These include the following -

1. Linking a suspect to a crime scene
2. Establishing paternity or maternity
3. Proving a sibling or half sibling relationship
4. Determining genealogical roots or ethnic origin

The DNA testing methods employed vary and depend largely upon the types of results being looked for as well as the types of samples that are available.

Complex

The entire DNA chain is far too long to examine completely - human DNA, for example, has about 3.3 billion base pairs. However, most of the DNA sequence is common between humans. Therefore, only the smaller, variable part, is used within the testing framework.

The earliest forms of DNA testing required a sample of blood the size of a coin. As processes have evolved and developed over time the required sample size has shrunk considerably, such that a DNA sample can now be taken from traces of saliva on the back of a postage stamp that has been licked.

Before a DNA comparison can take place the DNA must be isolated and purified from the host sample, i.e. removed from the surrounding cell into which it has been placed. This is achieved by disolving the cell walls with a detergent, digesting proteins in the cell with enzymes. Following this the DNA can be purified, concentrated and tested.

Short Tandem Repeats (STR)

The most common DNA testing method today is STR - short tandem repeats.

Human DNA comprises several regions of repeated sequences. These are always located at the same place in each chain, however, they vary from individual to individual. Short tandem repeats are repeated sequences of two to five base pairs in length. Their use in DNA profiling has proven to be excellent with chances of misidentification being one in the billions.

The CODIS database set up by the FBI holds thirteen standard locations of the DNA chain per individual to aid in identification.

Incidentally, DNA testing is not as quick or as straightforward as the forensic shows on television portray it to be - testing typically takes as long as three to five days.

Forensic DNA testing specifically relates to using unique DNA marker information to tie an individual to a crime or scene, in much the same way that fingerprinting has done in the last century or so.

The advantages of using DNA testing are that DNA can be left in many places that fingerprints cannot, i.e. in sex crime cases.

The downside is that forensic DNA testing can prove expensive. In the above types of case the cost can easily be a few hundreds of pounds and upwards.

In murder cases the costs rise sharply, often into the thousands.

Obviously, fingerprinting as a means of identification is much swifter and cheaper. Therefore, forensic DNA testing tends to be reserved for the more complex cases.

Complex

Forensic DNA testing is much more complex than standard DNA testing as the samples being examined are often from crime scenes rather than those taken from a living host. As such, there is a high probability they have become degraded and therefore of limited or restricted use.

However, where good quality DNA information is obtained, it is invaluable - courts quite readily accept forensic DNA evidence as the chances of misidentification are next to none when it has been examined under the correct protocols.

CODIS

In 1992 the FBI in America launched CODIS - the Combined DNA Indexing System - a tool that is widely referenced in today’s popular forensic television programmes.

CODIS is a central database that contains DNA information from criminals from across all the States of America.

The value of this is that many offenders go on to repeat their crimes. Therefore, data from crime scenes can quickly be matched with CODIS in order to identify known criminals with ease.

Forensic DNA testing has revolutionised criminology in much the same way that fingerprinting did when it first became widely used.

Unsolved Cases

DNA information from unsolved crimes is retained and referenced as new DNA samples are taken from the populace in an attempt to close old cases.

Suspects have been convicted or acquitted based purely on forensic DNA in areas where there may have previously been an element of doubt as to their guilt or innocence.

Forensic DNA has even been applied retroactively to famous old cases where there has been public interest.

There have been examples of executed prisoners being exhumed for testing in order to either prove their guilt once and for all, or to open their way for a posthumous pardon.

An American technology expert said, ‘He didn’t use expensive equipment or a specialised laboratory - he used $10 of ingredients you could buy and whipped up his gummy fingers in the equivalent of a home kitchen.’

Jelly Babies

Despite the low level of technology required, the mixture, which is very similar to that used in jelly babies, successfully fooled 11 different commercial fingerprint scanners.

In an online presentation Tsutomu Matsumoto demonstrated two methods of creating a fake finger using his gelatin mix.

The first method was to use gelatin and molding plastic to create the fake fingerprint from an authorised source.

This process typically took less than an hour and yielded a success rate of between 70% and 95%.

Security experts, however, point out that Matsumoto’s technique requires access to the ‘victim’s’ finger in order to produce the jelly model and so therefore does not produce a large threat.

Access Undenied

The second method detailed by Matsumoto does pose some interesting questions however.

Using a microscope to lift fingerprints before using computer technology to enhance and clean up the image allowed Matsumoto to then print the resulting image onto a transparent sheet.

Said sheet is then used to expose a photosensitive printed circuit board which in turn is then etched in order to create fingerprint impressions in the board.

Finally, the gelatin mix is then poured over the etched print and allowed to cool which creates the gummy finger.

If this method is employed then it allows the operator to use any fingerprint left by someone on a variety of surfaces.

Forensic Criminology

Obviously, this could have repercussions of the world of forensic criminology where certain acts are not as time sensitive as using a fingerprint scanner, for example….

The appeal of fingerprinting is that it is an easy system to operate, is used widely around the globe and can 100% accurately identify an individual (contrary to popular opinion, fingerprints are not completely and utterly unique, though the exceptions are down to genetic quirks and are so rare as to be negligible).

Automated

Fingerprints are also easily stored on automated systems, allow quick searches and sharing of information between different bodies.

This has allowed law enforcement groups to have easy access to the fingerprints of possible suspects the world over.

It is estimated that around 5-15% of the world’s population has their fingerprints stored on record.

With computers able to trawl through massive amounts of data in seconds, this gives law enforcement officers a huge advantage in their investigations.

Other identification methods are judged against these qualities and so it would seem that fingerprinting is liable to be the identifier of choice for some time to come, at least until databases of dna become more populated.

Crime Scene

In cases where a full fingerprint is extracted from a crime scene the matching with a suspect, once in custody or prior if their prints are already on record, is a formality.

A full print in association with a set of ten prints taken from the target are rarely, if ever, questioned in a court of law.

Where partial prints are concerned there is obviously some room for error and so it is down to the investigator to then find as many markers on the print as possible to prove a match beyond reasonable doubt.

However, identifying individuals is not as easy a process as popular crime shows on television would have you believe.

13 DNA regions

To make a positive identification on one particular individual, forensic scientists scan 13 DNA regions that are known to vary between people. This data is then used to construct a DNA profile (or DNA fingerprint as it is more colloquially known). This amount of data is not enough in itself to produce a single result, however - there is a slim chance that this DNA data may be shared by a very small number of other people.

DNA profiling

There are a variety of uses for DNA profiling that will be covered here at ForensicsFAQ.

These include the following -

• Identifying victims of crime and accidents
• Matching suspects to crimes
• Clearing people previously accused or convicted of crimes
• Confirming paternity in cases where fatherhood is disputed
• Discovering good organ donor / recipient matches
• Identifying other non human organic items of evidence at crime scenes

Though it has been popularised in the last decade or two, it was actually known of in the 1800s.

Two scientists, Francis Crick and James Watson, won the Nobel Prize in 1962 for their work on the structure of DNA.

They had discovered the double helix design of DNA as well as base pairing in 1952.

Alec Jeffreys

Twenty or so years later, in the 1980s, a British geneticist by the name of Alec Jeffreys discovered that previously though strands of junk DNA were in fact unique to individuals (except identical twins).

He further discovered that these genetic markers remain unchanged throughout a person’s life and are present in every cell.

From this point onwards, DNA testing would become a reality.

So what excactly is DNA?

Essentially, DNA strands dictate who we are, our capabilities and our appearance.

DNA can be found in pretty well every living organism and consists of four basic blocks, known as bases.

These bases are chemical components known as nucleotides - they are commonly referred to by the first letter of each compound - A, T, G and C (Adenine, Thymine, Guanine and Cytosine).

DNA is a twin strand of such nucleotides. Each bond in just one way - A with T and G with C. These twin strands are known as base pairs.

DNA Fingerprint

Knowing what DNA is, and how it can identify individuals, Jeffreys created a process for displaying DNA data in a pattern form on x-ray film.

This can be used to create a unique “DNA fingerprint” which can then be used by criminologists to compare suspects to DNA samples discovered at crime scenes.

The first successful use of DNA fingerprinting led to the conviction of Colin Pitchfork in 1983.

Since then, DNA fingerprinting has become more widespread, being used not only in a number of criminal cases but in cases of paternity too.

Whilst it offers answers (sometimes unexpected, as in cases of white parents having a coloured baby), it also poses many questions too - here in the UK there is concern over the collection and storage of DNA data.

For instance, there are reports of schoolchildren having samples taken and people accused, but never charged with crimes, being DNA fingerprinted.

These have raised legitimate privacy concerns.