DNA really the gold standard of forensic evidence?
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.
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.
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.
trial, the prosecutor, Rex Wild QC, hammered this point again and
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.
is DNA really as good as its been cracked up to be? Research
in the US has now thrown doubt on its utter reliability.
to the Los Angeles Times, it all began in 2001 when an Arizona
crime lab analyst, Kathryn Troyer, was running tests on the states
DNA database and turned up two crims with remarkably similar genetic
profiles. Astoundingly, one was black and the other white.
mens DNA matched at nine of the 13 locations on chromosomes,
or loci, commonly used to distinguish people.
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.
Troyer ran comparisons across the database and found dozens of similar
matches each seeming to defy impossible odds.
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.
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 youd
bet the farm on, are at least comprehensible to those of us who dont
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.
brings us back to the trial and conviction of Brad Murdoch. Its
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.
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.
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 wouldnt 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
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.
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 Murdochs guilt
and made the third piece of DNA, the just maybe, possibly, perhaps,
match from the Kombis steering wheel and gearstick, look very
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 Murdochs
lawyer, Id 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.