The Effectiveness of DNA Fingerprinting
by Lamise Kassem
DNA identification is the latest in a long line of controversial forensic
science methods (fingerprints, lie detectors, voice print). More than the
others, however, this advance stimulates the imagination in ways akin to
the mythical theft of fire from the gods. DNA-based techniques address growing
fears about violent crime. They also respond to even more basic hungers;
they seem capable of vanquishing the age old foe of uncertainty itself.
For many years now, scientists have argued about the reliability of forensic
DNA fingerprinting which is considered to be a powerful tool, some kind
of technology that was able to link blood, semen, or hair left at the scene
of crime to a suspect's DNA. The question is, " Is DNA fingerprinting
effective?" Experts have fought in so many courtrooms and have argued
about this new technology. A flow diagram shows the process of DNA fingerprinting(refer
to Bioscience, Vol.43, no.3, March 1993) A question arises about the result
of this process and that is, "Are we getting the right (1) result ?"
Many lawyers tried to find something to act upon this new technology that
would make it an unusable tool in predicting who is the suspect.
The lawyers attacked the scientists on how they could calculate the odds
that two people can have the same DNA profile. A DNA fingerprint is not
unique, as say, a fingerprint. There could turn out to be two unrelated
people to have the same set of alleles at the studied sites. The scientists
were able to resolve a problem like this by calculating the probability
by using the multiplication rule. The multiplication rule is basically
where they estimate the frequency with which each allele occurs in the general
population and then they multiply that frequency to possess the odds of
the match (Scientific American, October 1994, pg.34). Before performing
this calculation, it must be assumed that the alleles of different markers
are inherited independently of one another.
After doing the multiplication, is it likely that DNA profiles match?
If we arrive at a probable result, then the strength of this evidence is
usually measured by a "match probability" the probability that
an unrelated individual chosen randomly from an appropriate population will
match the crime profile. ( Nature, Vol.368, 24 March 1994, pg.285). We
must also take into consideration that even if we have a small match (2)
probability that does not in itself, establish guilt. We can see the results
of this tool by examining the examples below. In the first example, the
circumstances are real, however, the DNA fingerprint evidence allowed investigators
to distinguish between suspects : who were the father and his ten year old
son. What basically happened was that a brother and sister were stabbed
and the sister was also raped. She died as a result of the attack but the
brother even though he was in a critical condition, recovered. The father
was accused formally for the rape and murder of his daughter and the attempted
murder of his son. The father was questioned and his defense was that his
son had raped the sister and was stabbed by her during the struggle. Conventional
serological testing of the semen stains may or may not unequivocally distinguish
between father and son.
The DNA pattern from the father matched that of the semen evidence, whereas
the DNA pattern of the son did not. Even though we might say since the
father and son are related and they share some of the markers, it turns
out that there are genetic differences that were specific to each family
member. These differences allowed each piece of evidence to be unambiguously
attributed to a specific individual. The father subsequently was found
guilty and sentenced to life in prison. The results in this case highlight
the power of DNA (3) fingerprinting, and the legal issues raised during
the prosecution of this case are typical of those surrounding DNA-based
identification technology as it is currently used today.
Another example of the utility of DNA fingerprinting is where evidence
is left at the scene of crime. The evidence may be examined for the purpose
of identifying the person or persons who might have committed the crime.
It might seem that DNA analysis is most efficient in sexual assault crimes.
In this crime, the biological evidence(e.g semen collected from vaginal
swabs) and the location of that evidence is a convincing argument for the
guilt of a suspect whose DNA matches the DNA results. Even more important
is the often overlooked exclusionary or nonmatch result. Generally, if
there is not a match between the evidence and suspect, a charge of a sexual
assault becomes difficult to support in most circumstances. When charges
are dropped as a result of DNA nonmatch, there is no forum for the examination
of the results as there is in a match and subsequent trial.
A figure (Bioscience, Vol.43, No.3, March 1993, pg.150) shoes a result
of DNA matching according to the samples that were studied. DNA samples
were taken from three known individuals, which are shown on the autoradiograph
: the victim, suspect 1, suspect 2, and from an additional control sample.
From the autoradiograph, DNA samples on the lane (4) labeled "evidence"
match the lane labeled "suspect 1". From looking at such results,
the whole process is relatively easy to evaluate. There maybe many individuals
who if not for the DNA evidence, would be as likely to be the guilty person
as the defendant. If there were 500,000 such individuals, for example in
a large city, the probability that the defendant is innocent after taking
the DNA evidence into account would be at least one third (Nature, Vol.368,
24 March 1994, pg. 285). The reasons for the power of DNA-based identification
system depends on two facts :
1). LOW INCIDENCE PROBABILITIES
2). MULTIPLICATION OF INDEPENDENT OBSERVATIONS
Even though it turns out that DNA fingerprinting is a powerful tool, its
high efficiency and reliability of its results must not lead to over-interpretations
of their meaning. Positive results are still based on probabilities and
statistical analyses which always provide information about uncertainties
rather than about facts. Also when the results reveal that one in a billion
or in a trillion, this does not mean anything since this process deals with
long calculations, which may not be justified. Also, in addition to DNA
fingerprinting, more evidence is needed to reach the verdict of either innocence
or guilt. (5) The result of all of this has showed that DNA-based identification
is very important and has already been built in the forensic community and
has become a viable part of the judicial system both in the United States
and abroad. The Federal Bureau of Investigations performs 2,500 tests a
year for federal, state and local prosecutors, tests that have helped to
convict or exonerate tens of thousands of suspects.(6)
1). Identity Testing in Forensic Science Biosciencs, Vol.43, no.3, March
1993 Kevin C. McElfresh, Debbie Vining Forde, and Ivan Balazs.
2). How convincing is DNA Evidence ? Nature, Vol.368, 24 March 1994 David
J. Balding and Peter Donnelly.
3). High Profile Scientific American, October 1994 John Horgan.
4). DNA Fingerprinting : Academy Reports Science, Vol.256, 17 April 1992
5). Science in Court : A Culture Clash Science, Vol.256, 7 August 1992 Leslie
6). Statistical Evaluation of Dna Fingerprinting : A Critique of NRC's Report
Science, Vol.259, 5 February 1993 Devlin, Neil Risch, Kathryn Roeder.