DNA Profiling

DNA Database

South Africa’s National DNA Database

South Africa does in fact currently have a National DNA Database (for Criminal Intelligence) which holds the DNA profiles of certain suspects arrested and thereafter convicted, for recordable offences and DNA profiles collected from some crime scenes. South Africa is in the early stages of recognising the importance of maximising the size of its National DNA Database in order to enhance its capacity to solve cases with DNA, which will ultimately facilitate crime reduction. This is because the greater the number of DNA Profiles on the Database, the greater the chance of solving crimes and catching criminals. The expansion our National DNA Database requires certain changes in our law, and currently Parliament is reviewing an important new Bill called the Criminal Law (Forensic Procedures) Amendment Bill [B2-2009] which, when passed, will ensure that every person arrested for an alleged offence as well as all convicted offenders, will have their DNA profiles loaded onto the Database. These profiles will continually be searched against DNA profiles collected at crime scenes, to try and find a “match” or “hit����� between the profiles in order to identify a suspect. Given the number of repeat offenders in South Africa, as well as the recidivistic nature of many crimes, there is a strong possibility that eventually, the individual who committed the crime being investigated was convicted of a similar crime and already has his or her DNA profile on the National DNA Database. Even if a perpetrator is not identified through the DNA Database, crimes may be linked to each other if the same DNA profile is found at different crime scenes, thereby aiding an investigation and eventually leading to the identification and conviction of criminals.

How the science is used in crime detection:

Forensic science provides the link between a crime scene and a suspect. Since 1901, fingerprinting has been used to track offenders. However, currently, the international forensic tool of choice is DNA profiling, as evidence may be collected in many forms such as hair, blood, saliva, semen and perspiration. While blood, saliva and semen are still the main sources of DNA for forensic testing, trace amounts of DNA, for example from epithelial cells from the surface of the skin, can now be acquired from touched objects. Scientists can use the saliva on the rim of a glass, or the skin cells and hair shed on a cap, to compare with a suspect’s blood or saliva sample. Similarly, DNA collected from the perspiration on a hat or scarf, discarded by a rapist, can be compared with DNA in the saliva swabbed from the bite mark on a different rape victim.

The following diagram illustrates the comparison of three DNA profiles. Two are from suspects and one is the DNA profile obtained from evidence collected at a crime scene. It is clear that the evidence taken from the crime scene matches the DNA profile of Suspect 2, as the sequence of numbers (the DNA profile) is identical to the evidence.

Simplified example of the comparison between a crime scene sample and two suspects.


The designations D3, vWA and FGA represent three different chromosomal locations (STRs) under analysis.

In criminal investigations, the sequence of numbers from a DNA profile found at a crime scene may be compared to that of a known suspect stored on the DNA Database. Alternatively, where there is no suspect for a particular crime, DNA samples collected at a crime scene may be compared with DNA profiles stored on a National DNA Database. The Database is the resource which contains DNA profiles of people suspected and convicted of offences, as well as DNA profiles obtained from evidence left at crime scenes. A match or ‘hit’ between the crime scene evidence and a database profile may identify a new suspect. This can help to identify or rule out a potential suspect at an early stage thereby saving valuable police and other crime detection resources, leaving them free for other investigations. For this reason, a National DNA Database is considered to be one of the most powerful tools in crime prevention and detection used in the world today.

The time, effort and expense required to develop DNA databases are justified by the facts that:

  • Criminals tend to re-offend. For example, 90 percent of rapists and 50 percent of armed robbers have a previous conviction.
  • The severity of crimes committed by repeat -offenders often increases over time, with criminals committing their first offence between the ages of 16 and 19 years.
  • A small number of criminals are often responsible for numerous crimes. DNA databases can assist in linking these crimes to one another.