A person's DNA (de-oxyribonucleic acid) can be found from a single strand of hair, skin under a murder victim's nails or bodily fluids such as sweat, saliva, semen and blood. The chances of a sample of DNA being the same as another person, other than monozygotic twins is 1 in 24 million. This is why recent cases such as that of Sarah Payne rely so much on DNA samples found at the crime scene. In this case a single strand of Sarah's hair was found on Roy Whitting's sweatshirt and matching fibres from his sweatshirt were found on her shoe, although there was slight controversy as to whether the evidence was contaminated.
The method for extracting the DNA from a sample is a complex one. The technique was first developed in this country in 1985, the year of the first proven case. Firstly the DNA must be extracted from the sample of body tissue or fluid. This could be a very small amount in a criminal investigation where the offender could've taken every precaution to avoid being caught. A process called Polymerase Chain Reaction (PCR) then multiplies the sample and produces millions of identical strands of DNA. PCR is completed by a "repetitious, cyclic programme of heating and cooling the substance together with a heat resistant DNA polymerase and two specific DNA primers. The polymerase and primers reproduce a small region of a genome, the whole of the DNA of the organism." - quoted from DNA in Forensic Science by J. Robertson, A.M. Ross and L.A. Burgoyne.
Forensic scientists use PCR to produce plenty of DNA to carry out all the necessary experiments to obtain a conclusive result. The DNA is then broken up, using enzymes, into standard fragments. The enzymes used cut the DNA at precise sequences of A C G and T in the DNA. A few fragments are then chosen and separated by size on a gel. The gel is then exposed to radioactive samples of the DNA. This produces different sized bands and the bands that have the same sequence of DNA are prominent. Generally the DNA evidence found at the scene is tested and the suspect may have a blood test or a saliva sample from the inside of the cheek, which will also be tested. The two samples will be tested and compared several times. If the two samples have 5 bands or more that match the result is considered as decisive.
In 1992, after many years of arguments for and against the gene profiling, the National Research Agency accepted the method as a reliable one to help identify criminal suspects and shortly after the procedure entered the mainstream court system. Genetic fingerprinting is now so common it's difficult to avoid in everyday society.
The accuracy of gene profiling is very good. The chance of two people sharing one band of DNA is approximately 1 in 30. This may not seem like there is much chance of identifying a person, as there is a fairly high possibility of it being a number of people. However, the National Research Agency has a policy stating that at least 5 bands have to match for the test to be considered as positive. This means that the probability goes from 1 in 30 to 1 in 30 x 30 x 30 x 30 x 30, which is equal to 1 in 24 million providing the two samples aren't from blood relatives.
This is why genetic fingerprinting is considered as firm proof in a court of justice. Overall, the chance of getting a DNA test wrong is slim, providing the tests are carried out properly. However, just because someone's DNA is found at a crime scene doesn't prove that the suspect is guilty. There are many other areas of law that need to be investigated before a verdict can be decided.
Some offences that are committed rely almost solely on forensic evidence such as that of rape. If semen is discovered and recognized as that of the suspect then that is nearly a conclusive result. The suspect may claim that the other party consented to sexual intercourse, but would find it difficult to deny that intercourse took place.
Nevertheless, forensic evidence does have its problems. Firstly the tests have to be carried out in fully sterile conditions to ensure that the DNA isn't distorted in any way. It is crucial that the sample before PCR takes place is not contaminated as PCR will produce several thousands of exact clones and all the forthcoming tests could be inaccurate.
Also, over time mutation of a sample takes place, gradually decreasing the likelihood of identifying a suspect perfectly. There have been experiments carried out to test how quickly mutation takes place. Scientists have found that, with blood and semen stains kept at room temperature for more than four years, it was almost impossible to identify any reliable DNA information due to mutation. In some cases, such as rape, the evidence can disappear within a matter of hours. When testing for semen in a rape case, the DNA should be found within 20 hours of the crime. Following the first 20 hours, the probability of obtaining an accurate identification decreases hour by hour.
With the slim chance of 2 people's DNA making a full match, gene profiling is a definite way of proving whether a person was at a crime scene or not. Whether they actually committed the crime is a different matter. Providing all tests are carried out well and the DNA is extracted properly and without contamination then the results should be extremely accurate. On the whole, gene profiling has taken forensic science that stage further and it has developed to being a crucial part in identifying criminals all over the world. There remain ethical difficulties which have yet to be resolved, for example, the establishment of a DNA database, human rights and civil liberties, the use of DNA information in unrelated legal cases, etc, but these issues will be addressed in due course.