Forensic medicine is the branch of medicine concerned with the resolution of legal issues by the application of scientific medical knowledge. An important development in forensic medicine is genetic analysis, ie DNA profiling.

THE concept of “DNA fingerprinting” was introduced in 1985 as a method in the evaluation of human identification and relatedness. The forensic applications of DNA evidence were rapidly appreciated and are now often used in criminal investigations, for example, murder and sexual assaults like rape.

DNA evidence is also used in paternity and inheritance disputes as well as in the identification of victims in disasters. Such cases, especially those involving the rich and famous, are widely reported in the print and electronic media. Television and films almost always emphasise the singular role that DNA profiling plays in forensic examination, so much so that the common perception is that DNA always provides the answers and is conclusive.

It is hoped that this article will provide a balanced perspective on the role of DNA in forensic examinations. Although it focuses on sexual assaults, it is also applicable to other criminal investigations.

Forensic examination

A forensic examination is a detailed physical examination of the victim to document injuries and other evidence. Forensic evidence includes any combination of hair, saliva, semen, blood, urine and other evidence, for example, clothing on the body.

Each type of evidence may have specific relevance within the context of the assault in question. As such, the examiner needs an accurate account of the assault and the circumstances it took place in order to conduct a proper examination. This information may be made available by a police officer or the victim.

The forensic examination should be carried out promptly by a trained doctor, ideally in a forensically secure environment to avoid DNA contamination.

DNA evidence is unlikely to be found in women later than seven days after the sexual assault or three days in the case of men and children, but examination for injuries may still be useful.

There are some unique features in forensic examinations. The doctor has two duties i.e. to address the immediate needs and concerns of the victim and the need for rigorous evidence collection.

This dual function has to be understood by all parties. Whilst both functions can often be combined relatively seamlessly, there may be conflicts for the complainant and/or the doctor.


DNA is an abbreviation for deoxyribonucleic acid. It is found in almost every cell in the body and carries genetic information that is passed from one generation to another. The genetic information is in the form of a chemical code.

DNA has a double helix supported by four bases i.e. the purines, adenine and cytosine, and the pyrimidines, guanine and thymine. Adenine always pairs with thymine, and cystosine with guanine. The sequences of the base pairings determine an individual’s physical characteristics, for example, hair and eye colour, and the predisposition to inherited conditions, for example, thalassaemia.

Although 99.9% of human DNA is identical, each individual’s DNA is unique. Only identical twins have exactly the same pattern. Everyone inherits half their DNA from either parent. Siblings i.e. brothers or sisters inherit different combinations of DNA from the same parents and as such, are different from each other. However, they can have similar characteristics as some of the inherited DNA combinations are the same.

DNA can be obtained from any cell that contains a nucleus, for example, blood, semen, saliva or hair samples.

DNA profiling

The DNA profiling process involves:

·Chemical removal of the DNA from the sample,

·Separation of the two strands of the DNA helix,

·Enzymatic extraction of particular sequences producing a mixture of free DNA lengths,

·Sorting of the DNA sequences according to length,

·Addition of radioactive chemicals to the free DNA lengths,

·Photographing the free DNA lengths which emit x-rays that show up the sequences.

This produces the DNA fingerprint which appears as a series of dark lines that correspond to the sequences present.

The process is repeated several times with different enzymes. Provided a sufficient number of DNA sequences are used and there is sufficient variation of the sequences in a population, the unique profile of an individual can be obtained.

DNA profiling does not currently permit the examination of every single difference between the DNA of different people. However, techniques are available that look at specific areas of nuclear DNA known to vary widely between people. These DNA areas (loci) vary in length between different people. In DNA profiling, these loci are analysed and measured, using several techniques.

Although no two individuals can be found to have the same DNA profile, this cannot be excluded conclusively. That is why scientists’ reports contain the probability of a certain DNA profile matching a randomly selected person.

DNA profiling has, in general, helped in the conviction of many criminals and the clearing of the names of many innocent suspects. However, if there is an absolute reliance on DNA profiling in criminal investigations, the risk of a miscarriage of justice increases, which is obvious as one continues reading this article.

Forensic specimen collection techniques

The World Health Organization, in its publication, Guidelines for medico-legal care for victims of sexual violence, advise strict adherence to the following principles when collecting specimens for forensic analysis:

·Avoid contamination. Ensure that specimens are not contaminated by other materials. Wear gloves at all times. Modern DNA assay systems are very sensitive and may detect small amounts of extraneous material.

·Collect early. Try to collect forensic specimens as soon as possible. The likelihood of collecting evidentiary material decreases with the passing of time. Ideally, specimens should be collected within 24 hours of the assault; after 72 hours, yields are reduced considerably.

·Handle appropriately. Ensure that specimens are packed, stored and transported correctly. Analytical laboratories should be able to provide guidance on special requirements for specimen handling and storage. As a general rule, fluids should be refrigerated; anything else should be kept dry.

·Label accurately. All specimens must be clearly labelled with the patient’s name and date of birth, the health worker’s name, the type of specimen, and the date and time of collection.

·Ensure security. Specimens should be packed to ensure that they are secure and tamper proof. Only authorised people should be entrusted with specimens.

·Maintain continuity. Once a specimen has been collected, its subsequent handling should be recorded. Details of the transfer of the specimen between individuals should also be recorded. It is advisable to check with local authorities regarding the protocols for the recording of such information.

·Document collection. It is good practice to compile an itemised list in the patient’s medical notes or reports of all specimens collected and details of when, and to whom, they were transferred.

When there is limited or non compliance with the above principles, the value of DNA profiling in sexual assaults becomes more limited. Some of the common problems are discussed below.

No evidence

There may be no evidence in sexual assaults for a variety of reasons, for example:

·A condom may have been used;

·Penetration may not have taken place;

·Ejaculation may not have occurred;

·The evidence may have been washed away by the victim; and

·The evidence may have degraded with the passage of time.

Loss of evidence

There are several ways which lead to the loss of evidence, for example:

·Some seminal fluid components can degrade within body orifices,

·Semen can drain from the vagina or wash from the mouth,

·Sperm can lose motility,

·Body fluids can get washed away, and

·Dried secretions and foreign materials can fall from the body and clothing.

Prompt forensic examination is essential to avoid loss of evidence. It also helps in the early identification of the victim’s medical needs and concerns.

Time frame for evidence collection

The time frame for the collection of evidence in sexual assault is 72 hours. However, there are reports that some evidence may still be available after this time, for example, sperm may be found inside the cervix after 72 hours and traces of certain drugs may be found in the urine up to 96 hours after ingestion.

Other examples of evidence being found even after considerable periods of time include victims’ complaints of pain or bleeding or spotting visible injuries, and where there is a history of significant trauma from the assault.

Evidence collection outside time limits can vary due to factors such as the location of the evidence or type of sample collected.

The extension of time limits has been made possible because of the advances in DNA profiling technologies. These technologies have made it possible for scientists to analyse stored evidence from crimes that occurred years ago and to carry out DNA profiling from semen samples that contain very few or no sperm cells. As such, the importance of collecting all possible evidence cannot be over-emphasised.

Evidence preservation

Evidence is useless unless it is preserved. The transit time between evidence collection and storage of the evidence kits has to be minimal.

In order to avoid potential degradation of evidence, it is vital to transport kits containing liquid samples and other wet evidence in a timely fashion.

There has to be secure storage sites and the storage requirements should be consistent. The storage requirements depend on the specimen types collected, for example, evidence kits without blood or other wet evidence generally do not need refrigeration, unlike blood and other wet evidence.

Dried blood samples on collection cards do not require refrigeration. Urine needs to be refrigerated or frozen when stored.

All those involved in evidence collection and storage have to strictly adhere to the prescribed collection and storage requirements. They also have to be conversant with the specifics of the procedures and their responsibilities.

Evidential value of forensic examinations

It has to be always borne in mind that forensic examination is just one aspect of the total evidence in a criminal investigation. Its value has to be considered in the light of other evidence. This is succinctly put by two academics and a judge below.

Liz Kelly and Linda Regan of the Child and Woman Abuse Studies Unit, London Metropolitan University, in their publication Good Practice in Medical Responses to Recently Reported Rape, Especially Forensic Examinations stated:

“Whilst for children a forensic examination can establish that a sexual assault has taken place, since consent is not an issue, this is not the case for adults (or adolescents). Where the assault is by a stranger, there is a small chance that DNA evidence may identify them, and where the accused denies sexual contact, the same evidence may prove that it took place.

“In most cases, however, the defence is likely to be one of consent and all forensic evidence can do here is support, but not prove, the account of the complainant.”

Lord Hope of Craighead, in a case decided by the Privy Council of the United Kingdom viz: Pringle v. R (Jamaica) [2003] UKPC 9 (January 27, 2003), stated:

“A valuable description of the process of DNA profiling and of the procedure which should be adopted where use is made of DNA evidence was given by Phillips LJ in R v Doheny [1997] 1 Cr App R 369. As he pointed out, the cogency of this evidence makes it particularly important that DNA testing is rigorously conducted so as to obviate the risk of contamination in the laboratory and that the method of analysis and the basis of the statistical calculation should be transparent to the defence so far as possible.

“It is just as important that the true import of the conclusion that results from this exercise is explained to the jury as accurately and fairly as possible, and the jury are likely to need careful directions about the approach which they should take to this evidence in the summing up?

“Let it be assumed that the evidence about the random occurrence ratio is that one person in 50,000 has a DNA profile which matches that which was obtained from the crime scene. The fact that the defendant has that profile tells us that he is one of perhaps fifty thousand people who share that characteristic.

“One can then say, having regard to the population of the area, what the statistical probability is that he was the perpetrator. But that is all that can be said about it. The question whether the statistic points to the defendant as the actual perpetrator will depend on what else is known about him.

“If it is plain from the other evidence that he could not have committed the crime because he was elsewhere at the time, the fact that the defendant’s DNA profile matches that on the sample taken from the crime scene cannot be said to show that he did commit it.

“That proposition will have been negatived by the other evidence. So the probative effect of the DNA evidence must depend on the question whether there is some other evidence which can demonstrate its significance. And it is for the jury, not the person who gives the DNA evidence, to assess its significance in the light of that other evidence.”


When used in accordance with the principles of the collection of samples for forensic analysis, DNA plays an important role in forensic examinations.

However, it is not the magic bullet that will solve crimes. Other evidence is just as important or even more important, depending on the circumstances.

The principles and statistics of DNA profiling have to be understood by all parties concerned i.e. the complainant, suspect and those involved in the investigatory and judicial process.

Source: The Star Online (Dr.Milton Lum)