Legal Research > Papers by Public Defenders

DNA Evidence Considered


This paper, by Brian Hancock, Public Defender, was delivered to a group of barristers for a CPD program in February 2009.

Note: There are a number of papers by Andrew Haesler SC and Peter Zahra SC on the Public Defenders' website, which are acknowledged as sources for this short paper.
I have also relied on Linzi Wilson-Wilde, 'DNA Profiling in Criminal Investigations'

A. What is DNA?

1. The human body  is made up of different types of tissue; for example: bone, blood, muscle and skin. Each type of tissue is made up of a collection cells. The cell is made up of a nucleus, containing the deoxyribonucleic acid (DNA), surrounded by the cytoplasm, enclosed in the cell membrane. The cytoplasm contains structures, one of which is the mitochondria, responsible for supplying energy to the cell.

2. Not all cells in the body have a nucleus: red blood cells don't, however white blood cells do, and it is from these cells that a DNA profile can be derived when analysing blood.


3. The DNA molecule is often referred to as the 'blueprint for life'. It carries all the genetic information of an organism. The sequence of the genome is different for all organisms and also between individuals of the same type of organism.

4. The DNA double helix is wound around proteins, called histones, which are tightly packed into structures called chromosomes. Chromosomes are located in the nucleus of a cell. Humans have 46 chromosomes: 22 pairs and two sex chromosomes.

5. Watson and Crick discovered DNA's molecular structure in 1953; it consists of two complementary strands which bind together and twist to form a double helix, like a twisted ladder, with two strands as backbones - the phosphate-sugar or deoxyribose - and numerous rungs joining the two strands - two of four bases: adenine, thymine, guanine and cytosine. Adenine always binds to thymine and guanine always binds to cytosine.

6. The base pairs - rungs - are repeated along the strands in a strict order, creating a sequence for each fragment, rather like a repeated sentence; some sequences are long - dozens of nucleotides in length - and some are short - one to six nucleotides in length. There are approximately three billion nucleotide base pairs in the human genome.

7. Short Tandem Repeats (STR) are repetitive DNA whose core sequence varies from one to six nucleotides in length in the population. Different people have different numbers of repeat core units and therefore will have a different length of DNA. STR sites on the DNA are given a designation to represent the site.

8. The most commonly analysed STR sites in forensic science are the tetranucleotide repeats whose core-repeat sequence consists of four base pairs. The more sites analysed, the greater the discriminating power of the analysis.

B. What is an evidentiary sample?

9. Tissue and cells, deposited on a surface or object, can be collected, broken open and the DNA extracted, measured, amplified, analysed, interpreted, and later used as evidence.

10. Biological material containing DNA, deposited during the course of a crime, may be collected as an evidentiary sample from a number of locations. At a crime scene, a collection swab might be applied to a surface such as a wall, window, door, floor or bench-top. The interior of a suspect car, especially the door handles, gear stick and steering wheel might be swabbed. Objects found at or near a crime scene, such as clothes, mobile phones, bottles, letters, cigarette butts and toothbrushes, might be collected for later analysis. Doctors who conduct medical examinations of complainants in sexual assault cases routinely obtain internal and external swabs of relevant parts of the complainant's body as part of the established S.A.I.K. procedures.

11. Contact DNA is obtained from an individual coming into contact with an object or surface. Some individuals - shedders - have a greater propensity to shed a greater number of cells. Friction, as opposed to just touching, can aid in transferring a greater number of cells.

12. Samples with a very small number of cells present are sufficient to develop a profile, as long as there are a sufficient number of cells containing DNA of a high quality.

13. The biological materials which are possible sources of DNA include:

a) Blood - white blood cells;
b) Semen - spermatozoa, or skin cells shed during ejaculation;
c) Vaginal secretions - cells shed from the vaginal wall;
d) Saliva - cheek (buccal) cells from inside the mouth; saliva may be found on drinking vessels, cigarette butts, masks, stamps, envelopes and food;
e) Hair - cells found attached to the root of a plucked hair or adhering to the shaft of a hair;
f) Sweat/skin - skin cells that had been shed; they may be found on clothing, person items or handled items; also known as trace DNA or contact DNA;
g) Bone - cells are found in the marrow;
h) Tissue - cells;
i) Teeth - cells in tooth pulp;
j) Urine/faeces/vomit - shed cells, often degraded by bacteria.

14. There are different rates of success of DNA analysis of evidentiary samples, depending on the source. For blood and semen, the approximate success rate is 90%; for saliva: on a cigarette butt, 70%; on a drink bottle, 40%; and on an alcohol bottle, 25%; for hair, 25%; for trace or contact samples: on clothing, 30%; on objects with extended contact (eg: mobile phones), 15%; on objects with limited contact (eg door handles), 10%; and on objects with single contact (eg: smudge), less than10%.

15. Handling errors in the chain of custody of the exhibit, of errors in the laboratory, may lead to contamination of the evidentiary sample, thereby negating its integrity as evidence. Mix-ups in samples should show up on the documents. Investigating police may have been careless in collecting the sample, or not secured it correctly, allowing contamination.

16. In cases involving DNA evidence, consideration ought be given to whether or not the circumstances call for an examination of the chain of custody of the exhibit. All documents supporting the initial collection of the evidentiary sample, the 'booking up' in the police exhibit book, the transfer to the DAL and its receipt there, and laboratory procedures can be checked.

17. DNA evidentiary samples are mobile, once obtained. In R v Ton [2002] NSWCCA 337, a malicious wounding in gaol, the suspect's sock turned up in the victim's cell. In Regina v Lisoff [1999] NSWCCA 364 a re-trial was ordered. Lisoff was subsequently acquitted; his defence was that the blood found on L's trousers had been planted; it contained transfusion products and thus arguably had been taken from the victim after the assault and after he went to hospital.

18. In some cases, issues of secondary transference may arise. We shed tens of thousands of skin cells each day. The material may be found in everyday items with which others may also come into contact: sharing hats, driving the same car. In some circumstances, it has been possible to find material containing the DNA of an individual to become removed from one item and deposited on another item, making a 'false' connection between the individual and the second item, with which the individual may never have come into contact. Taking a swab from a crime scene is an example of a deliberate transfer of material expected to contain DNA.


C. What is a Reference sample?

19. An evidentiary sample is of no immediate forensic use without a reference sample with which it may be compared. The surrounding evidence in the case will connect the evidentiary sample to the crime; a known individual, usually, but not necessarily, being the suspect in the offence, will be the source of the reference sample. The DNA profiles developed from each sample are compared. By comparing the DNA profile developed from the evidentiary sample with that of the reference sample, a suspect may be excluded or not.

20. Legislation in NSW, in particular the Crimes (Forensic Procedure) Act 2000, allows police to obtain a DNA sample from those suspected of a crime and from those convicted of serious offences. The Act allows police to request a suspect undergo a forensic procedure; it allows police to order a person under arrest to provide a non-intimate DNA sample. The Act also empowers a magistrate to make an order that a suspect provide a forensic sample (ss 24 and 25).

21. An application to a magistrate for an order for the taking of a sample requires a positive finding by the court the person is a suspect. In Orban v Bayliss [2004] NSWSC 428 Simpson J stated, at [31]: The conditions that must be met, before an order can be made, demonstrate that the purpose of the legislation is not to enable investigating police (or other authorised persons) to identify a person as a suspect; it is to facilitate the procurement of evidence against a person who already is a suspect.

22. In Maguire v Beaton (2005) 162 A Crim R 21, a Magistrate's order for the taking of the plaintiff's fingerprints and palm-prints was set aside by Latham J. because there was insufficient evidence to justify the making of the order pursuant to s 24.

23. In R v White [2005] NSWSC 60 the accused had pleaded not guilty to murder of Phyllis Jean O'Brien at Katoomba on 29 August 2003. The judge-alone trial proceeded before Studdert J. who gave a ruling upon the admissibility of DNA evidence obtained on the analysis of a cigarette butt discarded by the accused at the police station at Katoomba. The police had little evidence to connect the accused to the offence; but he was in the police station after being arrested on warrants. A smoker, police contrived to be in a position to collect his discarded cigarette butt in the police station. The issue of the admissibility of the evidence of analysis involves consideration of s 82 of the Crimes (Forensic Procedures) Act 2000 which renders inadmissible evidence which has been obtained in circumstances contrary to the Act other than as provided for in s 82(4) and (5).

24. In White Studdert J., in admitting the evidence, stated:

"Having weighed up the various matters to which s 82(5) directs attention, I am of the opinion that the desirability of admitting the evidence the subject of challenge here outweighs the undesirability of admitting such evidence even if it be assumed, contrary to the view I have formed, that the evidence was obtained in contravention of the Act. For the above reasons, I consider that the evidence the subject of the challenge should be admitted and I so rule.

I do not overlook the provisions of s 138 of the Evidence Act. Evidence obtained improperly is not to be admitted by reason of that provision 'unless the desirability of admitting the evidence outweighs the undesirability of admitting [it]'Section 138(3) invites consideration of matters of a like type to those matters to be considered under s 82(5). Mr Hanley submitted the demands of s 82(5) were greater than the demands of s 138(3). Whether that be so or not, it seems to me, again assuming the evidence was improperly obtained, that the discretion that would thereby be enlivened under s 138 should be exercised in favour of admitting the evidence."


25. DNA profiling has also been used in 'mass screening', where large numbers of volunteers are asked to provide a DNA sample for comparison to a specific unsolved crime. The circumstances where this is practical may be rare. Australia's most highly publicised mass screen was in Wee Waa commenced on Saturday 8 April 2000, in respect of a sexual assault that occurred in the town on 1 January 1999. A total of 495 DNA samples were collected. The offender who was later arrested was convicted of the offence.

D. What is a DNA Profile?

26. Cells, chromosomes and DNA molecules are not visible to the naked eye, a fact that precludes expert or non-expert visual comparisons available in other types of evidence, such as fingerprints, handwriting or photo identification. In order to make the necessary comparison of two (or more) DNA samples, profiles are developed of each. Essentially, the comparison is made of the profiles, not of the physical DNA.

27. A profile is developed in several stages from a small part of the DNA sample: firstly, extracting; secondly, measuring; thirdly, amplifying; fourthly, analysing; and fifthly, interpreting.

28. 'The DNA extraction process liberates the DNA from the cell and removes all other cellular material and fluid, leaving the purified DNA in solution. There are a number of methods of extracting DNA . . . The main function of the quantitation process is to provide the scientist with some approximate information regarding the concentration of the DNA in the sample in order to maximise the chance of obtaining a DNA profile . . .' - Wilson-Wilde.

29. Polymerase Chain Reaction (PCR) is a method of amplifying or copying the DNA in the laboratory to obtain sufficient quantities for analysis. DNA can be replicated exponentially, allowing results to be obtained from degraded samples, and from less quantities of DNA. PCR also allows DNA typing of old samples such as ancient bones.

30. During the analysis stage after further processing, the amplified fragments are sorted by length; each is given its allelic designation by comparison with the length of known alleles. The number of alleles at each site is counted; the number will differ from person to person.

31. In 1999 the forensic laboratories of Australia agreed to conform to 10 specific sites for DNA analysis. This was the first step in developing a national DNA database. The Profiler Plus kit in use in Australia amplifies nine designated loci (sites) and the sex-indicating marker Amelogenin.

32. Two resulting numbers are produced for each of nine sites, and with sex chromosome included as the tenth, they form the 'DNA profile'. The profile can't be strictly said to be unique in the population, because not all the sites on the DNA chain have been analysed; only a very small number.


E. What is a matched individual?

33. Normally, a statement (Certificate of Analysis) from the Department of Analytical Laboratories (DAL), Lidcombe, will be the method of the stating the match conclusions between the evidentiary sample profile and the reference sample profile. The analyst will make the comparison and deliver his or her expert opinion on whether or not a match is made between the studied profiles. Supporting documentation and tests undertaken (probably by others) to support his or her opinion are generally not provided without further request or subpoena.

34. Often more than one evidentiary sample relating to the alleged offence will have been analysed, for example clothing of a complainant and items from a SAIK kit.

35. The Certificate will include one or more of several conclusions: (a) a partial match; (b) no result; (c) the suspect is excluded; (d) the suspect is not excluded as a contributor; (e) the suspect is not excluded as a source; (f) the suspect has the same profile as the evidentiary sample.

36. If the opinion is (f), then either: (i) the two samples are from the same individual; or (ii) the samples originate from different individuals, and by chance happen to match (a random match). The science behind DNA matching does not exclude chance matching. However, the more DNA sites tested, the smaller the chance of a random match.

37. It is this characteristic of DNA evidence which, in the context of all the other evidence in the case, qualifies it as a powerful form of circumstantial evidence when a suspect's profile is matched to an evidentiary profile. Conversely, as an exclusionary tool, can be powerful evidence of innocence where a suspect is excluded as the source of the sample.

38. The likelihood of a random match is calculated using probability theory, databases of known profiles, and population genetics.

39. If I bought one ticket in a lottery in which one million tickets were sold, my chance of winning the first prize is one in a million. This is a low probability. Conversely, if I toss a coin, the chance of getting a head is one in two, a high probability. These probability calculations are precise because the total number of possibilities in known: one million, or two.

40. In New South Wales, databases of a small sample of DNA profiles are kept on computer. The sample forms a basis for establishing probability of certain profiles occurring in the general community. It is like opinion polling for political purposes: a sample of voters is questioned to estimate relative support for opposing candidates; not every voter is questioned for an opinion poll, but the election result is potentially predicted.

41. Where the profiles are the same, the analyst will include a sentence such as the following: This profile is expected to occur in fewer than 1 in 10 billion in the general population.

42. The expression '1 in 10 billion' expresses an extremely low probability. That is, the probability of another person having the same profile is extremely low; put another way, the probability of the reference sample and the evidentiary sample originating from the same individual is high. However, it is not correct to conclude that 'random match probability' is an expression of the probability of the guilt of the accused; such reasoning has been condemned as the 'prosecutor's fallacy'.

43. Where the offender is suspected to come from a genetically isolated population group, or the offender could be a close relative of the accused, some adjustment may be made to the probability ratio to take into account that a population with a smaller gene pool may produce individuals with a higher proportion of similar DNA profiles. In R v Bropho [2004] WADC 182, a judge-alone trial of an old rape allegation, a child had been born of the complainant, who was an unreliable witness. DNA paternity testing pointed to the accused. Once problems with the number of loci used, and the fact that close relatives were also suspects were taken into consideration, the judge concluded he could not use the DNA evidence as reliable corroboration.

44. In cases where the validity of the database supporting the low probability of the profile being a random match is challenged, it is the racial characteristics of the offender, not of the suspect, which dictates the validity of the database; see R v Pantoja (1996) 88 A Crim R 554, per Hunt J.

45. Some prosecutions result from a 'cold hit', where a profile on an offenders' database can be linked to an unsolved crime. In R v Stone [2004] NSWSC 224, investigators found nothing to connect the accused with the murder; there was no identification of the offender. On 10 July 1990 the body of the victim had been discovered by her partner in their residence. A statement was obtained from the owner of a service station located nearby, who encountered a man at about 11 am on the relevant date. This man claimed to have been in a fight and was seen in the vicinity. His face was substantially bloodstained. He was the accused. While serving another prison sentence the accused was required to provide a sample of his DNA, details of which were to be entered in a data bank. In May 2002 police, pursuing investigations into the unsolved murder of Ms Henderson, arranged for the blood on the pillowcase and shirt which had been stored in the laboratory freezer to be tested against the data bank. A matching profile with the sample obtained from the prisoner was established.


F. What is DNA evidence?

46. When the Crimes (Forensic Procedures) Act 2000 was introduced into the NSW Parliament, Police Minister Paul Whelan stated: It is important to note that DNA will be only one tool in the police officer's kit. They will still need to assemble a brief of evidence against the offender; DNA alone will not convict. [Hansard, NSW Legislative Assembly, 31 May 2000, p 6293].

47. In R v Pantoja (1996) 88 A Crim R 554, Hunt CJ at CL stated: It is important to emphasize that a match obtained by any blood tests - DNA or otherwise - between the suspect and the offender does not establish that the two are the one and the same person. It establishes no more than that the accused could be the offender. However, any blood test which positively excludes the suspect as an offender, if there is a reasonable possibility that the test is correct, must necessarily exclude the suspect completely notwithstanding that a match has been obtained by other blood tests.

48. Where there is no DNA evidence linking the accused to the offence, this is not evidence of non-connection; it is simply an absence of evidence one way or the other.

49. Where there is DNA evidence tending to inculpate the accused, and other evidence, either to the same or to the opposite effect, the jury must consider the DNA evidence in the context of all the other evidence.

50. In cases involving DNA evidence, attention ought to be paid to the correct directions. They should carefully conform to the precise evidence. In R v Pantoja (1996) 88 A Crim R 554, Hunt CJ at CL further stated: The jury was required to be told that it had to be satisfied (beyond reasonable doubt) that there was a match between the two profiles. If they concluded that there was a match, then it would then have to be made clear to the jury that the only inference they could draw from such matching was that the appellant could not be excluded, and that therefore it was possible that he was responsible for the semen stains. Further, it would also need to have been made clear to the jury that the matching results could not in the absence of other evidence, prove beyond reasonable doubt that the appellant was the person responsible for the stains.

Brian Hancock
Public Defender
28 February 2009