| Until the
1990s, the only sure-fire way to establish the identity of an
individual was to examine his or her fingerprints. That is
because each individual's fingerprints have a unique pattern.
Now, DNA is rapidly becoming the method of choice when it
comes to linking individuals with crime scenes and criminal
assaults.
Most people know that
our inherited characteristics come from chromosomes and genes.
Since the 1950s, science has also known that the chromosomes
consist of self-replicating molecules known as
deoxyribonucleic acid, or DNA, and that our genes consist of
subsets of these very-large molecules. Now, as the end of the
century rapidly approaches, we are on the verge of learning
which genes result in which characteristics. While this
knowledge promises untold rewards in the treatment of disease,
agricultural production and an understanding of life itself,
it is also being used as a means to uniquely identify
individuals and to link individuals to criminal activities.
The universally
accepted theory underlying DNA analysis is that every person
(except an identical twin) has certain elements of his or her
DNA that are unique. Different methodologies allow experts to
identify these distinguishing elements. The most common of
these methodologies is "RFLP," or the
"Restriction Fragment Length Polymorphism
Technique." By comparing an individual's known DNA with a
sample of DNA from a crime scene (for example, in a droplet of
blood or a strand of hair), an expert can give an opinion
concerning the likelihood that both samples came from the same
individual.
This sort of
technology is extremely complex; few people are able to
understand it. And yet the major crime labs and, increasingly,
local police agencies, are becoming adept at comparing DNA
samples left at the scene of a crime with DNA taken from a
suspect -- and concluding on that basis that the suspect is
the culprit.
DNA evidence is also
proving to be a powerful tool in determining the innocence of
prisoners who were tried before DNA testing in its current
form was an option. If bodily samples, such as blood or semen,
were collected from the crime scene or victim and that
evidence is still available for DNA testing, a showing that
the prisoner's DNA doesn't match the crime-scene DNA can be a
powerful reason to conclude that the prisoner is innocent and
should be released. A number of prisoners who were sentenced
to death have been cleared through this technology.
The conclusion that a
DNA match proves the defendant's guilt is based primarily on
the assumption that the probability against one individual
matching another is in the hundreds of millions, or even
billions, depending on who is crunching the numbers. However,
as overwhelming as these figures may seem, it's still possible
to whittle them down to far less overwhelming odds -- or even
to no odds at all -- if it can be shown that the methods used
by the laboratories doing the testing were flawed in some
manner. It is this approach, among others, that the defense
team in the O.J. Simpson case used to mount its defense
against what appeared to be overwhelming DNA evidence
implicating O.J. Simpson as the guilty party.
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