Fri, 22 May 2009

Growth of the National DNA Database increases the risk of miscarriages

Brian Costello's Are DNA ‘cold hits’ resulting in miscarriages? is an analysis into whether false DNA matches will lead to miscarriages of justice, if they have not done so already. He questions whether chance DNA matches are occurring but are not being recognised.


Due to the current size of the NDNAD [National DNA Database], full profile chance matches (“adventitious matches”) are now actually expected to have occurred. A truly staggering 2.5 trillion odd comparisons have been made: 500,000 (Crime Scene Samples) x 5,000,000 (Subject Samples) = 2,500,000,000,000. Under ideal conditions, a full SGM+ profile is generally predicted to occur with a frequency of 1 in 1 trillion (although a figure of 1 in 1 billion is routinely given in court as it is perceived to be highly conservative in favour of the defence). With the NDNAD at its current size, and taking the 1 in 1 trillion chance match figure, a simple mathematical analysis indicates that two full SGM+ matches are expected to have occurred between a Crime Scene Sample and an unconnected Subject Sample. Put another way, it is expected that two suspects have been wrongly identified as linked to a crime scene by a false match.

Unfortunately, a simple mathematical analysis will not give the true picture. Two factors will increase the probability of adventitious matches: firstly, the condition of crime scene samples may lead to incomplete profiles; and secondly, individuals who are related are more likely to share the same profile than unrelated individuals.

A crime scene sample can be uploaded onto the NDNAD where results have been generated at only eight out of 10 loci (loci are DNA analysis sites), and a speculative search can be conducted where only six of 10 loci have produced results. Where results are obtained from less than the full 10 loci, the random match probability is reduced; it is more likely that a match will occur by chance alone. The comparison of incomplete profiles suggests that the prediction of only two adventitious matches could be a significant underestimate. Depending on the number of incomplete Crime Scene Samples uploaded, the number of expected adventitious matches could easily run into double digits. It would also appear that the problem of adventitious matches is set to increase with the expansion of low copy or low template DNA tests (“LtDNA”). LtDNA testing frequently produces incomplete profiles. If these incomplete profiles are uploaded to the NDNAD, or speculatively searched against it, the probability of obtaining adventitious matches will increase. Even more concerning is the fact that in a LtDNA test the entire DNA sample collected from a crime scene can be consumed, preventing further testing that would exonerate a person wrongly linked by an adventitious match.

People who are related share genetic material derived from their common ancestors. The closer the relationship the greater the chance their DNA profiles will match. In court, this would be expressed by a reduced match probability for a scenario that a relative committed the crime (e.g. match probability 1 in 1 billion for an unrelated individual, but one in several hundred thousand for a full sibling). It is a known fact that the NDNAD contains the profiles of a large number of related individuals. However, due to the level of duplication on the database – currently estimated to be around 13% of all samples – it is quite possible that two related (or unrelated) individuals share a full SGM+ profile, but the adventitious match has not been identified as it has incorrectly been attributed to duplication.


Given that further adventitious matches have been predicted, but none have been identified, there must be real concern that an innocent person has been wrongly convicted on the basis of an adventitious match; perhaps because they were not prepared to identify a family member and so entered a guilty plea, or perhaps because unlike Mr Easton they did not have compelling evidence of their innocence. If a miscarriage based on an adventitious match has not occurred yet, it appears likely it will occur in the future as the NDNAD grows ever larger. Greater use of the LtDNA technique will further increase the probability of such a miscarriage.

Brian Costello is completing a third six period of pupillage at One Inner Temple Lane and as a result of his concern is researching the discrepancy between predicted and identified adventitious matches. As part of that research he is interested in hearing from any defence practitioners who have:

If that's your case, then get in touch with him at

(Hat tip: Are DNA ‘cold hits’ resulting in miscarriages? was published in the excellent CrimeLine newsletter.)

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