability. According to the Los Angeles Times, it all began in 2001 when an Arizona crime lab analyst, Kathryn Troyer, was running tests on the state’s DNA database and turned up two crims with remarkably similar genetic profiles. Astoundingly, one was black and the other white. The men’s DNA matched at nine of the 13 locations on chromosomes, or loci, commonly used to distinguish people. Supposedly, the odds of unrelated people sharing those genetic markers were as remote as 1 in 113 billion. But the mug shots of the two men suggested they were not related. Intrigued, Troyer ran comparisons across the database and found dozens of similar matches – each seeming to defy impossible odds. When word of the discovery leaked out, defence lawyers began raising the possibility of false positives in DNA profiles and the FBI began what the LA Times described as “an aggressive behind-the-scenes campaign to block similar searches elsewhere, even those ordered by courts”. The technical debate is ongoing and complex, but we can at least say that the odds of a false positive, while not being of the sort you’d bet the farm on, are at least comprehensible to those of us who don’t have an astrophysics degree. And the number of loci compared makes a huge statistical difference. These days, labs usually compare 13 loci, but in the past, nine or sometimes fewer, was the standard. Obviously, the fewer the loci compared, the greater the chance of a false positive. Which brings us back to the trial and conviction of Brad Murdoch. It’s little wonder that the prosecution stressed that one piece of DNA: evidence of motive was risibly weak; the timelines were unconvincing; Joanne Lees’ account of the incident at Barrow Creek was wildly contradictory and had changed greatly over time. There was other DNA evidence – swabs taken from the gearstick and the steering wheel of the Kombi. This certainly was sampled before Murdoch was a suspect but it proved rather inconclusive, showing traces of Falconio, Lees and a third person who may or may not have been Murdoch. So the police faced a big problem: apart from the DNA on Lees T-shirt, every other piece of evidence was less than compelling and a jury might well believe that the tiny stain could have been accidentally acquired by Lees at any one of three places where she, Falconio and Murdoch crossed paths: a SA caravan park, the camel races, and the Alice Springs Red Rooster. I certainly wouldn’t like to go into a prosecution with evidence as slender as that, but the T-shirt sample had the virtue of having been tested before the cops settled on Murdoch as their suspect. It is from this fact that the ugly suspicion originated that the crucial third piece of corroborating DNA evidence – a match from inside the black tape of the makeshift handcuffs used to restrain Lees – was planted by the police, after Murdoch became the chief suspect, to firm up their case. As the defence, and various other commentators pointed out, they had ample opportunity to do so. And then there was the nature of the particular form of DNA test used to make the match and to firm-up those from inside the Kombi. This was the dodgy ‘low copy’ method not recognised by the FBI as valid, and the tiny sample was destroyed in the process of the test so there could never be a re-run. Unreliable this third piece of DNA evidence might have been, but in the context of the apparently unchallengeable match from the T-shirt it gained significance. It sharpened the likelihood of Murdoch’s guilt and made the third piece of DNA, the just maybe, possibly, perhaps, match from the Kombi’s steering wheel and gearstick, look very suggestive indeed. But what if that original DNA match from Lees’ T-shirt was no match at all? What if it was a false positive? If I was Murdoch’s lawyer, I’d be wanting to know just how many loci the NT crime lab (and the English ‘low copy’ lab) used in the profile that linked Murdoch to the crime.