Interpreting Complex Forensic DNA Evidence

Jane Moira Taupin

• Publisher: CRC Press
• Year: 2019
• eBook ISBN: 9781351023764
• Topic: Law
• Subtopic: Criminal Law
• Index: Law

Interpreting Complex Forensic DNA Evidence is a handy guide to recent advances—and emerging issues—in interpreting complex DNA evidence and profiles for use in criminal investigations. In certain cases, DNA cannot be connected to a specific biological material such as blood, semen or saliva. How or when the DNA was deposited may be an issue. The possibility of generating DNA profiles from touched objects, where there may not be a visible deposit, has expanded the scope and number of exhibits submitted for DNA analysis.

With such advances, and increasing improvements in technological capabilities in testing samples, this means it is possible to detect ever smaller amounts of DNA. There are also many efforts underway to seek was to interpret DNA profiles that are sub-optimal—either relative to the amount required by the testing kit and, potentially, the quality of the obtained sample. Laboratories often use enhancements in order to obtain a readable DNA profile.

The broad-reaching implications of improving DNA sensitivity have led to this next, emerging generation of more complex profiles. Examples partial profiles that do not faithfully reflect the proposed donor, or mixtures of partial DNA from multiple people. A complexity threshold has been proposed to limit interpretation of poor-quality data. Research is now addressing the interpretation of transfer of trace amounts of DNA. Complex issues are arising in trial that need to be reconciled as such complexity has added challenges to the interpretation of evidence and its introduction or dismissal in certain cases in the courts.

Key Features:

• Addresses DNA transfer, from person-to-person as well as to objects
• Outlines each stage required to produce a DNA profile from an exhibit—including collection, handling, storage, and analysis
• Discusses ethics, subjectivity, and bias—including cognitive dissonance—as they relate specifically to complex DNA evidence
• Highlights current techniques and the latest advances in DNA analysis, including advances in familial DNA searches

Interpreting Complex Forensic DNA Evidence provides tools to assist the criminal investigator, forensic expert, and legal professional when posed with a DNA result in a forensic report or testimony. The result—and any associated statistic—may not reveal any ambiguity, complexity, or the assumptions involved in deriving it. Questions from resolved criminal cases are posed, and the relevant forensic literature, provided for the reader to assess a DNA result and any associated statistic. Case studies throughout illustrate concepts and emphasize the need for conclusions in the forensic report that are supported by the data.

Complex DNA Evidence 1

BOX 1

• Simple versus complex DNA evidence
• Increased sensitivity
• Increased technology
• Illusion of scientific certainty
• History of exhibit
• The “silo” effect

1.0 Introduction

The moment forensic DNA evidence was introduced into a criminal investigation in the mid-1980s, investigators gained a powerful discrimination tool. Its sensational debut in a double murder case in England (Case 1.1 below) prompted a quick uptake by forensic laboratories.

The use of DNA evidence expanded over the years as analytical techniques became more sophisticated. In the beginning DNA analysis was implemented by only major laboratories that could afford the necessary equipment, and relied upon in serious crime cases. Today it is part of routine forensic laboratory work and may be used in less serious criminal cases such as theft. Indeed, DNA profiling is deemed necessary in any forensic laboratory that analyzes evidence that may have biological deposits.

At the same time, the word DNA has moved into the popular domain and language, such as, “It’s in your DNA”. This arises from the scientific fact that no two people have the same DNA (apart from identical twins) and from the idea of genetic inheritance. But forensic laboratories cannot analyze the whole genome of a person of interest. Nor has every person in the world been DNA profiled and placed on a database. There may be a future where both these activities are possible (and further chapters discuss these issues), but currently only certain areas on the DNA molecule are analyzed in forensic cases—routinely around 20 areas. Statistical calculations are performed on frequencies of the appearance of different variants on those areas compared to collected databases of different DNA populations or ethnicity, sometimes of a hundred people or so. Further statistical calculations are performed to provide an evidential “weight” or what is termed “statistical weighting”.

Crime scene samples are not pristine laboratory specimens. They may be of poor quality, insufficient quantity, contaminated with other substances, and/or old and degraded. While such deficiencies have always been a problem, improving technology has enabled more information to be obtained from these types of samples. Perhaps once they were not reported or solely used as part of an investigatory aspect, whereas today they may be reported in court as evidential.

The main questions a forensic scientist is required to ask:

• Whose DNA could it be?
• From what body fluid/matter has it originated?
• How did it get there?
• Have the results been reported in a transparent and balanced way?

There are assumptions and limitations when addressing these questions, as the following chapters describe.

It has been noted (Sense about Forensic Genetics, 2017) that as the amount of information that can be gleaned from the tiniest traces of DNA continues to grow, it is time to take stock of these increased possibilities and address the challenges that enhanced DNA technology could bring.

1.1 Simple versus Complex DNA Evidence

“Simple” DNA evidence involves sampling, analysis, and interpretation that are straightforward. For example, the deposit from which the DNA profile is obtained can be observed visually, and can be related to a specific biological fluid/matter which has been tested. As well, the amount of DNA is sufficient for the DNA testing kit, the DNA profiles themselves faithfully reflect that of the purported donor, and documentation that establishes continuity including absence of contamination issues is comprehensive.

By contrast, complex DNA evidence is more problematic. It is that DNA evidence used in a criminal trial that may require additional considerations in sampling, testing, and interpreting than what has been traditionally considered.

This complex DNA evidence is not confined to the DNA profile itself, but extends to the interpretation of such DNA profiles in the context of the case. There may be many DNA profiles generated from a particular crime scene, and these may need to be considered separately and then holistically as part of the context of the case. Alternatively, there may be only one DNA profile of interest, but that may be from a non-visible deposit and be partial or a mixture of DNA from multiple individuals.

Simple and complex DNA cases may be illustrated by two well-known criminal cases in the United Kingdom, the country where DNA profiling was first introduced into the criminal justice system.

The “simple” DNA case is illustrated by the first murder case solved by DNA profiling in the mid-1980s (Gill and Werrett, 1987; Wambaugh, 1989). The case was not viewed as “simple” at the time; rather, it was an amazing breakthrough. Today, forensic laboratories would regard it as routine, “simple” DNA evidence.

Case 1.1 The Pitchfork murders

A 15-year-old girl, Lynda Mann, was found raped and murdered in 1983, abandoned in the English Midlands countryside. Three years later, 15-year-old Dawn Ashworth was raped and murdered nearby. A 17-year-old kitchen porter with learning difficulties confessed to the second murder but not the first; police strongly suspected both had been murdered by the same offender. The semen on both bodies had blood group “A” and an enzyme profile that occurs in about 10% of the adult male population. Police asked geneticist Professor Alec Jeffreys of the University of Leicester to analyze the samples using his then-new technique of “DNA fingerprinting”. The technique had recently been publicized in the English media because it had resolved a parentage testing dispute in an immigration case (Jeffreys et al., 1986).

The semen on both bodies was indeed believed to be from the same man but DNA profiling excluded the kitchen porter. This led the police to conduct a world-first DNA-led intelligence screen of more than 5,000 local men, with villagers providing blood samples in a mass testing. This was at the time when reference samples for DNA profiling were collected using blood from the individual and not the saliva samples used today.

Colin Pitchfork persuaded a work colleague to donate a blood sample for him, but police discovered this ruse and it was subsequently found that the DNA profile obtained from the semen on the bodies matched Pitchfork’s DNA profile. Pitchfork pleaded guilty and was sentenced for the two murders in 1988; essentially, the DNA technique was not probed in court. He appealed in 2009 against the severity of his prison sentence of 30 years, and this was reduced to 28 years but only if public safety was assured (Pitchfork v R, 2009).

The first suspect in the Pitchfork case was thus the first in the world to be exonerated using DNA. It is the high discrimination power, the power to exclude, that is arguably the greatest achievement for criminal justice in DNA profiling.

The Pitchfork case led to the rapid introduction of DNA analysis into casework in England and Wales (Werrett et al., 1989).

Case 1.1 is “simple” for several reasons: identified semen was obtained from the internal cavity of the bodies of the deceased females, the DNA profiles were of sufficient discrimination and quality, and there was no uncertainty in transfer and interpretation.

A separation technique was developed in the Pitchfork case whereby sperm could be separated from other cells on the vaginal swabs from the deceased. This was necessary due to the large amounts of female cellular material obscuring any DNA results from the sperm (Gill et al., 1985). Although different DNA profiling tests are now in place, this type of physical and chemical separation is still used for internal medical swabs with identified spermatozoa.

1.2 Increased Sensitivity

The development of polymerase chain reaction (PCR) was implemented in forensic casework in the 1990s and increased the scope of crime scene samples that could be analyzed. PCR is known as the amplification step and essentially a molecular photocopier that can amplify very small samples into samples that can be detected and analyzed.

The PCR method was a boon for forensic science as it enabled the analysis of minute quantities of material. Coupled with automated fluorescent detection, multiple areas (loci) on the DNA molecule can be tested at once and visualized in an electropherogram (EPG), a diagrammatic plot of fluorescence intensity versus size and location of the fragment of DNA. This fragment is a repeating variable unit and called a short tandem repeat (STR).

The desire to maximize the investigative potential of DNA led scientists to develop ever more sensitive methods, so that genetic science could be applied to more and more casework, including “cold cases” that had previously yielded no clues. This is true today, with investigators (not just scientists but also the police) searching for new ways to identify a suspect with DNA analysis, where there is no evidence or “clue”, even with the high level of sensitivity now employed with routine DNA analysis.

The first low-level or low-template technique, at the time called low copy number (LCN), was developed by the Forensic Science Service in the United Kingdom in the late 1990s (Gill et al., 2000). Low copy number was viewed as a new weapon in the fight against crime. It used extra copying in the amplification PCR process and could achieve results from very small samples merely “touched” or from non-visible deposits. A re-opened “cold case” with a trial that queried the appropriate use of this technique occurred in the mid-2000s (R v Hoey, 2007).

Case 1.2 Omagh Bombing

The Omagh bombing occurred in 1998 in Northern Ireland. A devastating car bomb killed 29 people and injured 220 in the city. Sean Hoey was charged in 2005 because it was alleged his DNA was found on bomb timers collected during the crime scene examination (inferred to be “touch DNA”). However, the technique of LCN did not exist in 1998. Crime scene examiners did not necessarily follow the stringent anti-contamination measures needed for such a process. Justice Weir in the trial court of 2007 described the collection of exhibits as thoroughly disorganized and the police storage areas “a complete mess”. The way the DNA evidence had been recovered, packaged, stored, and transported was a concern. The forensic laboratory was no better, with labels routinely falling off items and experts not wearing masks or sometimes even gloves.

There was another concern, namely scientific opinion on the validity of the method. Justice Weir concluded that low copy DNA had not been appropriately validated by the scientific community. In his view, two articles published by the developers of the method were insufficient to validate the technique.

The accused was freed as a result of the court hearing.

The DNA evidence was indeed “complex” and not simple—complex n...

Table of contents

• Cover
• Half-Title
• Title
• Copyright
• Contents
• Preface
• Acknowledgments
• Author
• 1 Complex DNA Evidence
• 2 Complex DNA Profiles
• 3 Statistical Evaluation of Complex DNA Evidence
• 4 Transfer
• 5 Integrity
• 6 Familial DNA Searching
• Glossary of Terminology
• Index