Scientific Experts

• Author: Eric V. Gottardi/Jennifer A. MacLellan/Michael Lacy/Robin Flumerfelt
• Pages: 179-220

179

8

Scientific

Experts

I. Introduction ............................................. 180

II. DNA Evidence ........................................... 180

A. Overview .......................................... 180

B. The Science Behind the Practice ....................... 181

C. Leading DNA Evidence at Trial ......................... 185

D. Defence Considerations .............................. 188

E. Crown Considerations ............................... 189

F. Some Trial Issues ................................... 189

G. Conclusion ......................................... 193

H. Checklist .......................................... 194

III. Toxicology ............................................... 194

A. Overview .......................................... 194

B. The Science Behind the Practice ....................... 195

C. What the Expert Does ................................ 198

D. Trial Issues ......................................... 199

E. Conclusion ......................................... 203

IV. Latent-Print Identification .................................. 203

A. Overview .......................................... 203

B. The Science Behind the Practice ....................... 203

C. The Latent-Print Identification Process .................. 208

D. Other Potential Issues at Trial ......................... 210

E. Checklist .......................................... 214

V. Fire Investigations ........................................ 214

A. Overview .......................................... 214

B. The Science Behind the Practice ....................... 215

C. The Fire Investigation Process ......................... 216

D. Other Potential Issues at Trial ......................... 218

E. Conclusion ......................................... 220

© [2022] Emond Montgomery Publications. All Rights Reserved.

180 Part II Specific Types of Expert Evidence

I. Introduction

Whether you are having to look for weaknesses in a seemingly ironclad piece of DNA

evidence at a crime scene or calling a toxicologist to refute an accused person’s claims

of extreme intoxication, there is no doubt that in a long legal career it is quite likely that

you will find yourself having to come to grips with some of the evidence that scientific

experts can offer. Given the breadth and scope of each topic covered in this chapter,

we can only offer you a starting point—a survey of some of the legal hallmarks of this

evidence and a brief discussion of a few of the key issues that are commonly litigated at

trial. Counsel should keep in mind that forensic evidence is always developing. Courts

will expect counsel to turn their mind to all possible ways to collect forensic evidence

at the trial stage of proceedings.1

This chapter will focus on experts who purport to give opinions on topics that are

based in science: (1) DNA evidence, (2) toxicology evidence, (3) fingerprint evidence,

and (4) arson-based fire investigation evidence.

II. DNA Evidence

A. Overview

Forensic DNA evidence is a well-recognized subject of expert testimony that is often

described as the “gold standard” of forensic identification. Since its inception over

30 years ago, DNA evidence has been put to myriad uses: from proving an accused’s

identity, to undermining a complainant’s credibility, to exonerating the wrongfully

convicted. In some cases it may simply indicate that a person was in a certain location

at some point in time, but in many cases it can support a powerful inference that an

accused is the perpetrator.

While the practice of forensic DNA analysis is well established, it is complex and

ever evolving. Counsel must always remain vigilant to ensure that the methods used

and conclusions drawn from forensic DNA analysis are properly tested and contex-

tualized within the case as a whole. This, however, can seem a daunting task to the

busy practitioner. The issues that may be raised in the forensic DNA context are wide

in scope. This subsection provides a primer to counsel who are confronted with the

prospect of leading or challenging DNA evidence. While this chapter provides a start-

ing point, counsel should consult further resources—and, ideally, their own expert—

to better appreciate the issues raised that are particular to their case. The footnotes in

this chapter provide excellent secondary sources on DNA evidence.

1 See generally R v Hay, 2013 SCC 61.

© [2022] Emond Montgomery Publications. All Rights Reserved.

Chapter 8 Scientific Experts 181

B. The Science Behind the Practice

1. What Is DNA?

Deoxyribonucleic acid, more commonly known as DNA, is the hereditary material

in every living organism. It is a “genetic blueprint” that determines an individual’s

attributes and allows parents to pass on genetic information to their offspring.

DNA is composed of long sequences of chemicals called nucleotides. There are

four types of nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C).

These sequences form each person’s unique genetic code.

DNA is found in the nucleus of the cell, where it is packaged in structures called

chromosomes. Any cell from a given individual—from white blood cells to sperm cells

to skin cells—contains the same DNA pattern, which is unique to that individual.2

In recent decades, humans have developed techniques to examine the differences

in DNA structure between individuals. These scientific techniques form the basis of

forensic DNA analysis, which is the focus of this subsection.

2. Overview of Forensic DNA Analysis

Forensic DNA analysis is an identification tool that allows scientists to draw com-

parisons between two DNA samples. As Dr John Butler explained in Fundamentals of

Forensic DNA Typing:

[A] DNA profile by itself is fairly useless because it has no context. DNA analysis always

requires that a comparison be made between two samples: (1) a questioned sample, com-

monly referred to as “Q,” and (2) a known sample, referred to as “K.”3

The “known sample” is typically taken from the crime scene under investigation,

while the “questioned sample” is connected to a suspect. The purpose of forensic DNA

analysis is to determine the relationship between the two samples.

There are two steps to forensic DNA analysis: match identification and expressing

the statistical significance of a match.

a. Match Identification

The Ontario Court of Appeal has described the process of match identification in the

following terms:

The first use of DNA evidence is as evidence that the suspect’s DNA “matches” the

DNA found in blood, semen or tissue recovered at a crime scene. In this way, the DNA

2 John Walsh, “Forensic DNA Typing Evidence” in Kenneth Matthews et al, eds, The Expert: A

Practitioner’s Guide (Toronto: Carswell: 1995) ch 12 at 12-2.

3 John M Butler, Fundamentals of Forensic DNA Typing (Burlington, Mass: Academic Press, 2009)

at 9.

© [2022] Emond Montgomery Publications. All Rights Reserved.