DNA
 Deoxyribonucleic acid (DNA) is often called the genetic
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blueprint of life, because it contains all the genetic code for
the characteristics an organism inherits from its parents.
It also contains large sections often called ‘junk DNA’ that
do not appear to contain genetic information, but which are
also inherited.
Although no two people—except identical twins—have
absolutely identical DNA, the parts carrying genetic
information don’t vary very much.
The differences between people are much smaller than the
similarities.
The non-coding or junk DNA varies much more, and this is
what is compared when we use DNA for forensic
identification.
Samples for Forensic
Identification
 Forensic scientists are often able to collect
blood samples, pieces of flesh, hair samples,
mucus from the nose and throat, and, from
rape victims, semen samples.
 They can use these samples to carry out DNA
identification.
 This is because every body cell(except red
blood cells, which do not have a nucleus)
contains DNA in the nucleus.
Forensic Identification
 Identification using DNA is a powerful tool that
can be applied in many situations including:
 forensic applications
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Can the DNA found at a crime scene be matched to a person
on the national DNA database?
Is this blood spot from the victim or from the possible
assailant?
In a rape case, is this semen from a previously convicted
rapist?
 mass disasters, such as passenger aircraft crashes, the
9/11 terrorist attacks, the Bali bombings
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Can the various remains that have been recovered be
matched to a particular person known to have been on-site?
 identification of human remains
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Are these remains those of a particular missing person?
Who was the unknown child, tagged as body number 4,
recovered after the sinking of the Titanic in 1912?
What DNA is used for
identification
 Depending on the purpose and circumstances of
the identification, the DNA used comes from
either the chromosomes (nuclear DNA) or from
mitochondria (mtDNA).
 In both cases the identification depends on the
existence of segments of DNA that vary greatly
between individuals. Such regions of DNA are
termed hypervariable.
mtDNA
 mtDNA identification is less precise because persons from the
same maternal line have identical mtDNA profiles.
 mtDNA is used only when chromosomal DNA cannot be recovered
or when chromosomal DNA is degraded because of age.
 Identification using mtDNA is mainly applied either
 to identify victims of mass disasters where the names of the victims are
known but where identification of the remains by conventional means, such as
visual inspection or dental records, cannot be done, or
 to identify decomposed remains when the identity is suspected to be one of a
few particular missing persons. In both cases, there must be living relatives
on the maternal line to provide mtDNA for comparison with the mtDNA from
the remains.
Nuclear DNA
 Used when there is a need to match a DNA sample
from a crime scene to just one particular person.
 DNA samples from relatives are not required.
 Two ways to identify:
 DNA Fingerprinting – relies on hypervariable regions in the
non-coding regions of DNA
 DNA profiling – relies on short tandem repeats (STRS) in the
nuclear DNA. There are large number of STRs present on
different human chromosomes and these can be used to
identify one person uniquely (apart from identical siblings).
DNA Fingerprinting
 Was developed as an identification tool in
1985 by Professor Sir Alec Jeffreys
 Involved cutting DNA with restriction enzymes
and separating them on the basis of size using
electrophoresis.
 Fragments were visualised by a technique
known as Southern blotting
 Because of the variation in the DNA, the
enzymes cut DNA into different size fragments
meaning each DNA fingerprint is unique.
DNA Profiling
 Was developed in the mid-1990s.
 Has replaced DNA fingerprinting of nuclear DNA.
 Relies on the presence short tandem repeats (STRs)
on each chromosome. The number of repeats at an
STR locus varies between individuals, and each
variation is a distinct allele
 Each person has two alleles for a particular STR, one
inherited from their mother and one from their father.
 This alleles may be different, making the person
heterozygous; or they may be identical making the
person homozygous.
DNA profiling - STRs
 At each STR locus, one individual is either homozygous or
heterozygous and so can have a maximum of just two different
alleles. These alleles are inherited in a Mendelian fashion.
 The figure below shows that a person who is heterozygous 5/7 at
one particular STR locus has one allele with 5 repeats and another
allele with 7 repeats.
 Within the gene pool of a population, however, many different
alleles can exist at each STR locus.
STR Profiling
 To produce a DNA profile, multiple copies of the alleles at these
nine STRs are simultaneously produced using the polymerase
chain reaction and the various alleles are then separated and made
visible with fluorescent dyes.
 The resulting DNA profile is a series of coloured peaks at different
locations, with each peak being one allele of one specific STR. The
location of each peak indicates the size of the allele and hence the
number of repeats.
 Where sizes overlap, alleles of different STRs are distinguished by
fluorescent labels of different colours.
Why use DNA profiling rather
than DNA fingerprinting?
 Compared with DNA fingerprinting, DNA profiling:
 is far more sensitive and requires smaller quantities of DNA (even a
pinhead sized spot of blood can provide sufficient DNA) and the
STRs can be amplified by the polymerase chain reaction (PCR)
 is based on alleles whose sizes allow fragments differing by just
one base pair to be distinguished
 is carried out in a much shorter time — hours rather than days
 uses several single-locus probes rather than one multi-locus probe
 uses coloured fluorescent labels to visualise the STRs rather than
radioactive labels so that each different STR allele can be identified
by colour as well as by size
 produces less complex patterns that are more easily interpreted
DNA Profiling in Australia
 All Australian states use a common method of
DNA profiling for forensic purposes that involves
nine STRs from different human chromosomes.
 These STR markers were chosen for this purpose
because they are reproducible and robust, easy to
score, are highly informative and have low
mutation rates.
 In addition, a tenth marker (that is not an STR) is
used to identify the gender of the individual. This
gender marker is the Amel locus that is present
on both the X chromosome and the Y
chromosome.
 The Amel gene on the X chromosome is just 107
base pairs long while that on the Y chromosome
contains 113 base pairs. As a result, the gender
of a person can be identified from this marker.
Loci currently used for DNA
profiling in Australia
For simplicity, STR loci that start with the letter D are identified by their chromosomal
location only, for example, D13 or D7. In reality, the naming of STRs is more complex
because there are multiple STR regions on the one chromosome and these two STRs
(D13 and D7) are formally identified as D13S317 and D7S820.
STR Profiling
 A person shows either
one or two peaks at
each STR loci, where a
peak corresponds to an
allele, depending on
whether the individual
is homozygous or
heterozygous at that
locus.
 For the Amel gender
marker, if just a single
peak with a size of 107
base pairs appears on
the profile, the person
is female; if two peaks
are detected, one at
107 and the second at
113 base pairs, then
the person is male.
This person is female.
They are heterozygous for loci D3, vWA, FGA, D18 and D7.
They are homozygous for loci D8, D21, D5 and D13.
O.J. Simpson capital murder case,1/95-9/95
 Odds of blood in Ford Bronco not being R. Goldman’s:
 6.5 billion to 1
 Odds of blood on socks in bedroom not being N. Brown-Simpson’s:
 8.5 billion to 1
 Odds of blood on glove not being from R. Goldman, N. Brown-Simpson, and O.J. Simpson:
 21.5 billion to 1
 Number of people on planet earth:
 6.7 billion
 Odds of being struck by lightning in the U.S.:
 2.8 million to 1
 Odds of winning the Illinois Big Game lottery:
 76 million to 1
 Odds of getting killed driving to the gas station to buy a lottery ticket
 4.5 million to 1
 Odds of seeing 3 albino deer at the same time:
 85 million to 1
 Odds of having quintuplets:
 85 million to 1
 Odds of being struck by a meteorite:
 10 trillion to 1