No need to question the validity of DNA evidence

Whenever a crime is committed and DNA used to assist in the identification of the offender, the question of whether the DNA found at the crime scene is the same as that of the criminal must be asked. In other words, whose DNA it is?

As a geneticist, and having just returned from Interpol’s International DNA Users’ Conference in Lyon where internationally renowned forensic experts spoke repeatedly of the value of DNA Databases used for criminal intelligence purposes, I was astonished to read the article in the Times today (“Whose DNA is it anyway?“, Times LIVE, November 12) which quotes Prof. Muller as saying that “the probability that someone else would have the same DNA profile as that of the sample obtained at a crime scene was one in 2 million”. In South Africa our forensic science laboratory analyses 9 different locations in the DNA molecule to determine a forensic profile and obtain probabilities in the region of at least 1 in a billion.

After the passing of the legislation, South Africa will be using technology which is even more reliable as they will be analysing 15 locations in the DNA and will then obtain probabilities of another person, other than the suspect, having the same profile as that found at the crime scene of between 1 in 10 billion and 1 in an octillion – this is an extremely exact, valid and reliable science! Furthermore, it is incorrect to say that there is a 5% chance that two people listed in even a small database would have the same DNA profile. The value of DNA evidence lies in the fact that no two people share the same DNA (except identical twins) – that is if sufficient locations on the DNA molecule are analysed, which indeed they are if 9 or more are considered.

To answer the question can forensic DNA analysts determine whether the suspect is, beyond reasonable doubt, the source of the DNA left at a crime scene – yes, absolutely. What remains is however to determine how the crime was committed and whether in fact the suspect committed the crime. These are questions that only a court can decide after the investigating officer has provided the court with information on ALL forms of evidence in the case.

For example, were there any eyewitnesses, does the suspect live in the same geographical area, what was the possible motive? DNA evidence is invariably only one form of evidence in a case, but with the chances of anyone else having the same profile being very close to zero, it is extremely compelling.

In a country that has the highest incidence of sexual assault in the world and where a child is raped every 3 minutes, who could possibly argue against the fact that our DNA database should be expanded to include people suspected and convicted of this type of offence? Add to this the well-known fact that 90% of rapists re-offend… a DNA database of previous offenders will provide investigators with valuable leads in cases where there is no known suspect and assist in crime resolution.

The new DNA Bill cannot be passed soon enough.

In order to reduce crime in South Africa we need to make use of this cutting edge technology to ensure that criminals are held accountable for their actions.

Dr. Carolyn Hancock (PhD Genetics)

Director: DNA Project

One Response to “No need to question the validity of DNA evidence”

  1. We sent a copy of Dr Muller’s article and comments reflected in the press to the Chair of the DNA-database & DNA-legislation subgroup of the DNA Working Group of the European Network of Forensic Science Institutes (ENFSI), Dr.Ir. C.P. van der Beek MBA – Dr van der Beek is also the Custodian of the Dutch DNA-database in the Netherlands. His response was as follows:

    Just like Dr.Carolyn Hancock I was shocked to read the article in the Times (“Whose DNA is it anyway?“, Times LIVE, November 12) which quotes Professor Muller as saying that “the probability that someone else would have the same DNA profile as that of the sample obtained at a crime scene was one in 2 million”. I hope that the journalist who quoted Professor Muller has quoted him wrong (as unfortunately also has happened to me several times) because a quote like this can lead to a misplaced underestimation of the power of forensic DNA-databases. As already stated by Dr. Hancock, full DNA-profiles have a so called random match probability (= the chance that an unrelated person has the same DNA-profile) of less than one in a billion. Hence the chance that the DNA-profiles of 2 unrelated persons in a DNA-database are the same is practically zero. Only partial crime scene profiles may cause adventitious matches with DNA-profiles of persons in a DNA-database but custodians of DNA-databases and DNA-reporting officers are aware of this and will act accordingly when they find such matches.

    Further reading: ENFSI Document on DNA-database Management:…/enfsi…