Danielle Desmarais, Ph.D., Scientific Director, PRO-ADN Diagnostic inc.
Lambert Busque, M.D., FRCPC, Associate Professor of Medicine, Université de Montréal, and
President of PRO-ADN Diagnostic inc. www.proadn.com
INTRODUCTION
DNA testing has revolutionized the science of human identification. Using DNA technology, it is now possible to identify and differentiate virtually all the people who live on our planet. Yet its application in the field of criminal law raises a number of questions: Is the technique infallible? Does it have potential for error? What are its limitations?
1 DNA
1.1 Its Structure
Deoxyribonucleic acid (DNA) is the molecule that contains all our genetic information. It is made up of two complementary strands that are coiled together in a helix.
DNA comprises four different sub-units (bases) called Adenine (A), Guanine (G), Thymine
(T) and Cytosine (C), which pair together to form a very long chain. The DNA molecule is immense, containing 9 billion base pairs divided among 23 pairs of chromosomes that are passed on to us by each of our parents, with each parent giving 23 chromosomes. The random order in which the four bases appear determines all the physical traits and information required for the
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human body to function properly. Much like encyclopedias, our genetic code is recorded letter by letter, with its words, spaces, sentences, punctuation and information all categorized and organized.
1.2 The Characteristics of DNA
The DNA is identical in all the cells of our body, but unique to each individual (with the exception of identical twins, who have exactly the same genetic profile). To date, only 10% of the human genome code is known for the genes that are essential to maintaining life. The remainder of the human genome has not yet been coded and varies greatly from one person to the other.
DNA polymorphism is attributable in part to the presence of tandemly repeated sequences at specific sites (loci) on the genome. The number of repeats varies from one person to the next, causing variations in size known as alleles. For example, in human beings, one of these sites is made up of the AGAT sequence repeated between 7 and 15 times, depending on the person. Since each specific locus is present on both of the chromosomes in a given pair (except for the pair of XY sex chromosomes), each person has one allele inherited from the maternal chromosome and one from the paternal chromosome. As a result, even if multiple alleles are possible for a given site, each individual can only have two of these alleles. In heterozygous individuals (Aa), two different alleles are present, whereas homozygous individuals (AA and aa) have inherited two similar alleles.
Hence, by analyzing several sites, it is possible to draw up a virtually unique genetic profile that is much like the bar codes found on consumer products.
Apart from its great number of variations, the intrinsic characteristics of the DNA molecule offer numerous advantages in the field of human identification testing. Since the DNA present in all the nucleated cells in a given human being is identical, a wide variety of sources exist from which
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samples can be taken, ranging from blood, saliva, sperm, and vaginal secretions to hair, urine, skin, teeth, etc. Moreover, the DNA molecule is relatively stable and resistant to environmental elements, so that dried or frozen samples can be analyzed just as well as fresh samples.
2 DNA ANALYSIS
Analysis of DNA polymorphisms has pushed human identification testing to previously unimagined limits. Since Sir Alec Jeffreys first introduced the analysis of DNA polymorphisms in
1985
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, enormous strides have been made in human identification through DNA testing. This discipline is now a bona fide science that allows the match between two biological samples to be established with certainty.
2.1 Genetic Markers
While a vast number of polymorphic sites exist (genetic markers) throughout the entire human genome, only some 30 genetic markers have been thoroughly validated and used in reputable laboratories, both public and private, around the world. This standardization represents the culmination of many years of research and allows different laboratories to participate in international validation studies, compare their results and exchange data from their population studies.
Population studies are one of the most important aspects of genetic identification because they provide a means of collecting data on the frequency of each allele in different populations
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A.J. Jeffreys, V. Wilson and S.L. Thein (1985), "Hypervariable Minisatellite Regions in Human DNA," Nature ,
314:67-73.
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around the world. This step is a prerequisite to establishing both the rarity of a genetic profile and meaningful results. In fact, it is essential that we know the frequency of each allele in the target population in order to determine the probability of finding another individual with the same alleles.
2.2. Technologies Used for DNA Analysis
Genetic profiles are mainly constructed using two techniques: the so-called RFLP
(restriction fragment length polymorphisms) technique and the DNA amplification technique or
PCR (polymerase chain reaction). a) The RFLP Technique
The RFLP technique consists of cutting the DNA into pieces using a restriction enzyme on each side of the polymorphic region, leaving the region itself intact. The DNA fragments are then separated according to size through electrophoresis on a gel: the small DNA fragments migrate more quickly through the gel than the longer fragments. Once the migration pattern has been determined, the DNA fragments are then transferred through capillary attraction onto a membrane.
A probe (marked DNA fragment) that is capable of specifically recognizing the polymorphism under study is then used to visualize the complementary DNA fragments. The size of the repeat sequence can thus be determined according to its position on the membrane. The use of a single probe does not allow for the exclusion or positive identification of any particular individual. The final result is obtained by using four or five different probes, which gives cumulative discriminating power.
While highly efficient and reliable, this technique often reaches an impasse when the
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quantity of DNA available for analysis is very limited, or when the DNA is too severely damaged.
The PCR technique can offset these physical limitations and at the same time, provide faster human identification. b) The PCR Technique
The PCR technique, invented in 1988 by Dr Karry B. Mullis
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, makes it possible to replicate thousands of copies of a DNA sequence in vitro . The importance of this technology was underscored by the awarding of the Nobel Prize in Chemistry to its inventor in 1993.
Unlike the RFLP technique, which requires large quantities of DNA, the PCR technique only requires very minute quantities of DNA. DNA amplification is achieved by using two specific primers flanking both sides of the repeat sequence. The polymerase chain reaction is induced in three steps using a thermal cycler: denaturation, primer annealing and the synthesis of a new DNA strand. The new copies of the polymorphic region are produced by extending the primers using a heat-resistant enzyme. Since the amplification occurs exponentially, 1,000 copies can be created after some 30 cycles. The DNA fragments are first separated according to size, then visualized using different methods that make it possible to identify the DNA polymorphisms.
One of the major advantages of the PCR technique is the minimal quantities of DNA it requires: a few cells are all it takes. It is therefore possible to establish a genetic profile from a single hair or a few cells of buccal mucosa (e.g. on a cigarette butt, stamp, or envelope flap). A genetic profile can also be established from biological samples that have been damaged by solar
UV radiation or by microorganisms, since the DNA polymorphisms used in the PCR technique are
2
R.K. Saiki, D.H. Gelfand, S. Stoffel, S.J. Scharf, R. Higuchi, G.T. Horn and K.B. Mullis (1988), "Primer-Directed
Enzymatic Amplification of DNA with a Thermostable DNA Polymerase," Science, 239:487-491.
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small in size (100 to 350 bases). However, given their small size, the genetic sites used for this technique are less polymorphic than those used in the RFLP technique. This means that more loci must be analyzed in order to obtain a significant probability.
2.3. The Uses of Genetic Profiles
Genetic analyses are on their way to becoming the ideal tool for resolving questions of identity, especially in the areas of family law and criminal law. a) Family Law
DNA polymorphism analysis means that biological kinship can be retraced, since each individual inherits an allele from each of his or her parents for all the loci dispersed throughout the genome. In paternity suits, the maternal genetic traits present in the child can be determined using
DNA analysis and the presence of the remaining traits in the presumed father can be verified. In the field of family law, DNA analysis has improved the quality and accuracy of the conclusions that were previously obtained through blood typing (ABO) and through analysis of human leukocyte antigens (HLA)
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. It is now possible to identify or exclude men as fathers with such great accuracy that the conclusion is deemed to be certain. b) Criminal Law
Nowadays, recourse to DNA testing in criminal cases is commonplace, even though DNA was first used for identification purposes only ten years ago. The past decade has been witness to a
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For more information on this subject, see B.M. Knoppers, M.-A. Grimaud, L. Busque and D. Desmarais, "L'apport des technologies de l'ADN en droit familial : Aspects techniques et juridiques" in the Recueil de textes , 1977 Annual
Convention of the Barreau du Québec, pp. 329-346.
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burgeoning of activity in this field. For example, in 1995, Canada's Department of Justice amended the Criminal Code by Bill C-104 (Chapter 27), thereby authorizing police to oblige individuals who are considered serious suspects in a crime to provide a biological sample. It has now been proposed that a national database of genetic information be established for use in identifying the suspected perpetrators of crimes.
For each piece of biological evidence, DNA polymorphisms make it possible to establish a true genetic fingerprint that mirrors the specific arrangement of the different alleles. A specific
DNA profile can be linked to a particular person with certainty if all the DNA fragments can be perfectly superimposed. It therefore appears that the probability of finding two people with the same genetic profile depends on the number of loci analyzed and the frequency of the alleles found.
3. GENETIC IDENTIFICATION IN THE LEGAL SYSTEM
In the context of a lawsuit, DNA testing may be the determining factor for proving a suspect's innocence or guilt. Given the evident facility of DNA testing and the impact it is likely to have, it is imperative that questions be raised about the fallibility factors. DNA analyses performed for the purpose of determining paternity are not subject to the constraints inherent in criminal contexts, such as insufficient sample sizes or their degree of purity. Hence the controversy that surrounds the validity of DNA testing in criminal cases is essentially non-existent in family law cases.
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3.1. From DNA Tests to Second Assessments
DNA tests make an irrefutable contribution to the quest for truth, and constitute a particularly valuable means of solving serious crimes. Analyzing DNA polymorphisms is an extremely powerful and reliable tool. Yet the methods of analysis remain fallible, not to mention hard for non-scientists to understand, and it is sometimes difficult for second assessments to be made when the sample found at the crime scene is small. In an effort to prevent error, the DNA test must therefore be conducted in such a manner as to protect the rights of the defence.
Technically speaking, three requirements must be met in order to respect the rights of the defence: 1) reserve part of the biological specimen and/or its DNA before proceeding with the analyses per se, 2) ensure rigorous monitoring of all procedures noted in laboratory records, and 3) choose the most appropriate methods and technology so as not to compromise the validity of the
DNA test.
The fact that different types of laboratories exist — some under government auspices, others in the private sector — provides at least some guarantee of the accused person's right to make a full answer and defence to the charges. In Canada, most laboratories have adopted a code of ethics, inspired by recommendations put forward by American scientific associations, to ensure quality service and establish their credibility and repeatability
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. The existence of several qualified laboratories also promotes the principle of having a free choice of experts.
3.2.
Admissibility Standards
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In the United States, a number of organizations have established guidelines for laboratories performing DNA tests in order to ensure the quality of the results: the American Association of Blood Banks (AABB), the Technical Working
Group on DNA Analysis and Methods (TWGDAM), and the National Research Council (NRC). The scientists at PRO-
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Regarding the admissibility of DNA evidence, the theory of "general acceptance" by the scientific community ( Frye )
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was the first to be put forward in the U.S. courts, in 1988. The standard of
"general acceptance" has now given way to that of "reasonable reliability."
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RFLP and PCR technologies are highly reliable, but it is important to be aware that several methods of analysis exist, each with its own code of interpretation, specific statistical approach and distinct probative force.
The validation and international recognition of genetic markers are important reliability criteria for DNA tests. Moreover, since the genetic identification of a person translates into a probability rate based on the frequency of appearance of each allele in the target population, it is essential to have exhaustive databases to ensure that the probabilities are not subject to erroneous evaluation.
3.3. The Types of Results
Regardless of the complexity of the issue, DNA testing still involves determining whether or not similarities exist between two or more pieces of biological evidence. Table I summarizes the different types of results obtained, their possible interpretations and their probative force. a) Non-Matching Results
When the genetic profiles of the reference samples and of samples taken from the crime scene differ, a conclusion of exclusion is reached. For example, genetic analysis was the
ADN Diagnostic inc. are diligent in following these guidelines.
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Frye v. United States , 293 F. 1013 (1923): This was a case in which the lie detector test was used as a means of investigation and the theory of general acceptance was adopted as an admissibility standard.
6 R. v. Béland , (1987), 2, R.C.S., 398.
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determining factor in the acquittal of Guy-Paul Morin, who was unjustly convicted of murder
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.
More recently, David Milgaard, who spent 23 years of his life in prison for the murder of a young
Saskatoon nurse, was also found to have been innocent through DNA testing. b) Matching Results
When the samples present an identical genetic profile, the possibility of a probable match must be considered before concluding that there is a positive match. In most cases, it appears clear, given all the experimental controls in place, that the match is real and significant. However, the similarity between genetic profiles may be sheer coincidence owing to an insufficient number of genetic markers being analyzed. Some alleles are extremely common, while others are rare, making the product of the frequencies of a certain number of alleles (from 8 to 12 loci) necessary in order to affirm with virtual certainty that the match between a sample taken from a crime scene and that taken from a suspect is not simply a matter of chance. For example, a close relative of the criminal could be unjustly accused of wrongdoing: it must be borne in mind that there is a 25% probability of brothers and sisters having the two same alleles at a specific locus. The similarity may also be the result of a technical loophole in terms of the sample, the DNA preparation, the analysis, or even the interpretation of the results.
It is therefore important to verify that the match between the genetic profiles is not the result of these last two factors. In this regard, the services of a qualified expert would be useful to as he or she could envisage the possible loopholes and take concrete measures to confirm or quash any such doubts concerning the validity of the results.
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R. v. Morin , (1995) 37 C.R. (4th) 395 (C.A.O.).
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c) Ambiguous Results
Each case has its own particular features, given that the sample under dispute varies both in terms of the type of specimen, and the quality and quantity of the DNA. Some samples come from ambiguous sources owing to the degradation of the DNA, the presence of inhibitors (dye, oil, etc.), contamination, or the presence of multiple contributors (e.g. as in the case of gang rapes). In some cases, the DNA profile observed may prove to be an artifact (of weak intensity or non-replicable genetic profile). In other cases, it may be impossible to obtain a result. Under these circumstances, the role of the expert is to verify whether all steps (from specimen preparation to analysis) have been carried out in accordance with standard operating procedures.
3.4 Potential Sources of Errors and Quality Assurance
When DNA tests are performed under optimal conditions, they allow for highly reliable and accurate identification. However, as with all sophisticated techniques requiring long and complex manipulations, both human and technical errors are possible. It is important to ensure that preventive standards, rigorous protocols and quality control procedures are followed by all those involved, whether at the level of specimen collection, DNA preparation, the actual analysis or the interpretation of the results. Table II provides a summary of the possible sources of error. a) Sampling
A clear and thoroughly documented chain of custody must be maintained, commencing with the sample collection stage. The samples must be handled, analyzed and conserved in such a way as to protect them against loss, harmful changes and the risks of contamination. Due to the very
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high sensitivity of the PCR technique, it is sensitive to contamination by sources external to the
DNA, such as the DNA of the person handling the sample (police officer and laboratory technician), or the exchange of genetic material between different samples. Thus, each piece of biological evidence or DNA sample must be subdivided in order to conserve an unhandled portion, to allow for second assessments should they prove necessary. b) Analysis Procedure
All the steps in the analysis procedure must be rigorously controlled and documented.
Numerous quality control systems must be implemented in order to detect any technical problems
(equipment, reactives), contamination problems (experimental controls), and problems pertaining to the quality of the DNA. All these measures make it possible to ensure the accuracy of the results.
Moreover, the qualified expert must be able to make the necessary choices in terms of the techniques (RFLP or PCR) and work methods to be used in order not to destroy the evidence and to maximize the chances of success. c) Interpretation of Results
Interpreting the results is an important step that requires the utmost scientific rigour and a certain amount of experience. The results must be interpreted taking into account the technological and scientific limitations, as well as the quality controls. Whether unconsciously or consciously, the interpretation given to the results may be biased, because of artifacts (sub-optimal quantity or quality of the DNA), because of a false match caused by the presence of multiple contributors in the sample, or because the selection of results is coloured by the desire to avoid a non-match or non-
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incriminating result or certain pieces of biological evidence are omitted. It is therefore important to guard against a lack of objectivity because premature conclusions could have very serious consequences. d) Calculation of Probabilities
Probability calculations are based on complex genetic concepts that must be thoroughly mastered by the expert. The calculations vary depending on whether the genetic profile, at a specific locus, is heterozygous (Aa), homozygous (AA, aa) or mixed (multiple contributors). To prevent an erroneous calculation of the probability of a positive match between two genetic profiles, extreme caution must also be exercised, especially when samples from multiple contributors are involved.
Furthermore, statistical analysis will only be significant if pertinent databases are used.
CONCLUSION
The use of DNA polymorphisms for purposes of human identification testing is the most effective form of evidence offered by the field of medico-legal science. The accuracy and reliability of DNA testing make it one of the best tools for ascertaining the truth. However, DNA tests are complex and technically specialized, often disconcerting non-specialists. Although they are rare, errors can be made at several stages of DNA testing. DNA tests must therefore be analyzed with a highly critical eye in order to identify potential loopholes and confirm or disprove the validity of the results.
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