DNA and Crime Detection

DNA and Crime Detection

Today you can’t watch a crime show on television without seeing investigators request a cheek swab from suspects in order to analyze their DNA. Though the sequencing of DNA was successfully accomplished in the 1950s, DNA was not used in criminal cases until more recently.

The first use of DNA found at a crime scene occurred in Britain in 1986. A professor by the name of Alec Jeffreys was assisting the British police in solving two separate rape-murders. In each case, a 15-year-old girl was raped and killed, one in 1983 and another in 1986. The two murders were similar in pattern leading investigators to believe that the cases were linked.

The police thought they had their man when they arrested a young fellow with a history of mental illness, but Professor Jeffreys disagreed. Jeffreys analyzed semen collected from both bodies, and then Jeffreys asked everyone in the small town of Narborough, England to voluntarily submit to DNA testing. At first, the results were disappointing–no guilty party emerged.

As the investigation continued, townspeople began gossiping that a local baker by the name of Colin Pitchfork (real name!) had paid someone else to give a DNA sample for him. When police re-interviewed Pitchfork, he confessed to both crimes. When they specifically tested Pitchfork’s own DNA, police had a match and their man.

In the United States, we need look only as far back as 1995 to the trial that is sometimes known as the “trial of the century”–that of sports star O. J. Simpson–to find an early test of the use of DNA in American courts. Though Simpson’s DNA was found at the crime scene, the prosecution was unable to convince the jury of Simpson’s guilt.

The use of DNA in court was still very new in 1995. (The use of DNA was officially approved for court cases by the National Academy of Sciences in 1992.) The defense successfully convinced the jury that the evidence may have been contaminated. This case brought to light the importance of careful training of criminal investigators in the collection of evidence. As a result, it was soon specified that crime laboratories had to be certified for this type of testing in order for it to hold up in court.

By the late 1990s, forensics labs started to adopt a new method of analysis called STR (short tandem repeats) that cuts analysis time from weeks to days. Testing is also more accurate than it was originally. Early in the use of DNA for forensic evidence, the chance of error was one in one hundred thousand. The STR method brings the chance of error to a figure closer to one in a trillion.

The Use of Fingerprints

As DNA gradually edges out the use of fingerprints in criminal investigations, it is interesting to look back at the first time fingerprints were used in a murder case. With this type of evidence, too, it took thirty years of experimenting before it became accepted.

In 1905, two shopkeepers, a husband and wife, were found bludgeoned to death in their shop in Deptford, a small town outside London, and their cash drawer had been robbed of ten pounds. An Inspector McNaughton, who had been learning about and experimenting with fingerprint identification, was among those called to the scene. When he examined the cash box, he found on the underside of it one sweaty thumbprint. He sent it off to be identified, and this evidence soon led to the arrest of one of the two men who were responsible for the crime.

DNA is proving to be more accurate than fingerprint identification, but another major advantage has been the fact that this DNA evidence is being saved. Throughout the United States, a good number of court cases are being revisited as advocates for prisoners re-examine the cases and bring them back into court. If it is revealed that the prisoner’s DNA did not match that left at a crime scene, the result has been the freeing of a good number of wrongly-convicted prisoners.

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