DNA and Forensics

DNADeoxyribonucleic acid (DNA) is often called the genetic

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 IdentificationForensic 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 IdentificationIdentification using DNA is a powerful tool that can be

applied in many situations including:

forensic applications 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 Can the various remains that have been recovered be matched to a

particular person known to have been on-site?

identification of human remains 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.

mtDNAmtDNA 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 DNAUsed 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 DNADNA 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 FingerprintingWas developed as an identification tool in

1985 by Professor Sir Alec JeffreysInvolved 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 ProfilingWas 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 AustraliaAll 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