DNA Evidence in Criminal Law

DNA (deoxyribonucleic acid) is the determining factor in more than 40% of criminal cases nationwide. DNA evidence is used in the investigation of violent crimes where fluids (blood, semen or saliva) are present on or around the crime scene. DNA can also be extracted from hair follicles and sometimes from mucus membranes.

DNA evidence is an invaluable part of criminal investigations because, like fingerprints, DNA is unique to the individual. No two people (with the exception of twins) share the same DNA pattern, which allows forensic scientists to determine beyond a shadow of a doubt whether or not DNA collected from a suspect matches that which was collected from a crime scene.

DNA evidence is used for two main purposes: exclusionary and guilt. Often, there is more than one suspect in a violent crime, so court orders for DNA will be issued for all of them. If the DNA found at a crime scene doesn’t match any of the suspects, they are excluded from the investigation. If, however, it does indeed match, then the DNA evidence is one of the strongest cases against that individual.

The only problem is that DNA evidence is not infallible. Outside contaminents can drastically affect the results of a DNA test and DNA evidence that has been exposed to heat or moisture will also not reflect accurate results. Crime scene investigators and forensic scientists must be careful when collecting DNA evidence so as not to compromise the investigation.

Most people, when they think about DNA evidence used in criminal law, consider it to be a way for law enforcement to collar and convict a suspect. However, DNA evidence has also been used to prove that suspects (and even convicts) are not the perpetrators they were thought to be. Several defendants convicted before DNA evidence was used have since been exonerated based on DNA evidence tested now.