Friday, October 30, 2015

When not to trust scientists

Forensic DNA evidence is not infallible : Nature News & Comment

Given that it infuriates me that a substantial slab of people do not believe that those who work in the physical sciences are competent when working out things like a temperature record, or the effects of a gas in an atmosphere, I suppose I give the impression much of the time that I never question scientists of any stripe.

But when it comes to the biomedical sciences:  yes, there are many examples of scientists leaping in when they should be more cautious.

And when they get caught up in a court case, they are particularly prone to giving up caution, presumably  because they think they are promoting a good outcome.

This article in Nature shows the danger:
The term 'touch DNA' conveys to a courtroom that biological material
found on an object is the result of direct contact. In fact, forensic
scientists have no way of knowing whether the DNA was left behind
through such primary, direct transfer. It could also have been deposited
by secondary transfer, through an intermediary. (If I shake your hand
then I could pass some of your skin cells onto something that I touch

Contamination from secondary DNA transfer was raised as a possible problem in Nature in 1997 (R. A. H. van Oorschot and M. K. Jones Nature 387, 767; 1997).
It is known to happen, but has largely been dismissed by legal experts
as being rare outside the conditions of a laboratory. Experiments done
in real-world conditions seemed to support this, and concluded that
secondary DNA transfer would have little impact on interpretation of the
genetic profile.

It is important to recognize that DNA amplification kits have become
much more sensitive than they were in the past. As a result, the types
of samples being analysed have expanded. Investigators no longer need to
identify and request analysis of body fluids such as blood, semen and
saliva. They can swab surfaces for otherwise invisible cells left
behind, on the handle of a weapon or on a windowsill, perhaps, and ask
labs to generate a DNA profile from them. The new kits can generate a
full genetic profile of a suspect from as little as 100 picograms
(trillionths of a gram) of DNA.

These subtleties are not usually explained in court. Instead, a jury is told
that there is a one-in-a-quadrillion chance that the evidence retrieved
from the crime scene did not come from a defendant. Naturally, the
jurors assume that the defendant must have been there.

Given  the power of modern forensic techniques to pull a DNA profile from a
smudge of cells, secondary DNA transfer is no longer a purely
theoretical risk. In California in 2013, a man called Lukis Anderson was
arrested, held for four months and charged with murder after his DNA
was found under the fingernails of a homicide victim.

Anderson had never met the victim and was severely intoxicated and in hospital
when the man was killed. The same paramedics who took Anderson to
hospital responded to the murder. Most likely, the paramedics were
covered in Anderson's DNA, which they then inadvertently transferred.
The charges were dropped.

Experiments in our labs, under the supervision of forensic anthropologist Krista Latham,
show how easily DNA can be transferred to an object.

We asked pairs of people to shake hands for two minutes and then each
individual handled a separate knife. In 85% of cases, the DNA of the
other person was transferred to the knife and profiled. In one-fifth of
the samples, the DNA analysis identified this other person as the main
or only contributor of DNA to the 'weapon' (C. M. Cale et al. J. Forensic Sci.; 2015).

How significant is the result of a single study? Other analyses have shown
that DNA transfer can be unpredictable and can depend on environmental
conditions. We need more research on when and how secondary transfer can

1 comment:

Not Trampis said...

wow, interesting