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SILThe current technology is SGM Plus, introduced in 1999, which looks at 10 STRs.
From a CPS paper, but link not working:
B4. Adventitious (chance) DNA Matches
B4.1 Move to 16-marker system (recommendation 23)
Lines to take
SGM Plus DNA profiling is very discriminating between individuals. The probability of obtaining a match between the profiles of two unrelated individuals by chance is very low, of the order of 1 in a billion. However, it has not yet been possible to carry out the required statistical testing to be able to quote this match probability, and in practice a more conservative chance match figure of 1 in 1,000 million is used.
The Government has every confidence in the current SGM Plus profiling technology but recognises that it should keep its reliability and level of discrimination under review.
The NDNAD now contains the profiles of over 3 million individuals from our population of 60 million. We are not aware of any chance match between two full SGM Plus profiles for unrelated individuals having been obtained to date. However, as the size of the NDNAD grows and as more international comparisons are made with other countries' DNA profiles, the probability, although currently very small, will increase.
The SGM Plus profiling system looks at 10 STR (short tandem repeat) areas of DNA. It would be technically possible to improve the discriminating power of the SGM Plus profiling system further by testing for more markers, for example by developing a profiling system which looks at 13 or 16 STRs.
Recent research studies by the European DNA Profiling Group (EDNAP) and the European Network of Forensic Science Institutes (ENFSI) have demonstrated that the success rate for analysis of partly degraded samples (which may be found at crime scenes and which result in a partial profile with less than 21 markers) improved by using recently developed tests for markers based on mini-STRs (shorter lengths of DNA).
At a meeting earlier this year, EDNAP/ENFSI agreed that its strategy should be to incorporate tests for some of the mini-STRs into new profiling systems. It was also proposed that work should be undertaken to re-engineer the test for the existing markers in SGM Plus to make them easier to detect in degraded DNA.
For DNA profiling in the UK, this might mean the addition of tests for 3-6 of the new mini-STR markers to the current (possibly re-engineered) 10 SGM Plus markers, ideally in a single profiling technique i.e. the development of a technique which looks at 13-16 STR areas.
The international scientific community have agreed that the best way forward would be to develop a new 13-plex or 16-plex technique. This would also need to take account of the divergent requirements of the different European countries. The timescale for the development and introduction of a new multiplex profiling technique (one that looks at multiple markers) would be a commercial decision by the companies that make the multiplexes. It is estimated that the development of a new 13-plex or 16-plex could take 2-3 years. A 13 marker system would give match probabilities of about 1 in 10 (to the power 15) (compared to 1 in 10 (to the power 12) for SGM Plus).
Background Information
The technology used to obtain DNA profiles for the NDNAD looks at specific areas of DNA, known as short tandem repeats (STRs). STRs are known to vary widely between individuals by virtue of variation in their length and are therefore extremely useful for identification purposes.
The STRs are found only in the non-coding region of DNA and therefore provide no information of genetic significance e.g. about an individual's genetic predisposition to a medical condition.
The DNA technology used in forensic science has evolved enormously since DNA was first used in 1987. The first STR technique was introduced in 1994 and looked at only 4 STR areas. The next development was SGM (second generation multiplex) profiling which looked at 6 STRs. A multiplex is a profiling system which looks at more than one STR area.
The current technology is SGM Plus, introduced in 1999, which looks at 10 STRs. For each STR, there are 2 markers (or alleles), one from the individual's mother and one from their father. There is also a gender marker. A full DNA profile for the NDNAD therefore contains 20 markers and the gender marker.
When fewer than 20 markers have been determined - for example from degraded or incomplete samples from crime scenes - the level of discrimination is reduced accordingly.
The Government has every confidence in the current SGM Plus profiling technology but recognises that it should keep its reliability and level of discrimination under review.
The NDNAD now contains the profiles of over 3 million individuals from our population of 60 million. There is a very small probability of a chance match occurring between two full SGM Plus profiles for unrelated individuals, but we are not aware of any such chance match having been obtained to date. However, as the size of the NDNAD grows and as more international comparisons are made with other countries' DNA profiles, this probability, although small, will increase.
The risk of a chance match will also increase if the crime scene profile is a partial profile (i.e. does not have all 21 markers). Comparison of partial profiles from crime scene samples with full SGM Plus profiles from individuals on the NDNAD is thus more likely to result in matches being found relating to more than one individual. The evidential significance of a match between a suspect and a crime scene sample must always be considered in conjunction with other evidence available to the police.
B4.2 Current Practice in Preventing Adventitious Matches involving SGM Profiles
NB. The issue of how chance matches are avoided in relation to DNA profiles developed using the SGM system (which has now been replaced by SGM Plus) was raised at the last hearing of the Committee.
Lines to take
In relation to SGM to SGM matches (which test for only 6 STRs), Home Office Circular 58/2004 and the ACPO DNA Good Practice Guide advise that strong consideration should be given to upgrading the SGM suspect offender profile to SGM+ to ensure that the upgraded SGM+ suspect offender profile matches the crime scene profile before the matter comes to trial.
If the upgraded profile does not match with the crime scene profile, the NDNAD issues a Match Elimination Notification which indicates that the further analysis has eliminated the original SGM suspect offender profile from the SGM to SGM match. [The inference would be that the original SGM to SGM match was a chance match.]
It was suggested that a sampling exercise should be carried out to provide assurance that police forces were taking appropriate action to monitor and deal with SGM match notifications received from the Database.
The Home Office has recently written to several police forces to ask them about their procedures for considering the evidential quality of SGM matches e.g. whether such matches are upgraded to SGM+ and in what circumstances.
The forces have also been asked to undertake a case-tracking exercise of crimes with DNA SGM matches to look at how many SGM matches were upgraded to SGM+, how many continued to match the crime scene profile and, if not, whether the case went to court on the basis of other non-DNA evidence or whether there was insufficient evidence to proceed.
Background information
The SGM profiling technique was introduced in 1995 around the time that the NDNAD was established. SGM (second generation multiplex) tests for 6 STR areas or markers. It was subsequently replaced in 1999 by SGM Plus which test for 10 STR areas. Since June 1999, only SGM Plus profiles have been loaded on to the NDNAD.
In 2001, the National DNA Database Board recommended that consideration should be given, on a case by case basis, to upgrading any SGM profiles involved in a match, before taking any further steps, to minimise the risk of the match being adventitious (chance).
When a match involves an SGM profile, the police are made fully aware through caveats attached to the match report of its potential limitations as an intelligence tool for identifying suspects.
In 2004, the NDNAD and CPS agreed to a policy of charging on the basis of a Database match but only where there was sufficient supporting evidence. If the match involved an SGM profile, upgrading the profile to SGM Plus was recommended.
Link not working...
http://www.cps.gov.uk/legal/s_to_u/scientific_evidence/adventitious_dna_matches/index.html
