Posted on June 29, 2013 in Criminal Law
DNA testing under the microscope
With the proliferation of CSI type crime dramas over the last few
years, many people from Juries to the lay public have come to assume
that DNA testing is a quick and foolproof way of proving the guilt or
innocence of a criminal suspect. In fact, some experts even worry that
this “CSI culture” is invading the purview of the courts; leading juries
to expect and even demand DNA evidence from the prosecution even in
cases where such evidence would add little of substance to the analysis.
Adding to the confusion are the range of media reports, some of which
warn about the possible inaccuracies of DNA testing procedures and
others which hail the ways in which the technology has been used to
absolve and free convicted criminals.
However, despite the media hype and many public misconceptions to the
contrary, DNA testing is a highly reliable and accurate method of
matching a sample with a donor, or of proving that no match exists. But,
this accuracy depends entirely on the methods used, the care and skill
of investigators and lab specialists, and the presentation of the
resulting evidence by lawyers during trial.
What is DNA testing?
DNA testing is the process of comparing a sample of DNA, such as from
a crime scene, to a potential suspect; a match implying the suspect’s
presence at the scene. This is possible because every person’s DNA is
unique, distinct in some way from every other person who has ever lived
or who will live – with the exception of identical twins who share exact
DNA. However, while each individual’s DNA is unique to that person,
current testing methods do not allow for a complete DNA profile.
Methods are improving however. First developed in 1985 by British
scientist Alec Jefferys and colleagues, DNA testing has come a long way
from the early days, both in quality and cost. Analyses that once
required a fresh tissue sample the size of a quarter can now be
completed with just a few partially degraded cells from a cold case
file. Tests which were once impossible, such as distinguishing
individual DNA profiles from the mixed semen of multiple rapists, have
now become common place and DNA testing has even been used to compare
non-human samples, such as the seeds of a tree which were used to tie a
suspect to a crime scene in one recent Arizona case.
How accurate is a match?
The entirety of a DNA sample is not currently tested. Instead,
sections known to vary substantially among humans are sampled and
compared. While any single sample site has about a 7.5% chance of
matching another random human, insufficient for criminal identification,
the odds increase exponentially with every additional sample site
considered. At about 4 to 6 sample sites – called markers –
identification becomes reasonably possible. For good measure, the
standard FBI test compares 13 markers, leading to greater than 1 in 1
billion certainty that a suspect is in fact the person who left the
sample. In other words, by comparing 13 quality (which means highly
variable) markers, less than 7 people in the entire population will be
statistically probable matches. Given that the pool of suspects cannot
ever include the entire population of the planet, this level of
certainty can provide the evidence necessary for conviction beyond a
(For more details on these numbers please visit here)
What’s the catch?
Naturally, there are some caveats about the numbers listed above.
While the statistics are solid, they have been tested repeatedly by many
different mathematicians and scientists the world over, the accuracy of
any purported match comes down to process; and here is where CSI type
shows drive unreasonable expectations about DNA results. As with much of
the criminal justice system, human error is a major factor. Collecting
DNA samples from a crime scene often requires expert training and
professional equipment. Samples must then be transferred and stored in
approved ways to avoid degradation. A qualified lab must properly
utilize expensive edge equipment to analyze the sample to produce a DNA
profile. That profile is then compared against either a sample taken
from a suspect or against one of the many available databases of DNA
profiles, which imply a number of privacy and security concerns, and a
Every step of this process is fraught with potential error. Crime
scenes can be contaminated or evidence planted, samples can become
contaminated or degrade, labs might employ outdated techniques or
scientists might not have the requisite training, computer error could
be introduced, jurors might give too much weight to DNA evidence; and
all along the way, human malfeasance might undermine the validity of a
match. And all this stems from but a single situation; a single trial.
While for individuals the results of a single DNA match are of the
utmost importance, whether the evidence is being used to convict or
exonerate, for the criminal justice system – and its promise of fairness
and justice for the public as a whole – systemic problems become the
more critical consideration. On this level, a number of issues must be
dealt with if DNA testing is to remain a viable tool in criminal
Evidence from analysis of the UKs national DNA database shows that a
disproportionate number of the DNA profiles in the system come from
minorities. While it can be argued that this represents the increased
prevalence of minority convictions, the increasing use of so called
“cold matches” wherein DNA from every arrested person is periodically
checked against the database, leads to possibilities of
institutionalized racism and prejudice. Additionally, the increasingly
popular practice of collecting samples from arrestees prior to any
conviction is leading to a number of very serious privacy and security
considerations. DNA samples can, in addition to providing material for a
possible match to a criminal investigation, also reveal family
connections, expose health conditions, and even predict sexual
orientation or inclinations.
Protecting the future
DNA profiling has become an increasingly important feature of our
criminal justice system, both exonerating the innocent and convicting
previously unreachable suspects; however its future use is imperiled by a
growing number of developing problems with the practice. More must be
done to protect this valuable criminal justice resource from corruption,
error, and prejudice. The worrisome backlog of untested samples should
be cut down, and the public at large – many of whom ultimately become
jurors – must be better informed about the accuracy, and potential
shortfalls, of the science.
Content: The following blog was taken from
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DNA Testing has become more reliable and prevalent in criminal trials. This blog discusses that trend and the possibilities of future abuses.