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Interpreting complex DNA evidence– new statistics create debate

forensic_iconCriminal cases with potentially probative DNA material have been discussed in the media and debated in scientific journals in recent times.  The DNA material in contention is considered ‘complex’ in some ways and also cannot be related to a visible or particular biological fluid/matter. There could be a mixture of two or more individuals, low level amounts of DNA, or both.  New statistical methods – called ‘probabilistic genotyping’ – have been devised in order to try and interpret these complex profiles.

One statistical method developed in the United States (called TrueAllele®) is currently the subject of an admissibility hearing in New York State (1, 2 and 3).  John Wakefield is accused of strangling an acquaintance with an amplifier cord inside the victim’s apartment in April 2010.  The trial has been delayed several times, partly on the issue of the new DNA technology.  The attorney for the accused has called the statistics provided in the case “DNA voodoo”.

If a DNA profile is complete and from a single source, and unambiguous in nature, the profile may be considered to be optimal.  The DNA profile from the exhibit is compared to a reference sample and either there is a “match” or the person of interest is excluded.  If there is a match then a statistical probability is evaluated according to how rare the particular DNA profile is in the relevant population.

When a DNA profile from an exhibit indicates a mixture of two contributors – or more – then assigning a particular contributor type from the profile becomes more difficult.   When the DNA profile cannot be separated out into individual contributors from an examination of the profile then this mixed DNA profile is termed ‘unresolved’.  There is no traditional ‘match’ as the evidentiary DNA profile contains more components than the reference profile.

Sometimes it can be difficult to even determine the number of contributors to the DNA profile especially if there are potentially three or more contributors.  One study noted that 70% of four person mixtures could be interpreted as two or three person mixtures.  One of the authors of this study quoted by the magazine ‘New Scientist’ in 2010 stated “if you can’t determine how many contributors there were, it is ludicrous to suggest that you can tease apart who those contributors were or what their DNA profiles were” (4).

Furthermore, when the DNA profile is partial or low level (small amounts below the optimal required for analysis) then the profile may not faithfully reflect the actual DNA present on the exhibit.  There will be uncertainty whether there is a “match” or not with a reference sample.

The desire to interpret these complex DNA profiles has led to new statistical methods in the past couple of years.  These methods are either “Open Source” (freely available) or “Closed Source” (available under licence).  They differ in their statistical modelling and approach.  The interpretation of these unresolvable mixtures and low template DNA profiles has yet to reach a consensus within the forensic biology community (5).

It will be of interest to follow the progress of these new statistical methods in the forensic literature and the courts.

References

1.  Admissibility of DNA at issue, Steven Cook, May 8 2014, The Daily Gazette, available at www.dailygazette.com

2.  New DNA analysis may be used in Schenectady murder case, Steven Cook, April 12 2013, The Daily Gazette

3.  DNA test put on trial, Paul Nelson, September 16, 2013, Times Union, available at www.timesunion.com

4.  Fallible DNA evidence can mean prison or freedom, Linda Geddes, 11 August 2010, New Scientist, issue 2773

5.  A comparison of statistical models for the analysis of complex forensic DNA profiles, Hannah Kelly et. al., January 2014, Science and Justice, 66-70

Written by Jane Taupin, Forensic Science Consultant 

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