Published date01 January 2008
Date01 January 2008
Subject MatterNoticeboard
Cases and Comments
DNA mixtures—Australia
DNA, a chemical found in almost every cell of the body, is a person’s genetic
blueprint. An individual’s DNA profile is obtained by analysing a number of sections
assumed to be different, the characteristics of individual sections of DNA are
of bodily tissue or fluid left at a crime scene, most likely blood, semen, skin or saliva,
and comparing the DNA profile of the sample (the crime stain) with that of a sample
supplied by a known individual, usually a suspect, or a computer database of DNA
‘fingerprints’. Nowadays a DNA profile is usually created by examining at least 10
gene loci, that is, sites where particular genes are located on the DNA chain. DNA
testing does not, however, focus on genes. Except for the locus indicating the
of ‘junk’ DNA are repeated many times. The number of repeats differs a lot from
person to person. There may be a number of people who have the same number of
repeats at each of the examined loci and therefore share the same DNA profile.
Therefore, a DNA match between the crime stain and the reference sample does not
prove that the person from whom the reference sample was taken is responsible for
the crime stain. Forensic scientists calculatethefrequencyofthecrimestainDNA
profile in the population using empirical statistical data. If the random occurrence
ratio is one in 10 million and the population is 50 million, this means that there are
five people in the population with the crime stain DNA profile, one of whom is the
person from whom the reference sample was taken. DNA evidence may be expressed
as a likelihood ratio which compares the probability of obtaining a profile under
two competing propositions:
probability that the suspect left the crime stain
probability that the suspect did not leave the crime stain
When a crime stain is examined, forensic scientists expect to find a pair of alleles
at each gene locus, one from each parent. Sometimes testing discloses the
presence of more than two alleles at a locus. This indicates that the tissue or fluids
of at least two people have become mixed. It may be possible to distinguish a
major or minor contributor to the mixture. Two recent cases consider issues that
arise at trial when the DNA recovered at the crime scene is a mixture.
In RvBerry [2007] VSCA 202 two men, Berry and Wenitong, were convicted of
murdering a fellow prison inmate. The prosecution relied upon circumstantial
evidence, including a lot of DNA evidence. The appellants objected to the admissi-
bility of a small part of the DNA evidence. This came from a sock found in the
prison laundry. DNA was taken from two parts of the sock. Sample A was taken
from an area stained with blood, sample B from an unstained area. On testing, the
DNA profile from sample A (the blood stain) was found to be consistent with the
deceased’s DNA profile. The prosecution expert used a likelihood ratio to express
his opinion. He testified that it was 72 million times more likely that the DNA
profile was that of the deceased than that of a person selected at random from the
Victorian Caucasian population. No one objected to this DNA evidence. Testing of
sample B revealed a DNA mixture. Analysis showed that:
the DNA profile of the mixture was consistent with at least four
individuals having been contributors;
the largest portion of the DNA matched the DNA profile of Ali, a man
who had not at any material time been in the prison and whose DNA
profile was not detected in any of the other crime scene samples;
all other individual DNA profiles in the mixture were incomplete;
the deceased and the two appellants could not be excluded as
contributors to the mixture.
These points were agreed by the experts. The prosecution’s expert told the jury
that the DNA mixture in sample B was at least 180 times more likely if it contained
DNA from Ali + the deceased + the two appellants + one unknown person (the prosecution
hypothesis) than if it contained DNA from Ali + the deceased + three or more unknown
individuals (a possible defence hypothesis). This, he said, provided strong support
for the prosecution hypothesis. He calculated an alternative likelihood ratio on
the assumption that the defence hypothesis was that the DNA came from Ali + the
deceased + two or more unknown individuals. This ratio was a modest 95 times more
likely. The defence expert did not rely on a likelihood ratio to express his opinion.
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