The genetic code of a new human begins when sperm fertilises the ovum
of the mother-to-be and forms a brand new cell which carries half the
genetic heritage of each parent through structures called chromosomes. The
chromosomes comprise a chain-like structure of genes made from
DeoxyriboNucleic Acid (DNA) molecules.
Two strands of protein molecules entwine to form the famous
double-helix structure of the DNA molecule with all the information needed
to build, control and maintain a living organism in chemically coded form.
Genetic profiling or restriction fragment length polymorphism (RFLP) is
used to analyse DNA found in biological materials such as blood, semen,
bone and hair found at the scene of a crime. A chemical called a
restriction enzyme is used to divide the DNA into fragments, which are
then separated according to size by another laboratory technique,
electrophoresis. The separated fragments form a pattern of parallel bands
that reflect the composition of the DNA. In principle, the pattern
produced will always be unique to any person. This technique enables
scientists to compare two samples, one taken from the crime scene and one
taken from the suspect, and determine whether the samples came from the
It is that unique and individual genetic code that is helping law
enforcement solve unsolvable crimes. In the criminal justice system DNA
profiling can eliminate suspects as well as enhance the successful
prosecution of guilty offenders - but unfortunately DNA profiling is not
an infallible tool in the lawyers' arsenal.
1) Charges in the 1997 Arnott's biscuit extortion case were dropped as
a result of flawed DNA testing procedures at Queensland's John Tonge
Tweed Heads great-grandmother, Joy Ellen Thomas, 72, was accused of
threatening to poison the company's biscuits unless four Sydney detectives
took lie-detector tests concerning evidence they gave against her son,
convicted murderer Ronald Henry Thomas. The prosecution against Mrs Thomas
relied heavily on evidence from forensic biologist Barry Blair, who
conducted DNA testing for the Crown at the John Tonge Centre.
Blair testified that DNA recovered from a stamp on one of the extortion
letters matched Mrs Thomas's DNA but the stamp had not been tested for
saliva, so it could not be determined whether Mrs Thomas had licked the
stamp. Blair claimed that although saliva testing would have identified
the source of the cells it would have ruined any chance of recovering the
During the pre-trial hearing on April 24, 2002, Blair changed his
opinion about the key DNA evidence after a forensic biologist hired by the
defence, Lazlo Szabo, revealed the existence of a second DNA profile from
an unknown person also present on the stamp.
Szabo, from Tasmania's Forensic Science Laboratory Centre, believed the
presence of the second person's DNA was evident at twice the levels
recorded by the Blair. His report detailing the second DNA profile was
given to the Crown two months before the pre-trial hearing. Although the
presence of the second person's DNA was indicated within a table in
Blair's reports, it was not mentioned in the text because, as he testified
at the pre-trial hearing, the second profile was a "stutter" -
an anomaly in the testing procedures.
The presence of a second DNA profile was evidence that could have freed
Mrs Thomas months earlier but the Crown opted to remain silent about its
existence until they were forced during the pre-trial hearing to concede
the legal significance of two DNA profiles being present. Prosecutor Paul
Rutledge argued that the Crown's silence resulted from a difference of
opinion between the two experts.
Rutledge, who had successfully prosecuted Ronald Henry Thomas at his
1994 murder trial, withdrew the charges in Brisbane Supreme Court on
Friday 26 April 2002.
2) Frank Alan Button, 32, served ten months of a seven-year sentence
for the rape of a 13-year-old intellectually impaired girl before
independent DNA testing established that he was not the perpetrator of the
The young girl claimed she had been assaulted during a party at her
mother's home at Cherbourg, southern Queensland, on February 17, 1999. The
following day she told two school friends she might be pregnant because
she'd been raped. The school principal called the police and detectives
took her back to the house where the rape had occurred.
Frank Button was asleep in a drunken stupor inside the house when
police arrived. He became the prime suspect because he was wearing a shirt
similar to the one the girl described to the police. Button was taken to
the local police station where he was interviewed and subsequently charged
with rape and locked up.
This is an edited version of a paper examining the
difficulties of DNA evidence. The full text is available here.
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