Andrew Haesler SCDeputy Senior Public Defender
IntroductionDNA Deoxyribonucleic acid matching is a wonderful
investigative tool. DNA evidence can provide powerful evidence in support of a
prosecution case. DNA evidence can also provide convincing evidence of a
A profile taken from the DNA of a suspect can be
compared with the profile of a sample of DNA taken from a crime scene. It may,
depending on the other evidence, be compelling evidence of guilt. Profiles can
be stored on a computer database. They can be easily cross-checked and any
linkage investigated. A statistically validated “match” or “link” between these
profiles is evidence that they come from the same source.DNA is
corroborative evidence. It is particularly useful if supported by statistics
that the chance of someone else other than a defendant leaving the crime scene
stain is highly unlikely or highly improbable, by a ratio of one in ten
billion.DNA evidence can, however, be misunderstood. If unchallenged,
DNA evidence can be “significantly persuasive”. M Findlay and J Grix,
“Challenging forensic evidence” (2003) 14 Current Issues in Criminal
Justice 269 at 273. It appears to offer a degree of certainty that is often
missing from a criminal trial. It also has the cache of being the topic of the
moment on popular television.
DNA’s apparent certainty can be deceptive.
It can be misused and misapplied. It will not of itself solve the crime problem
although the chance of discovery may act as a deterrent. Jacobs J noted in
Griffith v The Queen (1977) 137 CLR 293 at 327: “The deterrent to an
increased volume of serious crime is not so much heavier sentences as the
impression in the minds of those who are persisting in a course of serious crime
that detection is likely and punishment will be certain”. DNA evidence is part
of a prosecution case; it is not a panacea.
We all want evidence and
results that make our already difficult jobs easier. We would love some expert
to ease the burden of judgment by saying, “this is the answer”. If only it were
that easy. There is still a lot we don’t know about DNA. “As we know, there are
known knowns. There are things we know we know. We also know there are known
unknowns. That is to say we know there are some things we do not know. But there
are also unknown unknowns. The ones we don’t know we don’t know.” Donald
Rumsfeld, US Secretary of Defence, Department of Defence Briefing, Washington
DC, 12 February 2002. We need to be acutely conscious of the limitations of DNA
evidence. Often the question is not, “Whose DNA is it?” but “How did the DNA get
This paper aims to address some of the issues that can arise when
DNA evidence is put before the court at trial. I cover six key areas:·
DNA science and technology;· Statistics;· DNA reports;· Problems
with the science and technology;· Problems with statistics; and·
Evaluating DNA evidence in context.
DNA science and
technologyDeoxyribonucleic acid (DNA) is found in all cells, except
red blood cells. DNA is said by some to be unique to all but identical twins.
However the fact, crucial for any understanding of DNA evidence, is that there
is no scientific proof of such uniqueness.
Only a few cells, invisible to
the naked eye, can be enough to obtain the DNA profile of an individual. DNA can
be extracted from the sorts of things regularly left at crime scenes, such as
traces of skin, blood, sweat, saliva or other bodily fluids. Most testing in
Australia focuses on the DNA found in the nucleus of a cell, although DNA can
also be extracted from the mitochondria. This article deals with DNA extracted
from the nucleus of a cell.
A crime scene sample is taken either by swab
or by collecting an exhibit for sampling. Usual police crime scene or hospital
sexual assault forensic exhibit procedures should be followed. The exhibits
and/or swabs are then transferred to a government analytical laboratory where
the swab is processed. A whole exhibit is examined visually and samples taken
from areas where it is presumed that DNA might be found. These samples and crime
scene swabs are then tested to see if it is possible to determine the type of
material found, for example, blood, semen, skin (epithelial) cells or saliva. It
is not always possible to determine the type of material. Sometimes only a
presumptive test can be done.
The item is then sent for processing. At
this point the machines and technicians take over. In all Australian
jurisdictions the processing of evidence for DNA follows a fairly standard
procedure based on commercially available kits:
· The DNA is first
· It is then measured. Too much DNA can skew a
sample. Too little can lead to a “no result”. Some products including cloth dyes
or cleaning agents can inhibit DNA analysis. Ultra violet light, heat, humidity
or bacterial action can destroy DNA. Ideally only a very small amount is needed.
Ideally between 0.5 and 1 nanogram of DNA per 20 microlitres A microlitre (ul)
is 1 millionth of a litre. There are about 15 drops of water in a millilitre (1
thousandth of a litre). is used. A nanogram is one thousand millionth of a gram,
which gives some indication of the sensitivity of DNA analysis!
extracted DNA is then amplified. The process is akin to a photocopier,
which not only turns out identical copies, but also does so exponentially. After
a few repeats we get millions of copies. The amplification process used in
Australia, known as PCR (Polymerase Chain Reaction), also enables multiple
points/loci on a person’s DNA to be analysed at the one time
· Those copies are then placed in a machine and
analysed. A process known as electrophoresis is applied and a series of
graphs and readouts obtained. These are then interpreted.
beauty of the science and technology of DNA testing is that the process results
in visual charts and computer readouts that describe what cannot be
What can’t be seen is this:
Every cell Except red blood
cells which have no nucleus; and sex cells (sperm and egg) which have only 23
chromosomes but combine to form 23 pairs. contains 23 pairs of chromosomes.
Those chromosomes are made up of and act as a storage mechanism for our DNA. The
DNA molecule or genome carries the entire genetic code of an individual. One
half is inherited from the mother, the other from the father. Each chromosome is
made up of genes, and spaces between them known as introns. Both in turn are
made up of a series of bases Adenine, Thymine, Guanine and Cytosine. Adenine
always binds with Thymine and Guanine with Cytosine: A-T & G-C attached to a
backbone of phosphate sugar. These bases occur in sequence Configured in the
famous double helix. and are depicted by the letters A, G, T and C.
sequence of bases follows regular patterns. It is the differing combinations of
bases, which make each person’s DNA potentially unique, as at some points (loci)
the sequences vary markedly from person to person.
By locating a
predetermined point on a gene (a locus) and measuring how many times a sequence
of bases (say ACTG, ACTG, ACTG, and ACTG) occur, a DNA profile is obtained.
Because they are less liable to deterioration the shorter sequences are used.
These are known as Short Tandem Repeats (STRs).
At each locus, or point
analysed, there should be two readings for the number of Short Tandem Repeats of
a particular sequence. These alternative sites on the chromosome are known as
alleles. One allele comes from the person’s mother, the other from their
father. If the number of STRs is the same, the individual is said to be
homozygous at that point/locus. If the number of STRs are different, the
individual is said to be heterozygous at that locus. There are two readings
because each chromosome is made up of two linked strands, one from the father
the other from the mother. If the parents share that number of STRs a single
result may be obtained at that locus. For example, if Mum is 12-14 and Dad
11-14, the possible combinations are 14-14, 11-12, 11-14 and
Multiple points, preferably from different chromosomes, are
tested. In Australia nine points/loci are tested as part of the Profiler Plus
system generally in use. In addition, a test is done of a gene known as
Amelogenin, which enables the sex of the sample’s donor to be determined. Each
of the 10 points/loci (that is nine loci plus Amelogenin) used in Profiler Plus
is given an internationally recognised identifier symbol: D3 S1358, VWA, FGA,
Amelogenin, D8 S1179, D21 S11, D18 S51, D5 S818, D13 S317 and D7 S820. The ‘D’
stands for DNA. The next number is the number of the chromosome. The ‘S’ stands
for Single Copy Sequence and the following number notes the order in which the
particular sequence was discovered.
A typical worksheet from the
Department of Analytical Laboratories notes the date, operator and method of
extraction: and the date, operator and result of amplification: and analysis.
All these tasks are undertaken by technicians under the direction of a
supervising biologist, who then undertakes the interpretation of the
Separate readout graphs and charts noting peak heights and areas
are also produced. The next worksheet will show the summary of case results and
the profile of the individual or sample taken from a crime scene. That profile
has two numbers corresponding to each of the known locations, for example, D3
S1358 - 15 and 16, meaning at point S 12358 on the 3rd chromosome there were 15
and 16 repeats of a particular sequence of bases. Further readings are given for
the other nine loci, and a reading for Amelogenin, which can only be either XX
(female) or XY (male).
These numbers can be computer coded and placed on
the DNA database. When the same series of numbers comes up on another part of
the database, for example, with a crime scene, suspect or convicted offender, a
“match” is called and the two results further interpreted to see if the
provisional match is justified. In other words, in the system used, no
differences between the two samples could be found.
If the numbers do not
match a suspect is excluded. If a suspect cannot be either matched or
excluded the result will be reported as “not excluded”.
inconclusive reading can result in the expert going further and giving either a
“not excluded as a contributor” or a “not excluded as a source”
finding. “Not excluded as a contributor,” means there was no match found, as
there was simply insufficient material either to match or exclude the person.
“Not excluded as a source,” means that it is possible a match may be made but
the DNA is too weak or too complex a mixture to reach a reliable conclusion.
That a person is “not excluded” can have no real relevance to proof of their
guilt or innocence.
A match between a crime
scene sample and an offender is reported if the same series of allele numbers
appears in the results for both samples. This is generally expressed in terms of
a match probability, “The suspect has the same profile (in the Profiler Plus
system) as the DNA recovered from point A at the crime scene. This profile is
expected to occur in fewer than 1 in 10 billion individuals in the general
population”. The statistics can also be expressed as a likelihood ratio, “It is
about 10 billion times more likely the suspect left the sample than if a random
person left the sample”. R v Karger (2001) 83 SASR 133 at
There are only six billion people on earth. How is such an
extraordinary figure arrived at, and what does it mean?
The first step in
calculating the probability of a match is to find out what the chances are that
another person chosen a random will have the same alleles at a certain locus. If
there’s a match at a point the search can be further narrowed by testing another
locus, and then another until all nine are examined. (In practice, a technique
known as multiplexing allows all loci to be tested at once.) As matches are
found at progressive loci, the argument becomes more convincing that the match
is not by chance, and that the two samples come from the same source. Of course,
if at any point the figures do not match, the suspect can be
How is this assessment turned into a calculation of
probability? What is done bears some similarities to an opinion poll. Samples of
DNA from hundreds of people are taken and an estimate made of the percentage of
people in the general population who have a certain allele at a specific
point/locus. Thus at locus D 13 S317, it may be found that 28% of people have
allele 12 and 32% have allele 11. The chances of a random match between a sample
and a person from the general population at that locus can then be calculated.
An expert in population genetics should prepare these estimates.
statisticians then apply a statistical formula known as the product rule. The
product rule involves multiplying the relative frequencies of each item matched.
When the results of each allele and loci are combined with the chances of a
random match at the remaining eight loci, the combination of probabilities, or
more correctly improbabilities, can become enormous. In Australia an adjustment
is made to the product rule to make allowance for the fact that the suspect may
come from a subpopulation whose usual genetic profiles may differ from that
found in the general population. The Balding-Nichols formula. A profile that is
common in the general population may be extremely rare in a subpopulation and
visa versa, thus skewing the statistical results. Balding and Nichols worked out
a mathematical formula that is said to correct for this problem.
jurisdictions, the United States for example, three different population
databases are used for Caucasians, Hispanics, and African-Americans. In New
South Wales, a single population database is used. The DAL uses a relatively
large population database of 739 samples. A New South Wales Aboriginal database
has recently been compiled using a population base of over 2500 profiles. In
other Australian jurisdictions, Asian and Indigenous databases are sometimes
JM Butler has noted: Forensic DNA typing, biology, technology
and genetics of STR markers, 2nd ed, 2005, Elsevier Academic Press,
Burlington, USA, p 500.
In New South Wales, the DAL biologists do not usually calculate their own
match probabilities. They use an Excel spreadsheet and a commercially available
program. They simply add in the figures, including the theta value (see below)
and up pops the match probability. It is generally so high for a nine loci match
that even allowing for an error range, figures well in excess of 1 billion are
The standard report prepared by the NSW Department of Analytical Laboratories
is usually brief and to the point: “I’m not into this detail stuff, I’m more
concepty”, Donald Rumsfeld again.
The explanatory note that accompanies the report contains these important
Other States follow a similar format. For example In South Australia a
Report from Forensic Science South Australia might say:
Standard information relating to calculations of likelihood ratios, match
probability and the type of Database used is attached.
As with all legal
documents it pays to read the fine print!
The reports generally note only
the results of the analysis they do not describe the process by which the result
was obtained. In South Australia provision is made in the Criminal Law
(Forensic Procedure) Act 2007 s 49 for tender of an evidentiary certificate
of the analyst, which in the absence of evidence to the contrary is proof of
matters certified. In R v Sing, (2002) 54 NSWLR 31: see also R v
Ryan  VSCA 176. the NSW Court of Criminal Appeal held that, unless
admitted by consent, the evidence from the expert who prepared the final report
itself was not admissible. To make the evidence admissible, the chain of custody
and handling of the exhibit from which the DNA was taken and analysed must be
proved — from collection of the sample to final analysis.
that, unless the defence do not require them, everyone who handled, processed
and analysed a sample should be called for cross-examination. The rationale for
the decision was the danger of unfair prejudice that might arise if relevant
witnesses were not called. A related issue was whether the expert’s conclusions,
reliant as they were on hearsay, were admissible in any event.
cases the defence will allow the report to be tendered without objection. There
should be more challenges. Evidence, particularly relating to the collection and
handling of DNA samples or exhibits from which they are taken, should not be
accepted uncritically. The opinion rule precludes the report’s admission unless
it can be shown that the opinion in it is substantially based on the author’s
training study or experience. Evidence Act 1995 (NSW) requirements were
dealt with comprehensively by Justice Heydon in Makita (Australia) Pty Ltd v
Sprowles (2001) 52 NSWLR 705 at . For non-uniform Evidence Act States
also Clark v Ryan (1960) 103 CLR 486, Wolper v Poole (1972) 2 SASR
419 at 421 per Bray CJ and R v Karger (2001) 83 SASR 1.
cases it is not the technology or the science but the supervising biologist’s
subjective interpretation of the results that is the crucial factor in assessing
whether a suspect sample and a crime scene sample “match”. What she/he is doing
is looking at the computer read-outs and coming to a conclusion. In some cases
the read-outs will be clear and conclusive, in some the readings will not be so
clear and in others they will be far from clear at all. Where professional
judgement and expertise are required to be exercised that evidence should be
tested, as there is often fertile ground for doubt.
Problems with the
science and technology involved in DNA analysis
A lot of time and
effort is wasted on testing and challenging the unchallengeable. The science
backing DNA analysis is good and getting better all the time. The technology has
been tested and cross-tested. There are protocols in place designed to ensure
that results are validated and possible contamination of results, or errors in
analysis are detected. Mistakes have occurred but they are so rare as to be
notorious. “UK DNA Mismatch” <http://www.scoop.co.nz/stories/HL0002/S00053.htm>;
and “Murder DNA tests botched”, The New Zealand Herald, 26 May 1999. The
police excused the error as “procedural difficulties in investigative analysis.”
The incident led to the establishment of the Eichelbaum-Scott Inquiry, which,
although it failed to find the cause of the error, stated that the most probable
explanation was accidental lab contamination. Deliberate corruption of results
has been noted in the United States, as has institutional bias. However, the
Australian analytical labs are justifiably jealous of their
Nevertheless in any contested DNA matter it is often
essential that the analyst’s expertise be established and the laboratory file
examined, if only to check that all procedures and protocols were followed. “The
cogency of DNA makes it particularly important that the DNA testing is
rigorously conducted so as to obviate the risk of error in the laboratory. The
method of DNA analysis and the basis of subsequent statistical calculation
should — so far as possible — be transparent to the defence. The true import of
the resultant conclusion [should be] accurately and fairly explained to the
jury” R v Doheny & Adams  1 Cr App R 369; and R v Karger
(2002) 83 SASR 135. Most DNA reports do not comply with the Director of Public
Prosecution’s duty of disclosure
following areas require close scrutiny.
Partial matchA partial
match reduces the opportunity for the full application of the statistical
equation used to calculate the likelihood of a “match”, known as the “product
rule”. In Australia the product rule is modified by the application of the
Balding-Nichols Formula to make allowance for genetic similarities within some
isolated communities, population substructures and the fact that random breeding
does not take place. See “Assumptions” below. A partial match creates the chance
that the missing portion may yield a result that would exclude the suspect. At a
certain point the match probability figures become so low as to be
A weak reading has similar
problems to that of the partial match. It is sometimes impossible to tell the
difference between a true reading at a locus and a glitch on the graph brought
about by the testing process. As a result alleles may be wrongly counted or
missed altogether. Most labs have a minimum peak height below which they will
not hazard an assessment. On occasion, a match will be given despite a low peak
height. Examination of low peaks can also disclose a potential extra contributor
to a sample, raising the possibility that this person may be the true culprit,
or the possibility of secondary transfer (see below).
The ACT Court of
Appeal recently examined the practice by which a lab “overloaded” a mixed DNA
sample and so enhanced minor peak heights, which would not otherwise fall into
interpretable limits. The retrial subsequently ordered will now have to examine
if this is a valid process. Hillier v R  ACTCA 3, Madgwick,
Weinberg and Dowsett JJ.
There is also a phenomenon known as a
stutter, where an artefact of testing appears as a peak, mimicking an
allele’s graph peak. Trained analysts claim to be able to ascertain the
difference between an allele and a stutter. Stutters have been, and will
continue to be, be interpreted as peaks with the consequence of a false match or
Similar problems can arise if only a single reading is
found at one locus. A single reading can mean the alleles at that point are the
same (known as ‘homozygosity’). It can also mean something has dropped out or
not shown up on the graph (known as allelic drop-out or a null allele). A false
positive or false negative reading can
Three or more alleles at a locus indicate
the presence of more than one contributor. It is often difficult to tell whether
the sample originated from 2, 3 or 4 people. Statistical models can help
analysts work out the probabilities of more than two contributors however a
number of possible combinations could be consistent with the findings. Despite
this, unless more than five alleles are found at a locus, the government
analyst’s generally assume that only two people contributed to a mixture. This
presumption has been proved correct in most cases. However, if this assumption
is false it can lead to a part of a mixture being matched wrongly to a
An assumption is then generally made that the taller peaks are
associated with the primary contributor and the shorter peaks the secondary. If
all the alleles can be matched to the crime scene or victim sample, these are
then taken out and the assumption made that the remainder match only the
suspect. Peak heights are often used in interpreting mixtures, but peak height
imbalances occasionally occur. Peaks with the same height are presumed to be
from the same person, but the reading of peak heights is far from an exact
science. Even in a single person sample, peak heights may vary, so assumptions
of regularity may be false.
In mixed samples alleles may simply be
undetectable or indistinguishable from background “noise”. Alternatively, as
mentioned above, alleles can simply “drop out” and not appear on the graph or
stutters can be confused with alleles from a minor
Most matches are correct, but
errors do occur. There may be innocent, accidental or malicious reasons for a
false match or a false exclusion. No matter how good the laboratory procedures
are, errors do occur. Protocols are in place to pick these up. It is rare, but
not unusual to find a worksheet noted “possible contamination”. This finding
will result in the re-testing of the sample. In every case I have seen a second
clean result was obtained. For most cases of potential contamination in the
laboratory the innocent explanation is just that. However, the lab will not know
if handling errors have occurred in the course of collecting or delivering the
exhibit. Nor will it know if the sample has been deliberately tampered with
before it arrived at the lab. In R v Lisoff  NSWCCA 364, the Court
of Criminal Appeal ordered a re-trial. Lisoff was subsequently acquitted,
presumably because of doubts that the victim’s blood found on L’s trousers may
have been planted. That blood was said to contain transfusion products and thus
had arguably been taken from the victim after the assault and
after he went to hospital.
Bias in the lab?
An example can be found in R v Button,  QCA 133. in which the
forensic scientist looked only for evidence that would implicate the accused and
missed crucial evidence pointing to the real culprit, because they did not do
the tests. Justice Williams described the various failures in the case as
resulting in “…a black day in the history of the administration of criminal
justice in Queensland.” Deliberate failure to investigate is rare but we must be
alert. The more likely cause of a failure to investigate alternatives is
pressure of work and a focus on output rather than using the genuine forensic
expertise of the analysts. The more procedures are automated the less the
analyst has to do with analysing the samples themselves. See S Walsh, “Is the
double helix a double edged sword?”, paper presented to UTS Speaks public
lecture, May 2005. The quest for volume can mean only one exhibit or part of an
exhibit is analysed. Sometimes as R v Button shows this is simply not
good enough. When an expert or technician is cross-examined about what was
tested it is sometimes prudent to find what was not
Inadvertent or secondary transfer
developments in DNA processing have enabled readable DNA to be obtained from
tiny samples, unimaginable even a few years ago. DNA can now be recovered from a
single cell and it is possible for as few as 30 cells to be processed in order
to give a readable result. Similarly, DNA can now be recovered from objects
where no bodily fluids are apparent, samples so small they can be obtained from
a fingerprint impression and from items such as knife handles or spectacles. R
van Oorschot et al (1997) 387 Nature 767. These finding have, however,
not always been reproducible, C Ladd et al, “A systematic analysis of secondary
DNA transfer”, (1999) 44 Journal of Forensic Science 1270–1272. See also
J Raymond et al, “Trace DNA: An underutilised resource or Pandora’s box?” (2004)
56 Journal of Forensic Identification 668–686. In some cases, enough DNA
can be recovered for analysis by conventional techniques. P Gill, “Biological
Evidence”, paper presented to 13th Interpol Forensic Science Symposium, Lyon,
France, October 2001; P Van Renterghem et al, “Use of latent fingerprints as a
source of DNA for genetic identification”, (2000) 8 Progress in Forensics
Given that we shed 40,000 skin cells each day, a
lot of our DNA can be left lying around. It appears that some of us are “good
shedders,” and some not. Experimental studies on Low Copy Number DNA have shown
that a simple series of handshakes can transfer DNA from the original source to
a third party.
Some experts are reluctant (if not dismissive) of
suggestions that the DNA found by their tests got there by way of secondary
transfer. They will say the secondary transfer tests involved smaller samples
than are regularly tested for. They will say that transfer generally requires
more than mere skin to skin contact. Although some studies support this
conclusion others do not. See notes 16 and 17 above. At the same time they will
triumph in their ability to get a sample from the smallest trace of material
left at a scene.
DNA is highly mobile and secondary transfer does occur.
Every time we speak and release spittle we have transferred our DNA. If, while
still moist, this spittle comes in contact with another object, transfer of a
few cells can occur. And only a few cells are enough to get a profile. After
all, the taking of a swab from a crime scene is but an example of secondary
transfer. We are dealing in such small quantities of material and such new areas
of science that assumptions, which presume against secondary transfer, must be
The Inquest in to the death of Corporal Kovco who was shot
with his own weapon while serving in Bagdad provided an incentive for the NSW
DAL to review aspects of secondary transfer. Recent studies reveal:
The trial of Barnes Unreported NSWSC trial (Wollongong), February 2004,
Buddin J. provides an example of secondary transfer. A young woman was found
dead in a park in Dapto, her discarded clothing covering her naked body. The
accused’s DNA was recovered from her bra. Other evidence pointing to Barnes’s
involvement inn the death was equivocal. Evidence established that about an hour
before her death the two had met outside a club. Both were drunk, and the
accused in particular was in a jolly mood shaking hands with a number of
complete strangers. The problem posed for the defence was, how did his DNA come
to be on the bra strap of a women who when they met was wearing a vinyl coat and
a singlet over her bra? The DAL analyst was dismissive of suggestions that
spittle sweat or skin from Barnes’s hand had got onto either the victim or her
jacket and then been transferred to the bra. The jury as evidenced by their not
guilty verdict were more accepting of the possibility of secondary
Problems with statistics
Statistics determine the
probability of an event occurring by looking at possible successes and dividing
them by possible outcomes. Predictions can be made in a general sense, but no
statistical analysis can say what the next outcome will be. What statistics can
do is give a model of expected behaviour. Those models can be independently
tested and validated for consistency and rates of error but they are tools and
models — they are not real. They must, of necessity, be based on a number of
assumptions, which in turn rely on statistical rules, and the developing science
of population genetics.
The first step in
calculating the probability of a match is to find out what the chances are that
another person chosen at random will have the same allele at a certain locus.
Before any match probability or likelihood ratio can be calculated the range of
possible outcomes must be found. This involves an understanding of population
genetics and the making of some basic assumptions in formulating the model or
database against which a suspect sample can be compared.
The basic model
assumes that there is an infinitely large population in which no one selects a
mate on racial or ethnic lines, a population in which there is no mutation, no
migration and the biological process known, as natural selection does not apply.
As none of these things are true, a basic model can overstate the value of the
statistical evidence. Allowance is now made for what we now know about human
genetics. Real populations do not breed randomly. Rather groups of people
(subpopulations) tend to breed within the group. Thus within that group certain
alleles may be more or less common than in the general population. A formula
worked out by Balding and Nichols is applied to make allowance for this fact. H
Roberts, “Statistical evaluation in forensic DNA typing”, in I Freckelton and H
Selby, Expert Evidence: Law practice, procedure and advocacy, 3rd Ed,
2005, Law Book Co, Ch at para [80A].77380] –[80A.480].
In addition, it is
· There is no genetic predisposition to crime, which might
skew the results
.· The samples used to compile the database are taken from a
random selection of the population and contain no close relatives.
databases are as good as a whole population survey. They can’t be, as we don’t
test everyone (yet!). The databases cannot be more than a model.
statistics take no account of other evidence. Such as “I have six brothers all
of whom live in the area”, R v Watters  EWCA Crim 89.or, “I have an
evil twin”. If there is a chance a close relative or same sex sibling has left
the sample the probabilities of a match are again quite different. For example,
there is a 1 in 10,000-match probability with a same sex sibling and a 1 in 100
million chance of a match with a first cousin. Butler, op cit n 14, p 511.
Variations among populations are truly random. They are not. (See discussion on
· There is no link between the alleles tested. An example
of linkage is the fact that blonde hair normally occurs in conjunction with blue
eyes. Linkage can skew the product rule calculations.
· We can tolerate a
degree of uncertainty. As lawyers, this is where our finely-tuned doubt
detection meters start to operate. How can there be uncertainty and not
Is a 9-locus profile unique?
A high match
probability or likelihood ratio carries with it an implication that no one else
has the same profile and that another match cannot exist. But statistical
probability cannot predict the next outcome. The circumstances that led to one
person’s genetic code may happen again by chance. A survey of DNA databases by
the National Institute of Forensic Science examined 33,858 profiles, and found
206 matching pairs! At each of the nine markers used on the Profiler Plus
system! This was significantly more than the statistical model predicted. S
Walsh and J Buckleton, Report on Duplicate Detection accompanying; J
Buckleton, S Walsh and R Mitchell, “Autosomal microsatellite diversity within
the indigenous Australian sub-population”, 2004, National Institute of Forensic
Science, Melbourne. New Zealand reviews of their database have also turned up an
unusual number of matches. See J Buckleton, S Walsh and C Trigg, Forensic DNA
evidence interpretation, 2005, CRC Press, Boca Raton, FL, USA, p 463.
Matching pairs can be explained as twins, brothers or duplicates, because
offenders use aliases or unrelated coincidence matches. Only by investigating
each match can the real reason be known. To date the investigation needed to
explain the matches has not been done. Unexplained matches illustrate the
proposition that the statistics are simply another tool.
This method of calculating the likelihood ratio from
mixed samples is generally more conservative than the product rule and is
regularly used by some laboratories, for example, in Tasmania but not New South
Wales. It involves looking at the sample as a whole and not first separating out
“known” components, such as the victim’s profile. That different methods of
calculation give different match probabilities illustrates the hypothetical
nature of any “match” conclusion.
We tend to mate
with people of similar genetic backgrounds, for example, it is unlikely for
someone from Brazil to have a child with someone from Iceland. In some societies
there is a cultural tendency to inbreeding, such as first cousin marriages.
Small or isolated population groups tend to have similar patterns of DNA.
Allowance is made for both direct and underlying relatedness in the population
in the DAL calculations by adding in an error factor, called theta or Fst.
Although calculated differently theta and Fst have approximately the same
Studies have shown that for European and Caucasian communities
these values are very low — less than 1%. Databases for the general Australian
community still use a conservative figure of 0.03 or 3%. This is incorporated
into the standard programme for calculating match probability.
into isolated communities genetically distant from the Caucasian model has shown
however that the 3% figure is not conservative. Because of Australia’s long-term
isolation prior to 1788 and restricted breeding practices in some Aboriginal
groups a theta value of 6% has been recommended. NIFS study, Evidence given by J
Buckleton in R v S (unreported, April 2006, NSWDC (Sydney), Norrish DCJ).
The research looked at the maximum genetic variation between surveys of tribal
groups and the error factor needed to allow a data base, based, for example, on
the Tiwi Islands to be used for assessing probability ratios for someone from a
mainland Aboriginal tribe. A variation or difference between people from
Northern China and South China including Vietnam has been assessed as high as
6%, Roberts, op cit n 31, 80A–621, citing Cavalli-Sforza et al. Even this figure
has been said to be too low. See R v Bropho  WADC
Where a close relative could be a suspect or
where the suspect or suspects come from a genetically isolated population, many
of the assumptions on which the mathematical calculation of a probable match do
not apply. Close relatives are more likely to have the same profile as a suspect
than the randomly selected person used as a basis of match probability
calculations because we share a much more limited genetic heritage than the
population at large. If a brother cannot be excluded specifically and may have
been involved then the match probability or likelihood ratios given by
application of standard formulae must be very conservatively revised. If the
brothers or cousins come from a small and genetically isolated group the figures
must again be reduced.
The problems were graphically illustrated in the
Western Australian case of R v Bropho.  WADC 182; see also R v
Watters  EWCA Crim 89. This was a judge-alone trial of an old rape
allegation. The complainant was not a reliable witness but a child had been born
at the time of the alleged rape. DNA evidence of paternity pointing to the
accused was crucial to the prosecution case. Initially the likelihood of the
accused being the father compared to a random person using a theta/Fst value of
3% was assessed at 1:3134. However, a theta of 13% reduced this to 1:358. Once
problems with a number of the loci used and the fact that close relatives were
also suspects were considered the judge could not use the DNA evidence as
“reliable corroboration”. Mr Bropho was acquitted.
Just because no DNA is found doesn’t prove that the defendant was
not there. “The absence of evidence is not the evidence of absence. It is
basically saying the same thing in a different way. Simply because you do not
have evidence that something does exist does not mean that you have evidence
that it doesn’t exist.” Donald Rumsfeld, US Secretary of Defence, again.
Although DNA can be obtained from a sample of less than 0.5 of a nanogram A
nanogram is 10 to the -9 of a gram, that is, 1/1,000,000,000th of a gram. of
human cells extracted from crime scene material, testing of samples has varying
success rates. When taken from fresh blood there is a 90% chance of the profile
being obtained. Saliva on a cigarette butt gives a 67% chance of the profile
being obtained. Saliva or sweat from a balaclava a 43% chance. Sweat left on an
object from a hand a 17% chance. Fallen hair with dead roots has only 25%
chance. The success rate is much higher for recently plucked hair roots. N
Cowdery, “DNA innocence panel”, College of Law Paper (O3/45), June
The failure to find DNA may simply mean that the testing processes
did not succeed. It helps neither the prosecution nor defence case. Finding
someone else’s DNA can, however, exclude a suspect. If the suspect and crime
scene samples differ at any of the markers measured on the Profiler Plus system
then they are from different individuals.
The significance of
There is a song by the rock band The Whitlams with the line,
“She was one in a million. So there’s five more just in New South Wales.”
“Up against the wall”, from the album, Eternal nightcap. Let’s be
realistic, if the singer was silly enough to let her go, it is most unlikely
that there are five more available and interested women in New South Wales at
all. The same fallacy applies in a DNA case when the probability ratio is not at
an extremely large level: It is wrong to say of a 1: X ratio, “Well there are
X more persons who it could be and they have not been eliminated. So there must
be a doubt, as it has been statistically proved that the real offender could
still be out there.”
The fallacy arises because this conclusion
ignores all other factors personal to the accused, which make him a suspect
above those other X persons; things such as opportunity, motive, proximity to
crime scene, age, sex, and physical description.
If, on the other hand,
there is simply no other evidence than the DNA “match”, the reasoning is not
fallacious at all.
The prosecutor’s fallacy
There is a clear danger of being overawed by statistics and falling into
fallacious reasoning. “If your experiment needs statistics you ought to have
done a better experiment”, Lord Rutherford quoted in NTJ Bailey, The
mathematical approach to biology and medicine, 1967, Wiley & Sons, New
York; “Statistics are like bikinis. What they reveal is suggestive, what they
conceal is vital”; Aaron Levinstein; “Statistics mean never having to say you’re
certain”, American Statistical Association T Shirt. An example comes from R v
Keir. In evidence the DAL analyst said: (2002) 127 A Crim R 198 at
The error was in the Crown, and later the trial judge, transposed this
statement and directed the jury: Ibid at .
Evaluating DNA evidence in context
can indicate that it is unlikely that another person would have the same DNA
profile as the defendant and the crime scene stain. Where that statistical
certainty is challenged careful attention needs to be paid to the nature of the
challenge. If the prosecution has not excluded the possibility of the accused
having a sibling or other relative who may have left the sample, or where the
expert called cannot support the statistical or population genetics behind their
opinion, then the apparent certainty of the model used may be
So too, if there is other evidence that contradicts the DNA
results. Examples recently accepted by juries (or the DPP in no-billing matters)
include alibi and where the victim’s description of the offender simply failed
to match in any reasonably acceptable way the description of the person matched
by DNA linkage to the crime.
Where DNA is the only evidence or is
critical to the case against the defendant, significant care must be taken when
evaluating its efficacy in proving the prosecution case.
Crimes (Forensic Procedures) Act 2000 (NSW) was introduced the Police
Minister Paul Whelan was explicit: Hansard, NSW Legislative Assembly, 31
May 2000, p 6293.
In R v Pantoja, (1996) 88 A Crim R 554 at 559, Hidden J agreeing.
Justice Hunt made the same point. Although our understanding of DNA has advanced
since 1996, the point still remains valid. A DNA link or match between the
accused and a crime scene stain demonstrates only that the accused could be the
offender. It does not establish that he was the offender.
In the same
case Justice Abadee J Ibid at 583 and 584. put it more empathically. He held
that the tribunal of fact must first be satisfied beyond reasonable doubt that
there is a match between the two profiles. That means only that the defendant
cannot be excluded and therefore it is possible he left the crime scene stain.
Further, the matching results could not, in the absence of other evidence, prove
beyond reasonable doubt the defendant was responsible for the crime scene
In R v Watters,  EWCA Crim 89. it was emphasised
that, there was no rule, that when the statistics reached a certain level a
prima facie case could be established. Rather, it was emphasised that the DNA
evidence must be evaluated in the context of the other evidence in the
Pantoja was decided in 1988. Courts have subsequently
expressed greater confidence in DNA and how it is presented. In R v
Galli, the Chief Justice said: (2001) 127 A Crim R 493 at .
There have been strong statements in support of the proposition that DNA
should be treated like fingerprint evidence.
In most cases I have reviewed there is some other evidence, insufficient of
itself to prove guilt, which the DNA evidence corroborates. Examples include:
R v Gum  SASC 311, where there were similarities in appearance
between the accused and the alleged rapist; R v Fitzherbert  QCA
255, where there was evidence of animosity and contact between the accused and
the victim; R v Butler  QCA 385 where the evidence was DNA and
opportunity and R v Weetra  SASC 337 where the accused lived nearby
and stolen property was found near his home.
I have yet to find a
superior court decision where DNA alone has been used to convict: eg where the
Crown could not prove the defendant was in Australia at the relevant time. There
are however cases where the barest of other evidence and a DNA link between a
defendant and a crime scene, to a very high probability, has been used to
convict. An example is R v Rowe: a police officer could at best give a
general description of his assailant but where blood from which was recovered
DNA linked to the accused was found on the officer shirt. As such it was not
strictly a DNA only case, although it came very close. Doyle CJ said:
I note that despite this view where DNA is the only evidence, the usual,
but not exclusive practise, of the NSW Director of Public Prosecutions is to
no-bill the matter. Cases where DNA is the only evidence are understandably
rare, but are becoming more common as more and more serious offenders are placed
on the offenders’ database. It was reported in The Advertiser of 28/8/2008 that
South Australia had 41,161 profiles on its database, a 100% increase on a year
before. Cold links are now being made, between crime scene stains and this
database, with increasing frequency. Examinations of “cold cases exhibits” have
turned up nuclear DNA from exhibits over 10 years old, see R v Stone
(2004) 144 A Crim R 568. Stone pleaded guilty in 2004 to a 1990 murder.
Sometimes cases are presented to court solely on the basis of this link. More
often the link leads to further investigations and other evidence such as
admissions, opportunity, identification or motive is presented. The DNA link
then provides powerful corroboration of that other evidence.
evidence sufficiently reliable for it to be used as the sole evidence against a
When an unadulterated suspect sample is compared with an
unadulterated crime scene stain it is highly unlikely that there is error.
However, because such errors have been known to occur it would not be wise to
presume that the DNA evidence is infallible. There are a number or reasons why
it is simply not possible to say that a match between a crime scene stain and a
suspect proves he or she is the offender. These include:
· There is still
so much we don’t know about DNA and statistics and population genetics, in he
particularly areas of linkage between genes The best illustration is that some
traits are not independent, for example, blonde hair and blue eyes. and
interrelatedness. Illustrated by R v Bropho  WADC 182.
potential for human error.
· The potential for contamination of samples. For
an excellent review of why courts cannot be complacent about contamination, see
K Edwards, “Ten things about DNA contamination that lawyers should know” (2005)
29 A Crim LJ
71. Most police forensic officers are well-trained
professionals but not all exhibits are collected by trained professionals.
Sometimes pressure of work or cost-cutting can lead to unacceptable shortcuts
being taken. Examples of improper techniques that can lead to contamination of
DNA samples include, improper bagging and storage of exhibits, for example,
bundling them all in a bag or back of the car, transfer of DNA by fingerprint
brushes, tweezers, or gloves which were used on more than one item.
Handling errors because of the conditions in which the DNA is kept or stored. It
is surprising how many samples, numbers and barcodes do not match with numbers
and barcodes recorded in notebooks or exhibit books. DNA profiles can be
obtained from such small amounts of body tissue or fluid, it is often hard to
avoid cross-contamination of samples. The most recent example arose in R v
Murdoch in the NT Supreme Court (the Falconio case) where DNA from the chief
of the laboratory doing the testing was found on a crime scene sample.
we are dealing with statistical models a chance match simply cannot be excluded
(and the possibility of a chance match increases if relatives or those from
certain racial groups may be involved).
· There is always a possibility of
tampering with the crime scene. This is not to impugn the police or Laboratory
staff. Other criminals could have left the sample and it would be naïve to
assume that all police can resist the temptation to plant evidence.
suspect’s sample could have arrived at the crime scene for a number of innocent
reasons — secondary transfer or prior contact with the scene or exhibit.
DNA technology allows only for comparison of computer-generated profiles of the
two samples of only a tiny fragment of DNA, There are over three billion pieces
in the DNA code. The Profiler Plus system looks at only 10 segments of that
total code. unlike fingerprints, where there is an actual physical comparison
between the fingerprint found at the scene and that of the suspect or
· There is danger that the statistical significance of a DNA match
can be overstated. Some of the risks attaching to DNA evidence are only now
becoming apparent. “The highly subjective nature of the mathematical process
remains concealed behind the apparent certainty of a bald statistic”, Mathew
Goode, “Observations on evidence of DNA frequency” (2002) 23 Adelaide Law
Review 45 at 66–67. Examples include; the Eichelbaum-Scott report on DNA in
New Zealand in 1999 The Rt Hon Sir Thomas Eichelbaum and Professor Sir John
Scott, Report on DNA anomalies for The Hon Tony Ryall, New Zealand Minister for
Justice, 30 November 1999, the inquiry in Victoria into how a female crime
victim’s DNA was found in the Jaiyden Leskie murder, the more recent problem of
contamination in the Gesah case The Age 22 July 2008 and 8 August 2008 and the
number of what are called “unresolved pairs” often found when the data bases are
searched for unexplained matches. Forensic DNA Evidence
Interpretation, Buckleton, Triggs & Walsh CRC (USA) 2004 page 463.
In 2005 Walsh and Buckleton reviewed Aboriginal DNA data bases for the National
Institute of Forensic Science, in an unpublished report “on Duplicate detection”
they noted that out of a sample group of 33,858 there were 1,575 matches, 206
occurred at 9 loci or greater. They explained these as being; coincidental
matches between unrelated individuals, the same person giving more than one
sample underran alias, close relatives matching or identical twins matching. The
report also noted that in New Zealand a similar review had found 64 unresolved
matches from a database of 50,000 people.
A DNA match thus shows that it
is possible to a very high degree of probability that the defendant is
the person responsible for leaving the stain. Despite the power of the
statistical analysis that accompanies DNA testing I argue this can never be
enough to prove a case beyond reasonable doubt in the absence of some other
evidence for the DNA to corroborate. As the court said in Pantoja all the
DNA match or link shows is that they could be the offender. In most, if
not all, cases there will be good reasons why the mere fact of a match and the
giving of a high match probability cannot be viewed with the certainty necessary
of itself to prove the prosecution case beyond reasonable doubt.
the prosecution can point to no other evidence to support or corroborate their
case against the accused, the case cannot be proved on the necessary high
Where there is some other evidence, common sense leads to the
conclusion that before a case can be proved beyond reasonable doubt all
relevant evidence (DNA and otherwise) must be considered in
How should a court evaluate the DNA evidence?
Supreme Court of British Columbia Australian Law Reform Commission, op cit n 45,
Part J, Law Enforcement and Evidence Chapters 39–46, [44.50]. In the United
Kingdom suggested guidelines can be found in R v Doheny  1 Cr App R
369 and in the Northern Territory in Latcha v The Queen (1998) 104 A Crim
R 390. has suggested that before DNA evidence is presented to a court it should
be made sufficiently clear that:
· the estimates are not intended to be
· they are the products of mathematical and scientific theory not
· they do not purport to define the likelihood of guilt;
they should only be used to form a notion of the rarity of the genetic profile
of the accused; and
· the DNA evidence must be considered along with all the
other evidence in the case relating to the issue of identification.
useful start for considering a direction on DNA evidence comes from the leading
South Australian decision of R v Karger: (2002) 83 SASR 135, Doyle CJ at
The case of R v S unreported, April 2006, NSWDC (Sydney), Norrish DCJ.
illustrates the point. A rape victim described her attacker in detail and by
name. That person’s DNA did not match the semen found. He was discharged. Later
a cold link was made to “S” who had been in custody for unrelated and quite
different offences. He was put on trial. He lived in the town where the rape
occurred. Apart from his Aboriginality he did not match the description of the
attacker. He did, however, have a close relative who not only matched the
description but also had the same first name as the originally nominated
suspect. That relative was not tested. The prosecution experts said that the
probability of “S” providing the sample was astronomically high. 1:370 billion
rounded down to 1:10 billion. It was still enormous even if a close relative
couldn’t be excluded. Doubt was cast on those figures by the disparity between
the descriptions, the untested relative, the acceptance by all experts that the
profile could not be said to be unique, disputes between experts about
Aboriginal Fst and the revelation of the unexplained matches on databases
reviews. The jury had a doubt despite the statistical ‘certainty’ of the match
probabilities given and “S” was acquitted. In no DNA case, should the question
of guilt be approached strictly on the basis of mathematical calculation. R v
Galli (2001) 127 A Crim R 493; and R v GK (2001) 125 A Crim R
DNA is just another piece of physical
evidence. The statistical evidence interpreting the DNA match is expert evidence
that can be used in deciding whether it has been proved that the appellant was
the source of the incriminating DNA. When it comes to prove a case it is
suggested that any DNA evidence and its accompanying statistics must be
evaluated cautiously with knowledge of its limitations and in context with all
the other evidence in the case. If there were no other evidence it is difficult
to see how a conviction could result.
A DNA profile and its accompanying
statistics are not real. They are scientific and statistical constructs. Even if
the DNA evidence and statistics are good, questions can arise about how the
match came about. This is an area that we as lawyers know and understand as it
involves questions of proof, evidence and weight of evidence. Like all evidence
DNA can be used, misused and abused.
Andrew Haesler *2008
Andrew Haesler SC is a NSW Barrister and Public DefenderAdmitted to practice
in 1981 Andrew worked as a Solicitor with the Redfern Legal Centre. He was the
Centre’s Principal Solicitor from 1985 to 1989. He also worked in Alice Springs
with the Aboriginal Legal Service and in Wollongong with the Legal Aid
Commission. Called to the NSW Bar in 1990 he became a Public Defender in 1995.
In 1999-2000 he was the Director of the Criminal Law Review Division of the NSW
Attorney General’s Department and appointed Senior Counsel in 2004. He has
given, and had published, many papers, on a variety of topics concerned with
Forensic DNA Typing, JM Butler, 2nd Ed. Elsevier (USA)
2005Forensic DNA Evidence Interpretation, Buckleton, Triggs
& Walsh CRC (USA) 2004DNA Profiling on Criminal Investigations, L
Wilson-Wilde, in Freckelton & Selby Expert Evidence, Chapter
80.Statistical evaluation in forensic DNA typing, H Roberts, in
Freckelton & Selby Expert Evidence Chapter 80A.Evaluating
Forensic Evidence: Essential elements of a competent defence review, WC
Thomson, S. (http Ford, T. Doom, M. Raymer & D. Krane.
Available at www.cs.wright.edu/itri/
Forensic Evidence, M Findlay & J Grix, 14 Current Issues in Criminal
Justice 269 at 273.DNA and the Changing Face of Justice, J
Goodman-Delahunty & D Tait, Australian Journal of Forensic Science
(2006) Vol 38 p.97.