DNA Testing Breaks New Ground for Courts

DNA testing has three major applications for forensic studies: identification of missing persons identification of victims of wars, accidents, and natural disasters and crime investigation. Annually, more than 20,000 forensic DNA tests are performed in the UK. Two out of three of all criminal cases using DNA evidence   involve sexual assault, the rest are cases dealing with burglary, murder, and other types of violent crime. During the last 15 years, DNA analysis became an indispensable police tool in fighting crime because it allows unambiguous identification of the criminal by traces of biological material left at the crime scene. It can also acquit innocent suspects based on DNA evidence.

Criminal justice system now relies heavily on DNA-based evidence. Since it was first used in the Enderby murder case (1986), thousands of perpetrators has been convicted of various crimes with the help of DNA evidence, and hundreds of wrongfully convicted people have been exonerated.

The most common samples collected at the crime scene are blood, semen, and saliva virtually any biological material or objects handled by a perpetrator can be now used for forensic DNA testing. Clothing, furniture, and other items which may have traces of DNA, are now routinely used for obtaining DNA evidence. The technology is so sensitive that it allows identification of a person by analysing DNA collected from a fingerprint left on the surface of an object or from a single hair left at a crime scene.

When a crime scene sample or a sample from a suspect is analysed, a DNA profile is produced. A DNA profile is a digitalised representation of an individual’s genotype with respect to the DNA markers tested. In the UK, all crime scene DNA profiles together with those of all suspects and arrestees for any recordable offence are deposited into a National DNA Database (NDNAD), which is the world’s first criminal DNA database. As of 2004, the UK national DNA database held over two and a half million DNA profiles collected from suspects and convicted criminals which is estimated to be about 40% of UK criminally active population as well as more than 200,000 crime scene samples. UK Police use the NDNAD as an investigative tool to help solving a wide range of crimes including murder, rape, sexual assault, robbery, terrorism, burglary and arson and have almost doubled their clearance rate for volume crimes such as house burglary, and motor vehicle offences. As each new subject sample profile is added to the database, it is checked against all contained crime scene sample. When a new crime scene sample profile is added, it is checked against the DNA profiles of all suspected individuals as well as against other crime scene sample records. Since its inception in 1995, the NDNAD had matched more than 200,000 crime scene samples to suspects and more than 20,000 crime scene samples to other crime scenes. Every week more than 300 crime scene samples are matched to the suspect and convicted criminal’s database.

However, a DNA match between a suspect and a crime scene does not automatically guarantee a conviction. DNA evidence is just another piece of evidence, although very strong one, and on its own is often not enough to convict someone of a particular crime. DNA evidence must always be taken in conjunction with other pieces of evidence and the weight of DNA evidence is impossible to estimate without taking into account the circumstances of the case.  Even when a strong match between a defendant and a crime scene sample is presented by the prosecution, non-DNA evidence may be pointed to someone else as the real perpetrator of the crime. This “other" evidence can decrease the weight of DNA evidence and increase the chances of successful defence.

The discovery of genetic fingerprinting 20 years ago had revolutionised the legal profession. Criminal and non-criminal justice systems were handed a very powerful tool to solve crime and resolve civil cases. Understanding how DNA testing can be applied in legal profession will bring benefits to both lawyers and their clients.