by Sue Carney, Independent Forensic Consultant, Ethos Forensics
On 24 July 2014, the UK forensic science providers implemented DNA17, the next generation of forensic DNA profiling techniques, and The National DNA Database began accepting DNA17 profiles. What are the advantages of this new system and what has been its effect on forensicevidence?
DNA17 arrived at a time when, in the minds of many forensic experts, criminal DNA profiling had reached reasonable limits of sensitivity and probative value. Since it was pioneered in the 1980s by Sir Alec Jeffreys of the University of Leicester, forensic DNA profiling has continued to advance. Colin Pitchfork, the first UK man to be convicted based on DNA evidence, was sentenced to life imprisonment in 1988 for the murders of Lynda Mann and Dawn Ashworth, thanks to DNA
fingerprinting, the earliest form of forensic DNA profiling. This early technique was time consuming and required vast quantities of biological material in comparison to today’s highly sensitive processes. The technique relied on variations in DNA sequence at areas targeted by DNA-modifying enzymes known as restriction enzymes. 1995 saw the next revolution in DNA profiling with the introduction of an analysis based, not on variations in sequence, but variations in length of well characterised regions of DNA known as Short Tandem Repeats. This STR profiling method coincided with the creation of the National DNA Database, which began to load the results of testing using the new STR profiling system referred to as SGM, Second Generation Multiplex.
SGM profiles examine six regions of DNA plus a gender-determining test and this method of profiling continued into the late 90s. By 1999, a new improved multiplex had been developed. SGM Plus examines the six SGM regions of DNA plus four additional regions and the gender test. These additional regions of DNA allowed for a test with far greater discriminating power in the identification of an individual.
Those legal practitioners well versed in DNA evidence will already know that under UK law, a DNA match is not considered conclusive. Instead, forensic experts attach a weight of evidence (known as a match probability or likelihood ratio, depending on the form of words used) to a full SGM plus profile match. Such a match is cited as being in the order of a billion times more likely if the biological material originated from the matching individual than if it were from another
unrelated person. It’s interesting to note that this likelihood ratio of a billion times more likely is a very conservative estimate. Figures are determined based on the relative frequencies with which the various components that make up a person’s DNA profile occur in the general population. The more regions of DNA that are examined, the more discriminating a profile becomes, hence the advantage of upgrading the six region SGM to the ten region SGM plus.
A good analogy to understand this principle, and one I have used in presenting DNA evidence at court on previous occasions, is to imagine the chance of encountering a specific type of car in a very large car park. If I want to know the chance that we’ll observe a silver car, one might consider that will be extremely likely, given that there are many silver cars. But, if we consider an additional characteristic by specifying that the car must have alloy wheels for example, then the chance of observing the required car is slightly decreased. In considering an additional characteristic, assuming of course that the presence of each characteristic is completely unconnected to the presence of the others, further decreases the likelihood of observing that particular car, and so on, until the
chance of a car fitting all the specified criteria becomes particularly rare.
Each region of DNA examined in a DNA profile can be considered analogous to those features of our chosen car. Each region of is inherited completely independently of every other region, since they are each carried on separate chromosomes. The more regions of DNA are examined, then the more characteristics are specified in a match, and the less likely it becomes that the specified characteristics will be encountered together from any source other than
the one from which that DNA originated. In determining the match probability, SGM plus considers the frequency with which every possible DNA component occurs in the general population. In developing SGM plus, the match probabilities for multiple test profiles were calculated, including consideration of the most frequent components at every region of the profile. Every match probability was found, in reality, to be considerably rarer than one in a billion. Other legal systems, such as that in the US, quote the actual match probability when presenting DNA evidence at court. In the UK, the one in a billion figure was considered to express sufficient strength of evidence and this capped estimate is used to describe the likelihood of all full profile SGM plus matches.
The implementation of DNA17 adds six new regions of DNA to the existing SGM plus profile. This means that actual figures are even rarer than those for SGM plus profiles. However, the standard likelihood ratio of a billion times more likely will remain unchanged. The extra discriminating power of the additional regions in a DNA17 profile results in an increased stringency of matches on The National DNA Database, and far less chance of an adventitious match.
DNA17 also brings an increased sensitivity to DNA profiling, which has shown improved results from degraded samples. Requests for the detection of so called “Touch DNA” are becoming commonplace. These are techniques, known as Low Template techniques, used to detect very low levels of DNA from items that might merely have been handled and which have no visible biological material. DNA17 blurs the boundaries between routine DNA profiling and Low Template techniques. Its improved chemistry means that sensitivity to low levels of DNA is part of the standard package. Since such low levels of DNA cannot be attributed to a particular body fluid or tissue type, it becomes difficult to determine how that DNA was deposited, or indeed whether it relates to a specific incident.
Furthermore, mixtures of DNA are being encountered with increasing frequency, as DNA17 shows us more of the previously undetected background DNA, including potential contamination. Even greater care in contamination prevention is advised at the crime scene along with cautious interpretation of such low level profiling results. For these reasons, careful consideration of the probative value of DNA17 profiles at the low level end of the spectrum will need to be exercised.
DNA17 is a Europe-wide system, which means compatibility with profiles generated in other countries in the EU and a more effective use of DNA records exchanged internationally. The Home Office has approved five different DNA17 kits for use in the UK. Interestingly, the various UK forensic science providers have each opted to validate different kits and expert systems. Although the profiles generated contain results at the same 17 areas of DNA, they have been generated in slightly different ways, which in the occasional rare circumstance, usually due to the presence of a mutation, might mean the occurrence of an
inconsistency. The forensic providers and DNA database staff are primed and ready to identify such circumstances, and it is safe to say that the advantages of this next development in forensic DNA profiling far outweigh the risks.
Ethos Forensics are fully conversant with DNA17, and have received expert training in the interpretational systems of the largest UK forensic science provider. As such, we have the required expertise in DNA17 interpretation and are available on behalf of the defence in criminal proceedings involving forensic DNA evidence.
Contact us for a no obligation quotation or to discuss the requirements of your case.
07796 546 224
Sue Carney, Independent Forensic Consultant.
A previous version of this article was published by The Solicitor’s Group in June 2014.