Evaluation of mixed-source, low-template DNA profiles in forensic science

Abstract

Enhancements in sensitivity now allow DNA profiles to be obtained from only tens of picograms of DNA, corresponding to a few cells, even for samples subject to degradation from environmental exposure. However, low-template DNA (LTDNA) profiles are subject to stochastic effects, such as "dropout" and "dropin" of alleles, and highly variable stutter peak heights. Although the sensitivity of the newly developed methods is highly appealing to crime investigators, courts are concerned about the reliability of the underlying science. High-profile cases relying on LTDNA evidence have collapsed amid controversy, including the case of Hoey in the United Kingdom and the case of Knox and Sollecito in Italy. I argue that rather than the reliability of the science, courts and commentators should focus on the validity of the statistical methods of evaluation of the evidence. Even noisy DNA evidence can be more powerful than many traditional types of evidence, and it can be helpful to a court as long as its strength is not overstated. There have been serious shortcomings in statistical methods for the evaluation of LTDNA profile evidence, however. Here, I propose a method that allows for multiple replicates with different rates of dropout, sporadic dropins, different amounts of DNA from different contributors, relatedness of suspected and alternate contributors, "uncertain" allele designations, and degradation. R code implementing the method is open source, facilitating wide scrutiny. I illustrate its good performance using real cases and simulated crime scene profiles. Reliability of Low-Template DNA Profiling P roblems with the courtroom use of low-template DNA (LTDNA) profiles were brought into sharp focus in the United Kingdom in 2007, with the collapse of a trial arising from the Omagh bombing in Northern Ireland in 1998. This crime killed 29 people and injured many more; consequently, early termination of the trial and acquittal of the defendant attracted widespread adverse publicity. The judge gave several reasons, but it was his critical appraisal of the LTDNA evidence that captured headlines. In response to the controversy, a report reviewing LTDNA evidence (1) was commissioned by the UK Forensic Science Regulator. The report found the underlying science to be "sound" and LTDNA profiling to be "fit for purpose," although admitting that there was lack of agreement "on how LTDNA profiles are to be interpreted."I suggest that these comments are somewhat contradictory: Without valid methods of assessing evidential strength, a technique cannot be fit for purpose in the criminal justice system. Fig. 1 shows part of the electropherogram (epg) giving the results from replicate LTDNA profiling runs in a crime investigation. The two epgs show substantial similarity yet also important differences: For example, the 17 allele at locus D19 is detected in Fig. 1 (Left) but not in Fig. 1 (Right), yet the reverse is true for the 11 allele. Is a technology that produces such variable results reliable? This is often asked by legal comme...

Computational aspects of DNA mixture analysis

“…Using the estimates in Table 2 we obtain a likelihood ratio of log 10 LR = 9.85 which is weaker than the evidence obtained with full peak height information but it is still incriminating for the defendant. Such an analysis is analogous to the one used in likeLTD as suggested by Balding (2013), where peak heights are used only to classify peaks as present, absent, or uncertain.…”

Does an English appeal court ruling increase the risks of miscarriages of justice when complex DNA profiles are searched against the national DNA database?

“…A proposal to use a Likelihood Ratio (LR) evaluation [27] was rejected as 'unvalidated'. However, it was successfully argued by the prosecution that 'scientific evidence' could be based on personal experience and observations of 'thousands' of cases reported, where many were mixtures.…”

The future of forensic DNA analysis

“…genotyping using what has been termed 'semicontinuous' (where only allele information is used) or 'fully continuous' (where alleles and peak height information are used) [57][58][59][60][61][62][63].…”