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4.14.2 - DNA Evidence

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What is DNA Evidence?

  1. "DNA evidence" refers to a type of expert evidence in which the DNA of a sample found at a crime scene (a "forensic sample") is compared with a DNA sample provided by the accused (a "reference sample").
  2. As DNA evidence is a type of expert evidence, the principles outlined in General Principles of Opinion Evidence apply (subject to any modifications noted below).
  3. DNA evidence examines and compares the DNA contained in chromosomes. DNA is consistent throughout a human body (R v Noll [1999] 3 VR 704; R v Jarrett (1994) 62 SASR 443).
  4. There are two ways in which DNA can be examined and compared:

    What does a DNA "Match" Establish?

  5. The DNA strands in chromosomes are held together by base pairs. There are approximately three billion base pairs, and about three million of those vary between individuals (R v Noll [1999] 3 VR 704; R v Jarrett (1994) 62 SASR 443).
  6. Due to the uniqueness of nuclear DNA, if all base pairs of two nuclear DNA samples were tested and matched, it would be possible to say that the samples came from the same person (or an identical twin).
  7. However, DNA testing does not measure and compare every single base pair of the relevant samples. It only measures and compares the length of certain strings of base pairs at known positions called "loci" (R v GK (2001) 53 NSWLR 317; R v Pantoja (1996) 88 A Crim R 554).
  8. This means that, even where there is a "match" between two samples, it is not possible to say with certainty that they came from the same person. One of the samples could have come from a different person who happens to have the same string length of base pairs at the loci that were tested (R v GK (2001) 53 NSWLR 317; R v Pantoja (1996) 88 A Crim R 554).
  9. Thus, a "match" between the accused’s DNA and a sample taken from the crime scene only establishes that the accused could be responsible for the forensic sample. That is, the evidence does not exclude the accused from responsibility (R v GK (2001) 53 NSWLR 317; R v Pantoja (1996) 88 A Crim R 554).
  10. For DNA evidence to have any further probative force, evidence must also be given by a suitably qualified expert about the probability of a match with a random member of the population (R v Noll [1999] 3 VR 704; R v Rye [2007] VSCA 247).
  11. Expert witnesses may state their conclusions about the probability of a match with a random member of the population in the following, equally mathematically correct, forms:
  12. As all of these values express the same information there is no reason to prefer one method of expression over another (provided the numbers are accurately calculated). Experts should express their conclusions in a way that the jury can readily comprehend (Aytugrul v R [2010] NSWCCA 272 per Simpson and Fullerton JJ (McClelland CJ at CL contra). See also R v GK (2001) 53 NSWLR 317; R v Doheny & Adams [1997] 1 Cr App R 369).
  13. To avoid the risk that the jury may be overwhelmed by exceptionally high probabilities or likelihood ratios, witnesses may be permitted to present the statistical evidence concerning the probability of a match qualitatively rather than quantitatively, by describing the probability of a match with the accused as "strong", "very strong" or "extremely strong" (see Forbes v R (2009) 167 ACTR 1).
  14. To reach these conclusions, experts rely on the frequency with which certain DNA string lengths occur in the general population (R v GK (2001) 53 NSWLR 317; R v Berry & Wenitong (2007) 17 VR 153).
  15. These frequencies are recorded in databases, which have been built up using previous genetic testing. A database (and the consequent results obtained from that database) will only be reliable if it contains a representative sample of the general community or, if the offender is from a specific ethnic group, the relevant ethnic community (R v Noll [1999] 3 VR 704; R v Pantoja (1996) 88 A Crim R 554).

    Samples from More than One Person

  16. Where a forensic sample contains DNA material from several people, an expert witness may give evidence on the probability of it matching a specified group of people (rather than having come from a group of randomly selected people) (R v GK (2001) 53 NSWLR 317; R v Berry & Wenitong (2007) 17 VR 153).[3]
  17. The expert may also give evidence on the likelihood of the DNA having come from one specified set of contributors (e.g., those alleged by the prosecution) rather than another (e.g., those alleged by the defence). This will allow the jury to assess the relative probability of the prosecution and defence hypotheses (R v Berry & Wenitong (2007) 17 VR 153).

    What does a Failure of DNA Samples to "Match" Establish?

  18. If accepted, evidence that the accused’s DNA sample does not match a sample taken from the crime scene establishes that the accused is not responsible for the forensic sample (R v GK (2001) 53 NSWLR 317; R v Pantoja (1996) 88 A Crim R 554).

    Standard of Proof

  19. Under the Jury Directions Act 2015, the only matters that must be proved beyond reasonable doubt are the elements and the absence of any relevant defences (Jury Directions Act 2015 s61. See also Payne v R [2015] VSCA 291 at [13]).
  20. However, in some cases, DNA evidence may be substantially the only evidence of one or more elements. In such cases, it may be appropriate for the judge to clearly identify for the jury the importance of the evidence. Judges should discuss the issue with counsel and hear submissions on what additional directions or comments are appropriate. Options include:

    Jury Directions

  21. DNA evidence is not a class of evidence that calls for special directions in every case (R v Berry & Wenitong (2007) 17 VR 153; R v Karger (2002) 83 SASR 135).
  22. Where DNA evidence is given, the judge must:
  23. Judges have traditionally accomplished these goals by giving a careful summary and explaining the limitations of the evidence (R v Berry & Wenitong (2007) 17 VR 153; R v Karger (2002) 83 SASR 135).
  24. Following the Jury Directions Act 2015, a judge is not required to summarise the evidence, but must identify so much of the evidence the judge considers necessary to assist the jury to determine the issues in the trial (Jury Directions Act 2015 ss65, 66).
  25. The need for directions on DNA evidence will depend on whether any directions are sought or whether there are substantial and compelling reasons for giving a direction in the absence of any request (Jury Directions Act 2015 ss14 - 16). See Directions Under Jury Directions Act 2015 for information on when directions are required. See also Judge’s Summing Up on Issues and Evidence.
  26. When directing the jury about DNA evidence, the following issues may need to be addressed if they are in dispute:
  27. The judge may also need to address any fallacies put forward by the parties.

    Is there a Match?

  28. The judge should tell the jury that it is a question of fact for them whether there is a match between the forensic sample and the reference sample (R v Pantoja (1996) 88 A Crim R 554; R v Karger (2002) 83 SASR 135).
  29. This will usually be determined by the jury using the evidence given by the expert witnesses (R v Pantoja (1996) 88 A Crim R 554; R v Karger (2002) 83 SASR 135).
  30. Where there is evidence that suggests that the reference sample does not match the forensic sample, and there is a reasonable possibility that that evidence is correct, the jury must find that the accused was not responsible for the forensic sample (R v Pantoja (1996) 88 A Crim R 554).
  31. In such circumstances, the jury should be directed to find that the accused was not responsible for the forensic sample, even if other tests have indicated that the reference sample and the forensic sample matched. The jury may only rely on a match if the reference sample matched the forensic sample at all loci that were tested (R v Pantoja (1996) 88 A Crim R 554).

    Was the Testing Accurate and Reliable?

  32. The jury should consider whether the testing was accurate and reliable (R v Pantoja (1996) 88 A Crim R 554; R v Karger (2002) 83 SASR 135).
  33. In some cases, this may require the jury to consider whether the evidence may have been affected by laboratory error (R v Karger (2002) 83 SASR 135).
  34. The jury should also consider any limitations in the database used by the experts to assess the probability of the sample coming from someone other than the accused (R v Pantoja (1996) 88 A Crim R 554).
  35. This may be particularly relevant where there is evidence that the offender is a member of a particular ethnic group that may have different genetic characteristics from the general community. The jury should be directed to consider these limitations and their effect on the statistical evidence (R v Pantoja (1996) 88 A Crim R 554).[4]

    What Are the Consequences of the Jury’s Findings?

  36. The judge should generally explain to the jury that evidence that the accused’s DNA sample matches a sample taken from the crime scene is not direct evidence that the accused committed the offence. It is only circumstantial evidence, and must be considered in light of the other evidence in the case (R v Karger (2002) 83 SASR 135; R v Vivona Vic CCA 12/9/94).
  37. Precisely what the jury should be told about the use they can make of this circumstantial evidence will depend on the content of the evidence:

    Warning Against Misuse of Evidence

  38. Expert evidence on the likelihood of a match with a random member of the population is apt to mislead the jury. In particular, it creates a risk that the jury will adopt the following erroneous reasoning (known as the "prosecutor’s fallacy"):
  39. This line of reasoning fails to recognise that even though only one person in a million has a DNA profile that matches the forensic sample, in a country the size of Australia (with over 21 million people), it is statistically likely that the DNA of at least 20 other people will also match that sample (R v GK (2001) 53 NSWLR 317; R v Doheny & Adams [1997] 1 Cr App R 369).
  40. It is not necessary to warn against the prosecutor’s fallacy in all cases. The judge should consider the need for a direction based on the following factors:
  41. If a direction is necessary, the judge should generally warn the jury against engaging in the prosecutor’s fallacy when the DNA evidence is led, and again in the final directions (R v Doheny & Adams [1997] 1 Cr App R 369; R v Karger (2002) 83 SASR 135; R v GK (2001) 53 NSWLR 317; Latcha v R (1998) 127 NTR 1).
  42. The judge may also warn the jury not to disregard the strength of the probability of a match. While DNA evidence is not capable of proving to a scientific certainty that the accused is guilty, it may be strong circumstantial evidence of guilt (R v GK (2001) 53 NSWLR 317).
  43. Where the probability of a match is sufficiently high, the jury should consider the fact that there is no evidence connecting any other (random) people with a matching DNA profile to the offence (R v GK (2001) 53 NSWLR 317; R v Doheny & Adams [1997] 1 Cr App R 369).

     

    Notes

[1] See further General Principles of Opinion Evidence.

[2] There are currently estimated to be 24,641,662 people in Australia.

[3] The expert uses population statistics to determine the probability of each string length that is measured. Provided each string length is independent, these probabilities are multiplied together to determine the probability of all string lengths being detected in a random sample.

[4] Care must be taken when addressing this issue to ensure that the focus is on the genetic characteristics of the offender’s ethnic group, rather than on the genetic characteristics of the accused’s ethnic group. See R v Pantoja (1996) 88 A Crim R 554.

Last updated: 9 March 2017

In This Section

4.14.2.1 - Charge: DNA Evidence

See Also

4.14 - Opinion Evidence

4.14.1 - General Principles of Opinion Evidence

4.14.3 - Fingerprint Evidence

4.14.4 - Handwriting Evidence