Flaws found in SA's DNA tests
DNA forensics is used widely in South Africa, but a recent supreme court case in Cape Town exposed serious flaws in its application, reports Rehana Rossouw
There are probably millions of people throughout the world who by now have heard of “the bloody glove” and its alleged proof of American sports superstar OJ Simpson’s involvement in the murder of his ex-wife and her lover.
The television coverage of the trial has had people hanging on the edge of their couches, even when the driest, deeply technical forensic evidence was presented, allegedly linking Simpson to the scene of the crime.
In South Africa, DNA forensics has wide application and is admitted to court as evidence in trials ranging from parenthood disputes to murders.
One of the accused in the brutal St James massacre trial last year was convicted with the help of forensic evidence linking him to a single bloody fingerprint at the scene.
But in a recent murder trial in the Cape Town Supreme Court, the prosecutor abandoned the DNA evidence route after it was challenged by an advocate for the defence who exposed flaws in the workings of the police forensics lab in Pretoria.
Evidence presented to the court was that one in 1 305 black people in South Africa would share the specific genotype of murder accused Johannes Motloutsi. Major Christo Weitz of the Forensic Science Laboratory, who initiated DNA testing for forensic work in South Africa, gave evidence for the state explaining how his staff had arrived at that conclusion.
DNA fingerprinting was introduced to South Africa in 1985, and the planning and purchasing of equipment for forensic work began in 1989. All testing is performed in Pretoria on samples collected by investigating officers across the country.
The forensics laboratory uses a Perkins Elmer 9 600 Cetus Thermal Cycler for DNA typing—a machine which is regarded throughout the world as the “Rolls Royce” of DNA amplifiers.
A PCR mix, or amplitape, is used to open the DNA chain for inspection and comparison.
“The technique is simple. When the DNA is heated up in the machine, the chain opens up like a zip and when it cools down, it closes like one,” Weitz said.
“When we warm the sample for the first time, the two chains pull apart from each other, leaving two identifiable pieces of DNA. When it is cooled, the pieces lengthen and the two chains double in size.
“We usually repeat the process 30 times and the one double chain is increased in size one-million times.
“That is what made this technique so attractive to us. Forensic samples are usually small amounts, so you can take a little DNA and multiply it to a workable
The forensics laboratory performs DNA testing on between 30 000 and 40 000 samples a year—a large quantity, but one which reflects South Africa’s high crime statistics.
To determine the likelihood of suspects being linked to the crime, the laboratory needed a database of ABO bloodgroups for comparison. At the time the laboratory was established, the existing database did not adequately represent a sample of the country’s population as it excluded the TBVC areas.
“We decided to establish a homogeneous database for the entire population of South Africa, one for four population groups: black, white, coloured and Asian,” Weitz told the court.
“We did not differentiate between Nguni, Sotho and Tswana or whites of German, Italian or Greek descent. When you get a sample from a crime scene it is blind, you don’t know from which racial group it is drawn.
“If, for instance, we had subdivided the white group into 20 subsections, we would have to work with so many parameters that it just isn’t feasible.”
The laboratory used 1991 census figures and population statistics provided by the Development Bank of South Africa to determine how much blood should be acquired for a representative sample. They decided a minimum of 100 blood samples per population group was sufficient.
“I delegated someone in the laboratory to get into a car, drive throughout South Africa and collect blood samples,” Weitz said. “I told him to try and get samples representative of the population of South
“When he returned, he did not manage to get a representative sample. For instance, we only have 46 blood samples of Asians, not the 100 we requested.
“But we proceeded with the analysis and decided to use what we had for our database. The one advantage we have is that unlike other databases worldwide, ours is made up of non-criminals.
“There is some debate in the forensic world around the issue that people who commit violent crimes have extra y chromosomes so it is an advantage to have non- criminal samples.”
Over the next few months, the forensic laboratory will add 5 000 criminal samples to enlarge its database, using only those for which the race is known.
The database being used has 412 black samples, 207 white, 114 coloured and 42 Asian, which does not reflect the prevalence of those groups in South Africa.
>From that sample, the lab determines the likelihood of one person in a population of 40-million having committed a crime.
Under cross-examination by Advocate Estelle Scholtz, Weitz admitted that his storage facilities were imperfect, his database insufficient and his regular testing of his machines incomplete.
Scholtz read to the court from the handbook supplied by Perkins Elmer which said a documented temperature verification test had to be performed monthly using a verification system provided by the manufacturers.
Weitz said the probe necessary for the test was not available in South Africa and admitted that this protocol was not followed in the forensic laboratory.
Many of the reagents necessary for DNA testing were also not available, but the laboratory bought the highest quality alternative chemicals and did not regard this as a deviation from the protocol.
“The storage facility, which should be set at four degrees Celsius, is also not working. To an extent, what we have in the laboratory is crisis management, it is not an ideal situation,” Weitz told the court.
He also referred to what is known in forensic circles as the “Hardy Weinburg principle” which holds that a sample has to be big enough to represent a population.
However, he pointed out, Kenya’s DNA database was established with a sample drawn from 12 people, so South Africa’s was more representative.
The bigger and wider the sample, the less chance there was of excluding genetic mutations which appear on a regular basis in some populations.
Weitz admitted that regional genetic factors were not taken into account when the sample was established. “For instance, intermarriage and incest could occur often in certain regions, but you can’t be specific about those elements until it is proven scientifically in a laboratory.”
Weitz said research had shown that the genetic differences between sub-groups of races were minute enough to be disregarded for the initial sample, but admitted that it could be improved.
“It’s going to be a difficult task to improve the database. It could take between 10 to 15 years to complete. But it’s going to be a lifelong task for me,” he said.