James M. Curran

Interpreting DNA Mixtures in Structured Populations

DNA profiles from multiple-contributor samples are interpreted by comparing the probabilities of the profiles under alternative propositions. The propositions may specify some known contributors to the sample and may also specify a number of unknown contributors. The probability of the alleles carried by the set of people, known or unknown, depends on the allelic frequencies and also upon any relationships among the people. Membership of the same subpopulation implies a relationship from a shared evolutionary history, and this effect has been incorporated into the probabilities. This acknowledgment of the effects of population structure requires account to be taken of all people in a subpopulation who are typed, whether or not they contributed to the sample.

Developing allelic and stutter peak height models for a continuous method of DNA interpretation

Traditional forensic DNA interpretation methods are restricted as they are unable to deal completely with complex low level or mixed DNA profiles. This type of data has become more prevalent as DNA typing technologies become more sensitive. In addition they do not make full use of the information available in peak heights. Existing methods of interpretation are often described as binary which describes the fact that the probability of the evidence is assigned as 0 or 1 (hence binary) (see for example [1] at 7.3.3). These methods are being replaced by more advanced interpretation methods such as continuous models. In this paper we describe a series of models that can be used to calculate expected values for allele and stutter peak heights, and their ratio SR. This model could inform methods which implement a continuous method for the interpretation of DNA profiling data.

Towards understanding the effect of uncertainty in the number of contributors to DNA stains

DNA evidence recovered from a scene or collected in relation to a case is generally declared as a mixture when more than two alleles are observed at several loci. However, in principle, all DNA profiles may be considered to be potentially mixtures, even those that show not more than two alleles at any locus. When using a likelihood ratio approach to the interpretation of mixed DNA profiles it is necessary to postulate the number of potential contributors. However, this number is never known with certainty. The possibility of a, say three-person mixture, presenting four or fewer peaks at each locus of the CODIS set was explored by Paoletti et al. [D.R. Paoletti, T.E. Doom, C.M. Krane, M.L. Raymer, D.E. Krane, Empirical analysis of the STR profiles resulting from conceptual mixtures, J. Forensic Sci. 50 (2005) 1361-1366]. In this work we extend this analysis to consider the profiler plus and SGM plus multiplices. We begin the assessment of the risk associated with current practice in the calculation of LRs. We open the discussion of possible ways to surmount this ambiguity.

A discussion of the merits of random man not excluded and likelihood ratios

DNA mixture interpretation is undertaken either by calculating a LR or an exclusion probability (RMNE or its complement CPI). Debate exists as to which has the greater claim. The merits and drawbacks of the two approaches are discussed. We conclude that the two matters that appear to have real force are: (1) LRs are more difficult to present in court and (2) the RMNE statistic wastes information that should be utilised.

A MCMC method for resolving two person mixtures.

In this paper a Monte Carlo Markov Chain (MCMC) method for resolving DNA mixtures containing at most four peaks per locus into a major and a minor contributor is presented. Unlike previous methods, this method can provide posterior probability assessments of the most probable genotype and a likely range for the mixing proportion. The proposed method is applied to two DNA mixtures where the true genotypes of the contributors are known. The method provides posterior probabilities of the genotypes of the contributes which concord strongly with the known facts.

Comparison of the variables affecting the recovery of DNA from common drinking containers

As the boundaries of forensic DNA profiling continue to expand, less obvious sources of biological evidence are being collected at crime scenes for DNA profiling. One example is the recovery of biological evidence from common drink containers, such as bottles and cans, which have been found at crime scenes. There are many variables that may have an impact on recovering a DNA profile from such exhibits. In this research, the effects of person to person variation, time, type of drink (including alcoholic and non-alcoholic beverages), and type of drink container, were assessed for their impact on the major analytical outcomes of the DNA process. The results show that the alpha-amylase activity varies from individual to individual and is reduced in the presence of some alcoholic drinks. A reasonable DNA yield was obtained from all samples, however, the concentrations exhibited significant person to person variation. The type of drink container influenced the DNA yield with cans giving a higher yield than bottles of the same drink type. To a reduced extent the presence or absence of alcohol affected the overall DNA yield and when partial or failed DNA profiles were produced they were more likely to be associated with alcoholic drinks than non-alcoholic drinks.

A comparison of statistical models for the analysis of complex forensic DNA profiles

Complex mixtures and LtDNA profiles are difficult to interpret. As yet there is no consensus within the forensic biology community as to how these profiles should be interpreted. This paper is a review of some of the current interpretation models, highlighting their weaknesses and strengths. It also discusses what a forensic biologist requires in an interpretation model and if this can be realistically executed under current justice systems.

The interpretation of elemental composition measurements from forensic glass evidence III.

In this paper we introduce a permutation testing approach to the interpretation of evidence which consists of elemental composition measurements, with glass evidence as an example. This work extends previous work of Curran et al. [J.M. Curran, C.M. Triggs, J.R. Almirall, J.S. Buckleton and K.AJ. Walsh, The interpretation of elemental composition measurements from forensic glass evidence, Science and Justice 37 (1997) 241-244.] and shows how we may remove some of the constraints that limited the applicability of the previous results. We provide the reader with tools for evidence pre-screening that may aid in the direction of further analyses of the data, rather than for the presentation of evidence interpretation in a court case.

Effectiveness of familial searches

The effectiveness of searching DNA databases for relatives of a profile of interest is examined by simulation and real experiments. The technique is found to be remarkably successful at locating parents, children or siblings in a considerable list of profiles.

Degradation of forensic DNA profiles

Selected profiles typed at the Promega PowerPlex 21 (PP21) loci were examined to determine if a linear or exponential model best described the relationship between peak height and molecular weight. There were fewer large departures from observed and expected peak heights using the exponential model. The larger differences that were observed were exclusively at the high molecular weight loci. We conclude that the data supports the use of an exponential curve to model peak heights versus molecular weight in PP21 profiles. We believe this observation will improve our ability to model expected peak heights for use in DNA interpretation software.