Dealing with DNA Evidence

A Legal Guide

Andrei Semikhodskii

• Publisher: Routledge-Cavendish
• Year: 2007
• ISBN:
  9781135390402
• Edition: 1
• Topic: Law
• Subtopic: Law Theory & Practice
• Index: Law

Giving the reader an in-depth understanding of DNA evidence in criminal practice, this text explains in clear language how DNA evidence is obtained and how it can be successfully challenged in court to minimize its impact or even dismiss it completely.

Since it first entered the criminal legal practice DNA has become an indispensable tool in fighting crime, as it allows both unambiguous identification of the criminal by traces of biological material left at the crime scene as well as acquitting innocent suspects.

This book:

• 
  
  

• outlines the various types of testing used to obtain DNA evidence
• highlights the weaknesses of DNA testing, presenting and discussing defence strategies for refuting DNA evidence
• shows how DNA should be treated as just another piece of evidence and how on its own it is often not enough to convict someone of a particular crime.

This book is essential reading for students and practitioners of criminal law and practice and forensic science and law.

Chapter 1

Introduction to criminal DNA analysis

1.1 The DNA revolution in the criminal justice system

Solving a crime is a difficult task. The challenge faced by a criminal investigator, though simple, is not trivial – correctly to identify the perpetrator and bring him/her to justice. This can be achieved in a number of ways, by examining eyewitness accounts and physical evidence recovered from the crime scene. Among the physical evidence, DNA evidence has possibly the highest probative value of all, on a par with fingerprint evidence, in identifying an individual.

According to the Locard exchange principle, one of the most fundamental concepts in forensic science, ‘the dust and debris that cover our clothing and bodies are the mute witnesses, sure and faithful, of all our movements and all our encounters’ (Locard, 1930). This means that when a person comes into contact with another person or substance, s/he bound to leave physical traces of his/her presence. The Locard principle denotes forensic science as the science of discovering associations between samples found at the crime scene and a reference sample.

The aim of forensic DNA analysis is to identify the source of biological evidence collected at the scene of crime. The seminal discovery in the early 1980s by Professor Sir Alec Jeffreys of a technique which enabled the use of DNA for human identification purposes has revolutionised forensic science. The criminal justice system now relies heavily on DNA-based evidence. All over the world, thousands of people have been convicted of various crimes with the help of DNA evidence, and hundreds of wrongfully convicted people have been exonerated. DNA analysis has become an indispensable police tool, as it allows unambiguous identification of the criminal by traces of biological material left at the crime scene and the acquittal of innocent suspects, based on DNA evidence. The importance of this silent but faithful witness in fighting crime cannot be underestimated.

Before the advent of DNA, biological evidence was analysed by serological methods, which are based on examination of blood groups and various human proteins. DNA testing completely replaced its predecessor within five years of its introduction. Now, DNA analysis is a routine tool of police investigation with police forces worldwide relying heavily on forensic DNA testing. In parallel with developing DNA testing technology, national criminal DNA databases have appeared in various countries. At the moment, UK and most European countries have DNA databases used by law enforcement agencies to combat crime.

There are several reasons which made DNA the method of choice of forensic examination:

• DNA has very high discrimination power. No two people, with the single exception of identical twins, have identical DNA. Current forensic DNA genotyping systems allow achieving discrimination power of one in several billion, ensuring that every DNA profile obtained is virtually unique.
• Individual’s DNA profile is constant and does not change with time. Many biometrical parameters used for human identification change during the lifetime of the individual. DNA does not. It is also impossible to replace one individual’s DNA with another’s.
• Different types of cell have identical DNA complements. DNA obtained from different sources from the same individual will have an identical pattern, regardless of the biological origin. A DNA profile of a seminal stain from one crime scene can be compared to that of a droplet of blood recovered from a different scene and if the semen and blood were deposited by the same individual, identical DNA results will be obtained.
• DNA is stable and reliable data can be produced from very old and decayed biological samples. DNA is more robust than proteins when subjected to harsh environments and is capable of withstanding both natural and man-made environmental injury. The high molecular integrity of DNA allows forensic scientists to analyse long-buried samples as well as samples that have been subjected to high temperatures and chemical treatment. Even when biological material is severely degraded DNA evidence can still be produced using modern forensic approaches.
• DNA is inherited. Family members have similar DNA profiles. Using sophisticated data analysis tools it is possible to identify a culprit with reasonable confidence by analysing DNA of close relatives.
• It is possible to generate millions of exact copies of DNA by specific enzymatic reactions which allow genetic information from exceedingly small amounts of biological material to be obtained. Modern day technology is so advanced that a single hair, skin flake or small droplet of sweat left at the crime scene is often sufficient to obtain a full DNA profile, which can be used to identify the perpetrator.

DNA testing can provide a vast amount of other information besides the conventional DNA profile. It is possible to determine the gender of the donor, his/her ethnic origin, hair, eye and skin colour, predisposition to certain diseases and even biological age. As new technologies develop and understanding about the function of human genes increases, this array will only expand. No other type of evidence is capable of producing so much information about the individual whose biological sample is analysed.

1.2 DNA – function, structure and location

1.2.1 Function and structure of DNA

Deoxyribonucleic Acid (DNA) is a complex biological molecule that encodes the complete genetic information of an organism. Written in our DNA is a record of each person’s individuality, a shared history of human evolution, and information that can provide insight into a person’s future health. In layman’s terms DNA is a software code which encodes all the information necessary for an organism to function, as well as the information required for its development and procreation.

Discovered by the Swiss biochemist Johan Friedrich Miescher in 1869 who called in ‘nuclein’, it was not until 1953 when James Watson and Francis Crick, working in Cambridge, revealed the structure of DNA and provided a simple model which explained how genetic information is passed from generation to generation. This fundamental discovery of the twentieth century opened a new era in biology and medicine and had an impact on various other fields from computing to forensic science.

According to the Watson and Crick model, one single DNA molecule consists of two intertwined strands making a double helix. Each strand has a polysugar-phosphate backbone from which protrude the nucleotides (bases) – A (adenine), T (thymine), G (guanine) and C (cytosine), the order of which determines the function of the genes. The two strands which form a DNA molecule are called complementary strands. The strands are held together by weak hydrogen bonds between the opposing bases thus forming base pairs (bp). The bonding between the opposing bases observes a strict base-pairing rule – adenine pairs only with thymine (an A–T pair) and cytosine with guanine (a C–G pair). This ensures that once the information on the nucleotide sequence is obtained for one of the strains it is easy to deduce the sequence of nucleotide of the complementary one.

Each human DNA molecule consists of various types of DNA sequences – sequences coding for genes, sequences coding for the elements which regulate the activity of genes, pseudogenes (or ‘fossil genes’, genes which are not needed any more and are not in use but which could have had some important function in the past) and sequences with no known function. The fraction of the coding DNA in the human genome is very small and constitutes about 1–2 per cent. The rest is thought to have either no function or the function is not yet identified.

The total DNA complement of a cell is called the genome. The size of the genome is usually stated as the total number of base pairs. Genome size is thought to parallel the complexity of an organism – however, this rule does not always hold. The human genome is approximately three and a half billion bp long, which is similar to that of many mammals such as mouse, rat or chimpanzee, while bread wheat has a genome which contains in excess of 15 billion bp. It is estimated that the total number of human genes is in the region of 25,000 to 40,000.

1.2.2 Genomic and mitochondrial DNA

DNA is present in almost every human cell. Within a cell, DNA can be found in two compartments, called organelles, the nucleus and the mitochondria. This compartmentalisation of DNA occurred at the very beginning of life on Earth and is a feature of all animal or plant species. This means that there are two evolutionary distinct types of human DNA – nuclear or genomic DNA, which is located in the nucleus, and mitochondrial DNA (mtDNA), located in mitochondria.

Genomic DNA is what most people think when they read about ‘DNA’ or ‘human DNA’. This type of DNA constitutes more than 99.7 per cent of the total cell DNA contents. Genomic DNA is organised into small structures called chromosomes, which are made of DNA and protein. The chromosomal number of individual cells in humans varies. Somatic cells are diploid, i.e., they have two sets of chromosomes, each given by one parent. All body cells with an exception of gametes (sex cells) are somatic cells. In humans, somatic cells have 23 pairs of chromosomes, making the total human chromosome complement of 46. Sex cells have a haploid (single) set of chromosomes. The sex cells are formed from a particular type of somatic cell by the process of meiosis, a specialised form of reductive cell division. This results in production of sperm cells in men and egg cells in women, both containing 23 chromosomes. When an egg is fertilised by a sperm, haploid genomes of the male and female gametes fuse resulting in a diploid fertilised egg, a zygote, containing 46 chromosomes.

mtDNA is located in mitochondria which are small cellular organelles serving as cell power plants. They produce energy for all cellular biochemical processes including cell division and chromosome replication.

Mitochondria are semi-autonomous self-reproducing organelles and have their distinctive genome. There are somewhere between 200 and 400 mitochondria in most human cells. DNA in a mitochondrion is organised into one small circular chromosome. Because of these unique features, mitochondria are believed to originate as a result of a symbiotic relationship between primitive eukaryotic organisms and an unknown species of aerobic bacteria. Mitochondria have an unusual mode of inheritance – they are passed only via the maternal line and although an individual gets an equal share of chromosomes from each parent, s/he inherits mitochondrial DNA exclusively from the mother.

All 100 trillion cells making up a human body originated from just a single cell – the zygote. Human cells are grouped into 220 different tissue types performing different functions but remarkably each and every one of them, with the important exception of gametes, is identical in terms of their DNA. This means that it will make no difference which cell or tissue type is analysed – blood, saliva, bones or muscle – barring mutations and some genetic abnormalities discussed below, the results of DNA profiling will be identical no matter what cell or tissue type is used for analysis.

Each sperm and egg cell is unique in terms of DNA and should one analyse them individually one would get different results from cell to cell. However, in practice, sperm cells are analysed millions at a time (e.g., from ejaculate or a vaginal swab) and the resulting DNA profile will be identical to that obtained from somatic cells as individual differences between each sperm will balance each other.

DNA in cells remains constant during the human life. The only variation in DNA between the cells is due to mutation. Although mutations are very rare they may affect forensic analysis and complicate interpretation of the results, especially in paternity disputes, kinship determination or identification of missing persons.

1.3 Human chromosomes and mtDNA

1.3.1 Human chromosomes

Humans have 23 pairs of chromosomes, 46 in total – two sets of 22 autosomes (chromosomes not involved in sex determination) and a pair of sex chromosomes (chromosomes which take part in sex determination). Chromosomes are numbered from 1 to 22 with chromosome 1 being the largest and 22 the smallest. The sex chromosomes are named X and Y. The X chromosome is among the largest human chromosomes while the Y chromosome is one of the smallest with only a handful of genes on it. Normal males (males with no chromosomal abnormalities) have one X and one Y chromosome, while normal females have two X chromosomes. The karyotype or chromosomal complement of an individual is indicated by providing the total number of chromosomes and the sex chromosome constitution. Thus, a karyotype of a normal male is indicated as 46, XY and that of a normal female 46, XX.

A matching pair of chromosomes is termed a homologous pair. Homologous chromosomes have the same structure and order of genes and very similar DNA sequence. One chromosome from a homologous pair is inherited from one parent and the other one from the other parent.

In rare cases, some individuals may have chromosomal abnormalities, which fall into two categories – numerical abnormalities, when the total number of chromosomes is different from 46 and structural abnormalities, when the total number of chromosomes may be 46 but one (or more) chromosome has some structural transformation. The most well known examples of numerical chromosomal abnormalities are Down’s syndrome, when an individual has three chromosomes 21, the phenomenon called a trisomy, and sex chromosome abnormalities when individuals have karyotypes like 47, XXX, 47, XXY, 47, XYY or 45, X. People with chromosomal abnormalities often have visible physical signs and impaired intelligence and can be easily identified by a medical professional. In some cases, however, depending on the type of abnormality, they may appear both intellectually and physically normal.

When a DNA profile from an individual with numerical chromosomal abnormalities is analysed, an extra DNA component can often be found and at first glance a single profile can be mistaken for a mixed one (DNA profile with more than one contributor). Although an experienced forensic scientist will be able to identify chromosomal abnormality in an individual by analysing his/her reference sample, when crime scene samples are profiled (which are in many cases mixed samples) it can add additional uncertainty, and the knowledge of whether or not a potential contributor has a particular chromosomal abnormality will assist in data interpretation. This is discussed in more detail in Chapter 5.

1.3.2 mtDNA

Mitochondrial DNA is a single circular DNA molecule of about 16,569 bp. An mtDNA molecule contains a coding region, which encodes only 37 genes, and a non-coding region, which is 1,122 bp long and contains elements necessary for gene regulation (Budowle et al, 2003). Usually, a mitochondrion contains 2–10 copies of mtDNA. The average number of mitochondria in somatic cells is about 200–400 but it can be as high as 1,000 while a sperm cell contains just a single mitochondrion.

Most interesting from the forensic point of view is the non-coding area of mtDNA. This region, which encompasses 610 bp, contains two areas which were found to be the most variable between individuals – hyper-variable regions 1 (HV1) and 2 (HV2), nucleotide positions 16,024–16,365 and 73–340 respectively.

mtDNA has increased stability due to the small circular double-stranded structure and extra layers of protective biological membrane, and is present in high numbers in each cell. Because of these features the main application of mtDNA testing is cases when little or no chromosomal DNA can be recovered from a sample (Buckleton et al, 2005c). When old or degraded biological material is analysed mtDNA is often the only source of DNA which can be isolated. mtDNA has been successfully used in analysis of bone tissues which might be several thousand years old. In forensic analysis mtDNA is an indispensable tool for identification of victims of mass disasters such as plane crashes, natural disasters or acts of terrorism or when a distant relative has to be used as reference sample (Buckleton et al, 2005c).

1.3.3 Inheritance of chromosomes and mtDNA

All humans except identical (monozygotic) twins are thought to be genetically different. Because monozygotic twins originate from a single zygote, they are genetically identical. A child gets one set of chromosomes from the father and the other one from the mother. There are two special cases to this rule. Boys inherit a Y chromosome from their fathers and then pass it on only to their male offspring (Figure 1.1). All children inherit mtDNA exclusively from their mothers but only girls then pass it on to all their children (Figure 1.2). These two modes of inheritance are called patrilineal and matrilineal respectively. As for the autosomes, there is 50 per cent chance of a child inheriting one of the parents’ two chromosomes. Only sex chromosomes do not strictly obey this rule – girls have a 50 per cent chance of inheriting a specific copy of the mother’s X chromosome but 100 per cent chance of inheriting the father’s only X chromosome. The same is true for boys with the exception that they have a 100 per cent chance of inheri...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• Table of Cases
• Table of Statutes and Regulations
• Chapter 1 Introduction to criminal DNA analysis
• Chapter 2 Forensic DNA testing
• Chapter 3 Interpretation and statistical evaluation of DNA evidence
• Chapter 4 Criminal DNA databases
• Chapter 5 The pitfalls of DNA testing
• Chapter 6 DNA testing errors
• Chapter 7 DNA evidence interpretation errors
• Chapter 8 DNA evidence during trial
• Chapter 9 Challenging DNA evidence in the courtroom
• Chapter 10 Post-convictional DNA testing
• Chapter 11 Ethical aspects of DNA testing
• References
• Glossary