DNA Fingerprinting

DNA fingerprinting is a technique to allow one to distinguish between individuals using their DNA. Although humans show identical sequences in the vast majority of the genome some specific sequences can be highly variable. It is therefore the likelihood that unrelated individuals would have different sequences at these particular sites in the genome which is the basis for genetic fingerprinting.


An example of such variable sequences are repeats consisting of between 3 and 5 basepairs. When analysing multiple regions containing variable repeats between two individuals a very high statistical power can be generated as one region containing a certain number of repeats does not relate to the number of repeats in any other region. Therefore, if one region in a certain individual has 15 repeats, a second region 30 and a third region 5, the chance of an unrelated person containing the same numbers of repeats is 1 in several million.


This technique can therefore also allow one to determine if two individuals are related. Naturally, identical twins contain identical genetic fingerprints, while relatives will share varying amounts depending on the closeness of the relationship - For this reason, genetic fingerprinting is often used in paternity tests. In 1912 a four-year-old boy called Bobby Dunbar vanished during a fishing trip. Eight months later it was claimed that another boy, Bruce Anderson, living with the family of William Walters, entrusted to them but his mother, was actually the missing child. The courts accepted this story, convicting Walters for the kidnapping and placing the boy with the Dunbar family where he was raised and known as Bobby throughout the rest of his life. However, in 2004 DNA tests on Dunbar's son and nephew revealed the two were not related, thus establishing that the boy found in 1912 was not Bobby Dunbar, whose real fate remains unknown.


Genetic fingerprinting is also employed in criminal cases to prove that the DNA found at a crime scene is statistically likely to be the same DNA present in a suspect. In 1988, Colin Pitchfork became the first murderer in the UK to be convicted as a result of DNA fingerprinting. Though he had originally evaded police by persuading a friend to give a fraudulent blood sample under his name, he was eventually caught out and sentenced to life in prison for the murder of two young girls. However, controversies do arise from this method, especially when a prosecutor gives some astronomical statistical figure for a DNA sample, which may have been contaminated or mislabelled, belonging to an accused. Indeed, there have been some wrongly ascribed “positive” matches for people who could not possibly have been involved in a crime and it appears several people in the US have been executed on DNA evidence now admitted to be faulty due to human errors in a laboratory. In December 2005 Robert Clark became the 164th person in the USA to be pardoned following post-conviction DNA testing, after serving 24 years of his sentence and in March 2009, Sean Hodgson was released after spending 27 years in jail for murder when tests prove DNA from the scene was not his.