Locard’s exchange principle, often understood as “ Every contact leaves a trace”, forms the foundation of modern-day forensic science. It is a useful operational doctrine, however, it cannot be reasonably expected that “every” contact will “always” leave a trace. As the practical realistic matter stands, an exchange may have occurred but not capable of being followed adequately or trace evidence may have fallen off after transfer.
Nevertheless, the principle is still a useful operational premise that could aid crime scene investigators to identify, convict or acquit suspects. In combination with biological evidence, whenever present, in the form of a DNA profile derived from blood, saliva, a single hair strand, or even skin cells left behind when a person touches a surface, this knowledge serves as a silent witness against perpetrators. To quote Paul L. Kirk, “this is evidence that does not forget”.
In 1984, Dr. Alec Jeffreys had his “Eureka” moment when he discovered certain patterns in a person’s DNA that could be used to distinguish one person from another. These patterns are called Variable Number Tandem Repeats or VNTR for short. These are regions in the loci of chromosomes that consists of a repeated sequence of DNA bases(A, T, C, G) i.e.. letters of DNA bases that repeat over and over again.
While the repeated sequences are usually the same from person to person, the number of times they are repeated differs. For example, the sequence GTCAACGTAGCT is repeated 10 times in person A but is repeated 13 times in person B.
Related individuals are likely to have a similar number of repeats to their close relatives but people who are complete strangers will have a different number of repeats and so different lengths of repeats.
DNA is extracted from a biological sample and is then amplified by a process called PCR (Polymerase Chain Reaction) which makes it possible to study the DNA further without the risk of using up the limited sample. Only the tandem repeats units (VNTR) are selected and amplified since these will be the distinguishable difference between individuals. The next step involves adding the DNA obtained to Agarose gel and passing an electric current through it in a process known as gel electrophoresis. Gel electrophoresis separate fragments of DNA according to size and produce a banding pattern which is revealed by a dye. The pattern of these bands that shows the presence of different size repeats makes up the DNA profile of a person.
The first notable use of DNA profiling in an investigation was in 1986 when the technique assisted in solving Dawn Ashworth’s case. Richard Buckland, who rather unexpectedly confessed to Ashworth’s murder, was later acquitted when the recovered DNA did not match his genetic code. The real culprit- Colin Pitchfork-was later convicted and sentenced to life when it was found that his DNA matched those of the crime scene samples.
The next year, Robert Melias became the first person to be found guilty with the help of DNA profiling when his DNA structure matched the semen stain on the clothes of a woman who was raped.
However, DNA profiling is not a one-stop solution to all criminal cases and forensic investigations: one must not overstate the utility of this common forensic method. A simple match of DNA on evidence does not always mean that the person was responsible for the crime. Such was the case of David Camm who returned home to find that his wife and two children died from gunshot wounds. Camm was arrested because the forensic analyst claimed that his t-shirt had been stained with his wife’s blood suggesting that he was the perpetrator. Camm was later released when the DNA of a career criminal-Charles Boney-was matched to a sweatshirt at the scene of the crime and the convictions took another turn.
The development of DNA profiling has undoubtedly aided in the progress of criminal and forensic investigation cases. But the line between the fiction of CSI shows and reality must not be blurred for a holistic review that takes into account other forms of evidence and explanations is necessary.
A “contact” does not always leave a perfect “trace”. And DNA evidence could be mishandled, deposited to the location even before the crime took place and the possibility of a secondary transfer where DNA was transferred to someone else and then carried to the scene is a possibility.
- Pakin Pongpaiboon
IBDP Student
Neerja Modi School