DNA is a powerful tool because each person's DNA is different from every other individual's, except for identical twins. Because of that difference, DNA collected from a crime scene can either link a suspect to the evidence or eliminate a suspect. It also can identify a victim through DNA from relatives, even when no body can be found. And when evidence from one crime scene is compared with evidence from another, those crime scenes can be linked to the same perpetrator nationwide. Moreover, DNA maintains its integrity so that evidence from crimes committed many years ago may still yield sufficient DNA to conduct an analysis.

The fact that a person's identity can be authenticated by an examination of that individual's genetic traits has revolutionised crime detection. Recent advancements in DNA technology are enabling the police forces worldwide to solve cases previously thought to be unsolvable. The FSL is in the process of exploiting the potential of this tool.

How does a "match"occur on the DNA Database?

The DNA from every collected sample, being it from a person or crime scene, is analysed to produce a 'DNA profile' for each individual or sample from the scene of a crime. The database relies on the fact that every person's DNA is unique (unless they are an identical twin) and that the chance of an identical match between the DNA sequence taken from two different people is less than one in a billion if they are not related. Following collection and analysis of a sample, a new DNA profile can be compared with the profiles stored on the database. A match can arise if, inter alia

• a new scene of crime profile matches the profile of an individual already on the database. This can help to identify a potential suspect very rapidly;
• a new individual's profile matches a stored scene of crime profile from an unsolved crime or crimes on the database. This type of 'speculative search' can identify a potential suspect long after a crime has been committed;

While blood, saliva and semen are still the main sources of DNA for forensic testing, trace amounts of DNA, for example from epithelial cells, are now able to be acquired from touched objects, such as the handle of a weapon, the steering wheel of a stolen car or the inside of a glove. Equally as important as the analysis of the suspected offender samples, is the analysis of the biological evidence collected from crime scenes, regardless of whether a suspect has been identified in that case. The saliva on the lip of a drink can used by a suspected criminal or the skin cells/hair shed on a woolen cap worn by a suspected criminal can be compared with a suspect's blood or saliva sample. Similarly, DNA collected from the perspiration on a hat or scarf discarded by a rapist at one crime scene can be compared with DNA in the saliva swabbed from the bite mark on a different rape victim.

In South Africa.....

The NDCID contains DNA data held on a computer and stored samples from the scenes of crimes as well as from people who have been suspected and/or arrested of a crime. Whilst the SA DNAD only has approximately 80 000 samples (compared to over 3 million in the UK), it is encouraging to know that a NDCID does in fact exist and can be built upon to achieve the number of entries as that of other countries. We also know that currently the FSL are not able to analyze all the cases with biological evidence that are submitted to them. This oftentimes leaves those cases for which there are no suspects unanalyzed in laboratory storage. The DNA Project will accordingly greatly assist the DNA Analysis Backlog to address these needs. Until the laboratories have the capacity to analyze every case with biological evidence, the NDCID in South Africa will continue to be underutilized. In addition, by helping to convict or rule out a suspect at an early stage, a DNA database would save valuable police and other crime detection resources, leaving them free for other investigations or to be deployed towards more crime prevention.