While the way crime laboratories test DNA may still improve, a law professor from the University of Fort Hare has warned that law enforcement and defence counsel should be cautious when evaluating the quality of evidence obtained in this way.
Prof. Lirieka Meintjies-Van der Walt from the Faculty of Law at the University of Fort Hare argues in a paper published in the Journal of Juridical Science.
She explained that typically this kind of DNA analysis is based on low amounts of genetic material found (between 100 picograms and 200 picograms in South Africa). A picogram is one trillionth of a gram.
These are then amplified through a process called Polymerase Chain Reaction to create larger samples for analysis but the reliability of this process has been questioned. It is a similar process to PCR testing that was used to test for Covid-19.
However, she points out that two risks of amplifying genetic material in this way are what experts call a stochastic effect which can make results more difficult to interpret and in some cases lead to the improper reporting of a DNA profile. Contamination is another risk.
The South African Police’s reliance on DNA evidence to prove their criminal cases became clear in the past few years as the police’s forensic laboratories suffered several setbacks leading to a backlog of thousands of cases. The backlog has in the meanwhile been cleared and the resumption of cases has seen notorious and serial criminals found guilty and put behind bars.
Prof. Meintjies-Van der Walt added in her paper that while standard operating laboratory protocols could prevent contamination and there are ways to detect contamination both of these would be difficult for a legal team to establish.
She said that initially, forensic scientists required large amounts of genetic material to produce a DNA profile. However, presently, investigators can retrieve DNA profiles from as little as three skin cells left behind when a criminal handles a gun, grips a steering wheel, turns a doorknob, or throws a brick.
“By merely touching a surface, a perpetrator can leave behind trace amounts of skin or epithelial cells that are invisible to the human eye. Scientists can work with these low template smaller DNA samples
by usually amplifying them at least 32 times instead of 28 times, as is the case in the ordinary Polymerase Chain Reaction (PCR),” Prof. Meintjies-Van der Walt wrote.
“Nevertheless, the interpretation of trace DNA, also known as low copy number DNA, such as DNA deposited by touching an object or a person, is far more complex and problematic than that of the relatively large samples previously used,” she added.
She said the complexity in analysing and interpreting trace DNA should serve as a caution to legal fact finders not to attach more weight to DNA evidence as they would to other available evidence
“The very nature of DNA transfer, as briefly noted below, is inherently complex. The lack of availability of substantial DNA samples resulting in trace evidence further complicates establishing DNA profiles.