Peter Gill

Peter Gill
Norwegian Institute of Public Health · Division of Forensic Medicine and Drug Abuse Research

PhD

About

272
Publications
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Publications

Publications (272)
Article
In sexual assault cases, it can be challenging to identify the type of body fluids/ cell types present in a crime scene sample, especially the origin of epithelial cells.Therefore, more labs are applying mRNA body fluid analysis for saliva, skin and vaginal mucosa markers. To address activity level propositions, it is necessary to assign probabilit...
Preprint
The application of qPCR to estimate the quantity of DNA present is usually based upon a short amplicon (typically c.80bp) and a longer amplicon (typically c.200-300bp) where the latter is used to determine the amount of degradation present in a sample. The data are used to make decisions about a) whether there is sufficient template to amplify? b)...
Article
Full-text available
Interpretation of DNA evidence involving mixtures is challenging when alleles from minor contributors coincide with stutters from major contributors. To accommodate this, it is important to have a good understanding of stutter sequence formation trends. Here, multiple stutter types were characterized based on MPS data from 387 single source samples...
Article
Full-text available
The association of body fluids/cell types and donors in mixed biological traces is an important, but challenging task required to evaluate the value of evidence given forensic propositions concerning the source of the DNA. The linking of a DNA profile with evidence from presumptive tests or RNA analysis is not straightforward. Coding region SNPs (c...
Article
Full-text available
Non-self DNA is normally present on skin due to DNA transfer occurring during daily activities. The understanding of persistence and accumulation of foreign DNA on the neck can assist in the interpretation of DNA evidence collected from an assaulted victim. Establishing the composition and level of non-self DNA present is relevant, especially in ca...
Article
Full-text available
When DNA from a suspect is detected in a sample collected at a crime scene, there can be alternative explanations about the activity that may have led to the transfer, persistence and recovery of his/her DNA. Previous studies have shown that DNA can be indirectly transferred via intermediate surfaces and that DNA on a previously used object can per...
Article
Full-text available
Semaan et al. (J Forensic Res, 2020, 11, 453) discuss a mock case “where eight different individuals [P1 through P8] could not be excluded in a mixed DNA analysis. Even though … expert DNA mixture analysis software was used.” Two of these are the true donors. The LRs reported are incorrect due to the incorrect entry of propositions into LRmix Studi...
Article
A comparative study has been carried out, comparing two different methods to estimate activity level likelihood ratios (LRa) using Bayesian Networks. The first method uses the sub-source likelihood ratio (log10LRϕ) as a ‘quality indicator’. However, this has been criticised as introducing potential bias from population differences in allelic propor...
Article
Full-text available
Probabilistic genotyping has become widespread. EuroForMix and DNAStatistX are both based upon maximum likelihood estimation using a γ model, whereas STRmix™ is a Bayesian approach that specifies prior distributions on the unknown model parameters. A general overview is provided of the historical development of probabilistic genotyping. Some genera...
Article
Likelihood ratios (LR) differences between the probabilistic genotyping software EuroForMix and STRmix™ are examined. After considering differences in the allele probabilities, the LRs from both software for an unambiguous single‐source profile were identical (four significant figures). LRs from both software for an unambiguous single‐source profil...
Preprint
Full-text available
A comparative study has been carried out, comparing two different methods to calculate activity level likelihood ratios (LR a ) using Bayesian Networks. The first method uses the sub-source likelihood ratio (log 10 LR φ ) as a 'quality indicator'. However, this has been criticised as introducing potential bias from population differences in allelic...
Article
Full-text available
The shedder status of an individual may be important to consider in the context of DNA transfer, persistence and recovery and in Bayesian networks where a person’s shedder status may have an impact on the outcome. In this study we compared two methods to determine shedder status: the handheld tube (HH) method and a fluorescent cell count (CC) metho...
Article
Full-text available
Bayesian logistic regression is used to model the probability of DNA recovery following direct and secondary transfer and persistence over a 24 h period between deposition and sample collection. Sub-source level likelihood ratios provided the raw data for activity-level analysis. Probabilities of secondary transfer are typically low, and there are...
Article
To answer the question “Are low likelihood ratios reliable?” requires both a definition of reliable and then a test of whether low likelihood ratios (LRs) meet that definition. We offer, from a purely statistical standpoint, that reliability can be determined by assessing whether the expected rate of inclusionary support for non-donors over many ca...
Article
Full-text available
The increased interest in the use of Massively Parallel Sequencing (MPS) technologies to type traditional autosomal STR markers raises multiple questions regarding interpretation of the results via probabilistic genotyping. To begin to address some of those questions, we examined the effects of using differing degrees of sequence information, pre-f...
Chapter
When low levels of DNA are analyzed, the interpretation of DNA profiles are complicated, because of stochastic effects leading to imbalanced heterozygotes. An extreme form of heterozygote imbalance leads to the disappearance of one or both alleles; this is called allele drop-out. The higher the peak heights, or the greater the quantity of DNA, the...
Chapter
The interpretation starts with an assessment of the case circumstances. To build the case, the scientist may be asked to analyze samples retrieved from a crime scene. The first step is to gather all the relevant information to formulate preliminary propositions given the case circumstances. What are the expectations if the prosecution proposition i...
Chapter
The forensic scientist has a dual role of investigative vs. evaluative. The scientist is said to be in the investigative mode when there is no suspect in a case. This will usually lead to a search for potential suspects on a national DNA database. In addition, some cases may be complex, with numerous case stains and suspects to investigate. A numbe...
Chapter
The interpretation of evidence is rooted in population genetics theory. The fundamental principle that underpins this is the Hardy–Weinberg equilibrium. The inheritance of genes follows laws of probability that enable probability estimates to describe the “rarity” of a DNA profile. Such calculations rely upon the Hardy–Weinberg equilibrium assumpti...
Chapter
The quantitative (continuous) model incorporates allele peak height information. This chapter describes the early development of the theory, beginning with the “Clayton” guidelines, which were empirical, taking into account heterozygote balance to distinguish major/minor contributors of mixtures. The “mixture proportion” is an important concept tha...
Chapter
LRmix is a qualitative model that is based upon the likelihood ratio formulation. It takes account of drop-out, drop-in, and any number of contributors. However, it does not explicitly model allele peak heights, stutter, or degradation, which is reserved for quantitative (continuous) models; the latter are further discussed in subsequent chapters....
Chapter
This chapter traces the early methods used to deal with the interpretation of low template DNA, starting with the “consensus” method, where replicate tests of a single extract are carried out; only duplicated alleles are reported. This method was restricted to simple profiles and was somewhat ad-hoc. The concept of drop-in was introduced. These are...
Chapter
Empirical evaluation of evidence, where allele peak heights are taken into consideration can only be carried out with well represented DNA profiles. A well represented profile is defined as one which is above the stochastic threshold. The methods shown here are restricted to two-person contributors to a mixture. Characterization of heterozygotes, s...
Chapter
The likelihood ratio is the only framework that can be properly utilized to interpret complex DNA profiles. Although a model used to calculate a likelihood ratio may be robust, there are no true likelihood ratios or true models. The magnitude of the likelihood ratio is a measure of the value or the strength of the evidence. The likelihood ratio may...
Chapter
This chapter unifies the concepts previously discussed to frame and develop the theory behind the LRmix model. LRmix is a qualitative model that takes account of drop-out, drop-in, and any number of contributors. However, it does not explicitly model allele peak heights, stutter, or degradation, which is reserved for quantitative (continuous) model...
Chapter
This chapter describes the use of EuroForMix to analyze massive parallel sequencing (MPS) data (both SNPs and STRs). Mixture analysis of SNPs is best carried out using quantitative models, especially if there are more than two contributors. Because there are no more than two alleles per locus, this estimation is difficult to achieve, hence some exp...
Chapter
This chapter is about validation. The purpose of validation is to define the scope and limitations of a software. There are three parts to a validation exercise: a)Conceptual validation, where the mathematical formulae that underpin a software are verified as correct. b)Software validation, to ensure that the software is running as expected; verifi...
Article
Full-text available
The value of the evidence depends critically on propositions. In the second of two papers intended to provide advice to the community on difficult aspects of evaluation and the formulation of propositions, we focus primarily on activity level propositions. This helps the court address the question of "How did an individual's cell material get there...
Article
Data from all sexual assault cases analysed at the Section of Forensic Biology at Oslo University Hospital in the period 2013-2015 were reviewed to study transfer and persistence of cells deposited on the body. Data were recorded on detection of both sperm and epithelial cells. The final dataset consist of 2141 samples from 765 cases. In this study...
Article
The interpretation of evidence continues to be one of the biggest challenges facing the forensic community. This is the first of two papers intended to provide advice on difficult aspects of evaluation and in particular on the formulation of propositions. The scientist has a dual role: investigator (crime-focused), where often there is no suspect a...
Conference Paper
A series of two- and three-person mixtures of varying dilutions were prepared and analysed with Life Technologies' HID-Ion AmpliSeq™ Identity Panel v2.2 using the Ion PGM™ massively parallel sequencing (MPS) system. From this panel we used 134 autosomal SNPs. Using the reference samples of three donors, we evaluated the strength of evidence with li...
Article
A series of two- and three-person mixtures of varying dilutions were prepared and analysed with Life Technologies' HID-Ion AmpliSeq™ Identity Panel v2.2 using the Ion PGM™ massively parallel sequencing (MPS) system. From this panel we used 134 autosomal SNPs. Using the reference samples of three donors, we evaluated the strength of evidence with li...
Article
Full-text available
Simulation experiments were used to show the impact of varying extraction efficiency, aliquot proportion, and PCR efficiency on the heterozygote balance of a range of diploid and haploid cells. Reducing either parameters introduces variance. It is well-known that the variance in heterozygote balance increases as the amount of DNA is reduced. Surpri...
Article
In court questions are often raised related to how trace DNA was deposited, directly during the crime or innocently for instance by secondary transfer. It is therefore of interest to have knowledge of the probability of transfer or secondary transfer in different situations. Factors that could influence transfer probabilities are background DNA and...
Research
Full-text available
This is a public guide to DNA in criminal investigations which provides widely accessible information on the capacities and limitations of current and some emerging DNA technologies used in criminal justice.
Article
The use of biostatistical software programs to assist in data interpretation and calculate likelihood ratios is essential to forensic geneticists and part of the daily case work flow for both kinship and DNA identification laboratories. Previous recommendations issued by the DNA Commission of the International Society for Forensic Genetics (ISFG) c...
Article
The investigation of the performance of models to interpret complex DNA profiles is best undertaken using real DNA profiles. Here we used a data set to reflect the variety typically encountered in real casework. The "crime-stains" were constructed from known individuals and comprised a total of 59 diverse samples: pristine DNA/DNA extracted from bl...
Article
The statistical evaluation of autosomal Short Tandem Repeat (STR) genotypes is based on allele frequencies. These are empirically determined from sets of randomly selected human samples, compiled into STR databases that have been established in the course of population genetic studies. There is currently no agreed procedure of performing quality co...
Article
As the profiling systems used in forensic analyses have become more sensitive in recent years, the risk of detecting a contamination in a DNA sample has increased proportionally. This requires more stringent work protocols and awareness to minimize the chance of contamination. Although there is high consciousness on contamination and best practice...
Article
Full-text available
The case of the ‘murder of Meredith Kercher’ has been the subject of intense media scrutiny since 2007 when the offence was committed. Three individuals were arrested and accused of the crime. Amanda Knox and Raffaele Sollecito were exonerated in March 2015. Another defendant, Rudy Guede, remains convicted as the sole perpetrator. He was implicated...
Article
The DNA Commission of the International Society for Forensic Genetics (ISFG) is reviewing factors that need to be considered ahead of the adoption by the forensic community of short tandem repeat (STR) genotyping by massively parallel sequencing (MPS) technologies. MPS produces sequence data that provide a precise description of the repeat allele s...
Article
A number of new computer programs have recently been developed to facilitate the interpretation and statistical weighting of complex DNA profiles in forensic casework. Acceptance of such software in the user community, and subsequent acceptance by the court, relies heavily upon their validation. To date, few guidelines exist that describe the appro...
Article
We have released a software named EuroForMix to analyze STR DNA profiles in a user-friendly graphical user interface. The software implements a model to explain the allelic peak height on a continuous scale in order to carry out weight-of-evidence calculations for profiles which could be from a mixture of contributors. Through a properly parameteri...
Conference Paper
The freely available open source software named EuroForMix implements a statistical model to explain the variation of the allelic peak height from the PCR-process. The model considers mixture proportion, peak height distribution properties, stutter proportion and degradation slope as unknown parameters. EuroForMix supports two inference approaches....
Article
A study of persistence of DNA from previous users' computer equipment and secondary transfer to a new user's hand was conducted. Four individuals (two male and two female) swapped their personal computer keyboard and mouse. Samples were collected from parts of the equipment (background DNA), and from the new users' hands on the first day and then f...
Article
A series of two- and three-person mixtures of varying ratios were prepared and analysed with Life Technologies' HID-Ion AmpliSeq™ Identity Panel v2.2 using the Ion PGM™ massively parallel sequencing system. This panel includes 136 autosomal SNPs and 33 Y-chromosome SNPs. Using the reference samples of the mixture donors, we evaluated the strength o...
Article
Interpretation of DNA mixtures with three or more contributors, defined here as high order mixtures, is difficult because of the inevitability of allele sharing. Allele sharing complicates the estimation of the number of contributors, which is an important parameter to assess the probative value. Consequently, these mixtures may not be deemed suita...
Article
With the introduction of new multiplex PCR kits and instrumentation such as the Applied Biosystems 3500xl, there has recently been a rapid change in technology that has greatly increased sensitivity of detection so that a DNA profile can routinely be obtained from only a few cells. Research to evaluate the risks of passive transfer has not kept pac...
Article
The introduction of Short Tandem Repeat (STR) DNA was a revolution within a revolution that transformed forensic DNA profiling into a tool that could be used, for the first time, to create National DNA databases. This transformation would not have been possible without the concurrent development of fluorescent automated sequencers, combined with th...
Article
The interpretation of mixed DNA profiles obtained from low template DNA samples has proven to be a particularly difficult task in forensic casework. Newly developed likelihood ratio (LR) models that account for PCR-related stochastic effects, such as allelic drop-out, drop-in and stutters, have enabled the analysis of complex cases that would other...
Article
Likelihood ratio (LR) methods to interpret multi-contributor, low template, complex DNA mixtures are becoming standard practice. The next major development will be to introduce search engines based on the new methods to interrogate very large national DNA databases, such as those held by China, the USA and the UK. Here we describe a rapid method th...
Article
There has been very little work published on the variation of reporting practices of mixtures between laboratories, but it has been previously demonstrated that there is little consistency. This is because there is no current uniformity of practice, so different laboratories will operate using different rules. The interpretation of mixtures is not...
Article
If complex DNA profiles, conditioned on multiple individuals are evaluated, it may be difficult to assess the strength of the evidence based on the likelihood ratio. A likelihood ratio does not give information about the relative weights that are provided by separate contributors. Alternatively, the observed likelihood ratio can be evaluated with r...
Article
Often in forensic cases, the profile of at least one of the contributors to a DNA evidence sample is unknown and a database search is needed to discover possible perpetrators. In this article we consider two types of search strategies to extract suspects from a database using methods based on probability arguments. The performance of the proposed m...
Article
Misleading DNA Evidence: A Guide for Scientists, Judges, and Lawyers presents the reasons miscarriages of justice can occur when dealing with DNA, what the role of the forensic scientist is throughout the process, and how judges and lawyers can educate themselves about all of the possibilities to consider when dealing with cases that involve DNA ev...
Article
There is general lack of awareness that high LR based on complex propositions e.g. three contributors, does not necessarily translate into probative evidence against a suspect. In some cases there is an increased chance of false inclusion of a person of interest. This is an issue for all LR-based models. One way to address this issue is to further...
Article
Abstract The validation of new short tandem repeat (STR) systems for forensic purposes is extremely time consuming and expensive. However, if a full understanding of biological processes was achieved, then this would effectively by-pass the need to carry out validation by traditional methods, since millions of DNA profiles could realistically be ge...
Article
Although likelihood ratio (LR) based methods to analyse complex mixtures of two or more individuals, that exhibit the twin phenomena of drop-out and drop-in has been in the public domain for more than a decade, progress towards widespread implementation in to casework has been slow. The aim of this paper is to establish a LR-based framework using p...
Article
The interpretation of DNA mixtures has proven to be a complex problem in forensic genetics. In particular, low template DNA samples, where alleles can be missing (allele drop-out), or where alleles unrelated to the crime-sample are amplified (allele drop-in), cannot be analysed with classical approaches such as random man not excluded or random mat...
Article
DNA profiling of biological material from scenes of crimes is often complicated because the amount of DNA is limited and the quality of the DNA may be compromised. Furthermore, the sensitivity of STR typing kits has been continuously improved to detect low level DNA traces. This may lead to (1) partial DNA profiles and (2) detection of additional a...
Article
High throughput analysis of buccal scrape reference samples using the Identifiler system on the Applied Biosystems 3500xl Genetic Analyser is described. This platform is much more sensitive than previous platforms, e.g. 3130xl. The range of signal detection is also much greater and this means that the system is more tolerant to a wide range of inpu...
Article
Full-text available
To support and to underpin the European initiative to increase the European set of standard markers (ESS), by the addition of five new loci, a collaborative project was organised by the European Network of Forensic Science Institutes (ENFSI) DNA working group in order to assess the new multiplex kits available. We have prepared allele frequency dat...
Article
An evaluation was carried out to determine the effect on routine forensic calculations when incorporating STRs D12S391 and vWA. These loci are co-located on the same arm of chromosome 12. It has been suggested that allelic association could result in over-estimates of strength-of-evidence calculations. In the first place, we argue that is very unli...