Every
person has a unique set of DNA that was inherited from their mother and father
and identifies who they are. The only
exception to that rule is identical twins.
In a crime scene DNA can be collected and using analytical techniques be
matched with a suspect to determine if they were present there. In the past twenty years, technology has
become so advanced that DNA testing has evolved also. There are basically four tests used today;
STRs, PCR, RFLP, and mtDNA. Some have
been used for a while and others are fairly new. How do they compare to one another and how
are they different? How do analysts use
these tests to solve crimes, and does finding DNA at a crime scene impact the
whole criminal process?
We inherit half of our DNA from our father
and half from our mother, and that makes up the blueprint for our looks.
Imagine that you have a pool of 13 potential suspects (stars)
implicated in a crime. Using STR
analysis, you discover that 6 of the potential suspects have the THO1 genotype
of 6-3, which is a genotype found at the scene of the crime. The other 7 samples have a different genotype
at the TH01 locus. So, in the analysis
of this one allele, you were able to exclude 7 suspects. (Strausbaugh, nd, p.
14).
STR (short random repeats) are short sequences of base pairs that repeat
end to end, like a train. "Although
these sequences occur in everyone's DNA, the number of times they repeat vary
between individuals" (Platt, 2003, p. 60).
To determine uniqueness of the STR, they are counted and separated for
analysis. "STR is one of the newer
and more flexible techniques; it has the advantage of being able to analyze
degraded and broken pieces of DNA" (Sapp, nd, p. 3).
PCR (polymerase chain reaction)
is a process to artificially increase DNA for analysis. This is useful, for example; if one drop of
blood is found, using this process will give the technician enough DNA to
effectively analyze. The problem with
PCR testing is that the possibility of contamination is greater within the
crime scene, and the lab doing the testing.
One of the earliest tests for
DNA testing is the RFLP (restriction fragment length polymorphism). (Sapp, nd, p. 3). This test, however, requires that large
amounts of un-degraded DNA be analyzed.
Because of the huge amounts in order to test, it is rarely used
today. Also, in order for the test to be
effective, samples should be fresh and clean, which can pose a challenge as
crime scenes are generally messy.
Afore mentioned tests use
nuclear DNA, an alternative is to use mtDNA (mitochondrial DNA) which is a form
of DNA not involving the nucleus of a cell.
"Unlike nuclear DNA, where 50% is inherited from each parent,
mitochondrial DNA is passed on intact from just the mother, which makes it
ideal for tracing ancestry" (Platt, 2003, p. 63). It also does not decay as quickly as nuclear
DNA, and can be found years back from bone fragments. This helps to identify victims that have
decomposed.
The CODIS (combined DNA index
system) is a database that stores DNA of people that have been convicted of
serious, violent crimes such as rape and murder. This allows law enforcement to cross
reference a suspect if they commit another serious crime. This system has helped to catch child
predators.
How can DNA evidence solve crimes? "DNA profiling is the use of
molecular genetic methods to determine the exact genotype of a DNA sample to
distinguish one human being from another" (Strausbaugh, nd). It is now routinely used for investigations
of crime scenes, disasters, and missing persons. Imagine a crime scene where there is fresh
blood, the DNA is extracted and the main suspect is an ex boyfriend, who claims
he has not seen her in months. However,
the analyst team discovers that a DNA match was found on her shirt, that proves
he was lying and the potential murderer.
Before DNA evidence became the preferred way to identify suspects, the
forensic specialist had to link suspects to the crime scene the hard way. "They often did this indirectly, for
example showing that the muddy shoe found in the suspect's car had a shoe print
identical to that left at the scene of a crime" (Strausbaugh, nd). Finding evidence such as this can suggest
that the suspect was at the crime scene, however it does not "prove"
that they were there. That's where DNA
evidence can help; it links the suspect to the crime scene.
The turning point in solving
most crimes today came when it was discovered that DNA testing could be used to
determine who was at the crime scene and who was not. Before that day, in the 1980's, fingerprints
were most commonly used. "Although
conventional testing procedures had gone a long way toward narrowing the source
of biological materials, individualization remained an elusive goal. Now DNA typing has allowed forensic
scientists to accomplish this goal" (Saferstein, 2011, p. 266). STRs have been the forerunner for DNA typing,
considering one only needs a small amount of a bodily fluid to test, since it
uses the PCR to maximize the quantity.
What makes this invaluable to forensic scientists is that the more STRs
they can characterize to one individual, the likelihood that they are the
perpetrator is greater. Other tests can
be used; however most of them have limitations.
Also, STRs use modern technology that makes testing faster and easier,
and with limited time to work with, this is gold.
References
Platt, R. (2003). Crime
scene: The ultimate guide to forensic science, DK Publishing, New York, New
York, 10014
Saferstein, R. (2011).
Criminalistics: An introduction to forensic science, 10th edition, Upper
Saddle River, New Jersey, 07458
Sapp, B. (nd). DNA
& typing, Retrieved December 17, 2011 from
http://www.bsapp.com/forensics_illustrated/forensic_text_adobe/text_unit_11_dna_typing.pdf
Strausbaugh, L. Dr. (nd). Crime scene investigator PCR basics kit, Retrieved December 17,
2011 from http://www.cpet.ufl.edu/sets/PDF/CSI%2OPCR%20manual.pdf
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