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From fingerprints to DNA

This article originally appeared in the October/November 2001 edition of Helix.

In the bedroom, a man in his fifties lies dead on the floor next to a bed with a broken glass bottle nearby; reddish-brown stains on the walls, a red stain on the carpet and the bathroom window smashed. What's been going on - accident or foul play?  Roger Beckmann explores the role of forensic science in solving crime.


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This is the sort of puzzle that could confront a crime scene investigator (CSI) from the police force. Crime scene investigators, often referred to as 'scientific police', are one part of the forensic team whose skills are essential in modern police investigation.

In the fictitious scene just described, police must determine whether or not a crime has occurred. The man may have died of natural causes, the stains could be from spilt red wine, and a cricket ball may have smashed the bathroom window weeks ago - or even after the death.

To put the pieces together, a team effort is required. Police officers will interview neighbours and gather information about the dead person. A doctor, or forensic pathologist, may be called to examine the body. The police forensic team must look for clues as to what occurred, and try to reconstruct a possible sequence of events.

The science of crime

Every day, many crimes are committed in Australia. Science has become an essential tool for solving these crimes.

Originally, the word 'forensic' meant anything relating to a law court. But today it refers to a whole new subject. Forensics or forensic science means using science to solve crime. But what exactly does a forensic scientist do?

At the scene of the crime, the crime scene investigators will thoroughly examine the area. They will take detailed notes and photographs, look for fingerprints, palm prints and sole prints; marks of tools and weapons; marks from shoes; fibres from clothing or material; fragments of paint and glass and body fluids.

Detecting fingerprints using a monochromatic light
Detecting fingerprints using a monochromatic light. Source: Australian Federal Police

Increasingly, CSIs are using specialised techniques, on site, to determine which areas to concentrate their investigation on. For example, they may use luminol, which detects traces of blood by reacting with the iron in the haemoglobin molecules that are within red blood cells. Luminol glows when it comes into contact with blood. Sometimes it reveals small traces of blood that would normally be invisible.

The presence of blood, however, does not mean it is from a human, or even that it had anything to do with a crime. After all, people do occasionally cut themselves, which could leave a tiny drop of blood on the carpet.


Is it like the TV shows?

Forensic scientists are quick to point out that TV dramas about forensic science are inaccurate.

The most common misconception is that most forensic work is involved with murders. Instead, it is involved with house burglaries, drug offences, fires, and vehicle accidents.

Secondly, it is not the forensic experts' job to confirm what police investigators would like to hear. Forensic science, like any form of science, involves having an open mind and being impartial. The crime scene investigator or the lab scientist cannot afford to hang on to pet theories or preferred suspects. He or she must work to uncover facts that can be used as evidence. Quite often, these facts may serve to rule out many of the suspects that the police have.

Analysis of objects determine if they were used in a crime
Analysis of objects determine if they were used in a crime. Source: Australian Federal Police

Finally, TV dramas tend to roll all forensic work into one person's responsibilities. In real life, forensic scientists are specialists. No single person possesses all the knowledge and expertise of every field of forensics. Solving crimes is very much teamwork, and modern teams are large.

Answers from the dead

Pathology is the study of disease, and cause of death. A forensic pathologist specialises in examining dead bodies to determine how and when death occurred.

As well as examining the body, either on site or through photos taken at the scene, the forensic pathologist may also use x-ray imaging and will usually conduct an autopsy or post-mortem. A post-mortem is a careful dissection of the corpse, which comes from the Latin language meaning after-death. The pathologist will also take samples of body tissues, like blood, liver or hair, for further analysis.

One of the first tasks with a body on site is to establish the time of death. The pathologist starts with temperature. The normal temperature inside a human body is 37oC. A pathologist will take the temperature inside the body, as well as the temperature in the place where the body was found. The rate at which body temperature falls after death depends on the external temperature, the clothing worn, the size of the body, and its percentage of fat.

After 12 hours, most dead bodies are the same temperature as their surroundings, therefore other methods are required for determining the time of death. There are many gradual changes that take place after death. These occur in a particular sequence, concluding with total decomposition. Skilled pathologists can usually use these changes to assess, roughly, when death occurred.

After the dead body has been examined at the scene, it is wrapped for transport. This is done very carefully because small details - like fragments of skin, hair or blood caught under a victim's fingernails during a struggle - may provide crucial evidence, linking a suspect to the crime. The remnants of explosive material from a gunshot may also be present on the skin, for example, near a bullet hole. Such residues, and a bullet wound itself can give information about the type of bullet used, as well as the distance and direction from which it was fired.

At the autopsy, pathologists start with carefully looking at the body and its clothing. They then examine the skin - looking for cuts, scratches, stabs, wounds or injection needle marks - and the nails.

Secrets from within

Although a body does not always show marks on the outside, there is often evidence inside that will be revealed during the autopsy. For example, a large blood clot within a major artery or vein, or even in the heart itself, may be a cause of natural death, which would rule out foul play.

Samples of a person's stomach contents can reveal the last food eaten. Blood analysis will show the existence of any poisons, illegal drugs or medicines. Urine in the bladder may also be analysed.


Print precision

Analysis of fingerprints is probably the most well known use of forensic science. Each fingertip has a pattern of fine skin ridges that are slightly different for every person - even identical twins.

Everywhere we go, we leave a telltale sign of our presence
Everywhere we go, we leave a telltale sign of our presence. Source: Australian Federal Police

A fingerprint is composed of grease and dried sweat left behind by the tips of the fingers. The palms of the hand also leave identifiable prints, as do the soles of the feet.

Fingerprints can be detected on a vast range of different surfaces using a variety of techniques. The police keep a huge national database of prints taken from charged criminals. New prints are taken by a laser-scanning procedure, where the hand is placed on a flat glass plate, and its print is stored and compared to other prints electronically.

DNA Fingerprinting

A new form of identification relies on DNA, which carries the genetic information of each person. Everyone's DNA is different (except for identical twins). DNA profiling or typing is sometimes called DNA fingerprinting because it allows police to identify an individual in the same way as fingerprints do. DNA can be extracted from any body fluid (blood, saliva, sweat, nasal mucus etc) or from fragments of a body (hair roots, torn skin or flesh).

DNA fingerprinting
DNA fingerprinting has become a powerful tool in helping solve crime. Source: Australian Federal Police

Forensic scientists do not look at the whole of a person's DNA sequence, but rather a sub-set of a DNA profile. DNA profiles are a very powerful means of determining whether two or more samples may or may not have come from the same person. If DNA profiles do not match, they came from different people. However, if they do match, there is still a very slight chance that they may have come from different people.

DNA is the same in every cell of the body, and stays the same throughout life. As such, DNA profiles taken at different times and places can be compared in order to determine whether or not they come from the same person.

DNA analysis does not enable scientists to build up a picture of a person from their DNA. The only characteristic that the DNA tells us is the sex of the person. DNA profiling is only used to compare different DNA samples, and to determine whether or not they could be from the same person.


Correlation is not proof

As in all areas of science, evidence from forensic investigation does not mean proof. A correlation between two things does not mean that one causes the other.

Searching for evidence
Searching for evidence that may help solve the crime. Source: Australian Federal Police

For example, you might observe that most people involved in car crashes wear seat belts, but this doesn't mean that seat belts cause crashes. Your observation is correct, but you cannot use it as evidence for the cause of accidents. You need to make further observations - for example, of people in cars who are not involved in crashes.

Pieces of evidence may not always agree. A fibre found at a murder scene may match a male suspect's jacket, but other evidence, such as DNA found at the scene, may belong to someone else. There are a number of possibilities. The suspect may have:

  • committed the murder;
  • been present at the crime, without committing the offence;
  • been present at the scene innocently or suspiciously before the crime occurred;
  • arrived after the crime and left in fright; or,
  • been nowhere near the scene and his jacket was used by someone else with or without his knowledge.

Therefore, all evidence must be taken together. Very rarely is one piece of evidence conclusive proof.

The author would like to thank the Australian Federal Police Forensic Services, in particular Mr Karl Kent, for generous assistance with this article.

Who Stole the Minister's Malibu? competition

ABC Sites

Forensic Dentistry bites back - and is a picture worth a thousand words? - The Law Report 2/5/2000

Expert Witness - Four Corners 20/10/2001

Peruvian Mummy - Catalyst 14/2/2002

Bite Mark - Catalyst 16/8/2001

Forensic maggots - Science Show 13/6/1998

Forensic Pigs - Quantum 20/5/1999

Dating Death - Great Moments in Science 1998

More Information

National Institute of Forensic Science

Australian Federal Police Forensic Services

The International Insititue of Forensic Studies

Victorian Institute of Forensic Medicine

Victoria Forensic Science Centre

Department of Forensic Medicine (NSW)

Queensland Health, Pathology and Scientific Services

Westmead Hospital and Community Health Services - Forensic Research Development Unit

The Forensic Science Society (UK)

Initial Findings Upon Examination and Autopsy of the body of an Apparent Alien Creature

16th Annual International Symposium on the Forensic Sciences

People Against Fingerprinting

Tara Moss is a forensic crime writer. The Crime Library and Cybersleuths contain information that is useful for those interested in crime research and history.


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Published 7/3/2002
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