DNA – Deoxyribonucleic Acid
The analysis of the DNA is known as DNA Profiling or DNA typing. DNA fingerprinting is not the appropriate term. Forensic experts tend to shy away from the term DNA fingerprints for a number of reasons. Traditional fingerprinting has developed a reputation over years of use and trials of being a unique identification method and is so recognised by courts. DNA profiling is still developing and is not as recognised by courts. There are reasons for this.
Traditional fingerprints are unique – at least as far as anyone has ever been able to check while identical twins do have the same DNA. Also the accuracy of DNA profiling in matching to people depends on the technique used and particularly the number of loci checked. While this is getting better so the chances of a false match are very small, the courts have not yet accepted it as absolute proof of identity. DNA fingerprinting also happens to be the registered trademark of Cellmark, a biotechnology company.
DNA profiling is a biological tool which allows the scientist to compare samples of DNA material. With the exception of identical siblings, the DNA of each person is unique. That is why it is so valuable as a form of identification. DNA analysis can reveal the genetic profile of an individual and compare this with samples from a crime scene to determine whether or not he/she may be the source of the biological material.
DNA profiling is also commonly in use for paternity testing, usually to determine fatherhood of a child when this is disputed. It may also be used in helping to identify whether objects have been handled by, or belonged to, a missing person.
What is DNA?
The initials stand for “deoxyribonucleic acid”, found in the cells of all living things, including the human body. The DNA is a very long molecule and is found in the nucleus of cells.
Although each person’s DNA is unique (unless he/she has an identical twin), the techniques for identification only look at small parts of the DNA.
It is important to realise that a person will have the same DNA throughout their body in every cell with a nucleus. That is, the same DNA profiling results for one person will be obtained whether testing blood or semen or muscle tissue.
For what cases is DNA profiling used?
It depends on many factors. DNA profiling is typically carried out when human biological fluid or tissue is found at a crime scene and used as evidence to link to, or exclude from, that scene, a possible suspect. Some examples are:
- A murder where it appeared that a struggle took place and blood from the murderer was left at the scene. This would be compared to a sample of blood from the suspect.
- A murder where a blunt instrument was used and a suspect was found with a club on which there were dried blood stains. The blood on the club would be compared to the victim’s blood.
- A rape case where seminal fluid from the offender can be taken from the victim. This would be compared to a sample of blood from a suspect.
DNA profiling can only be used if there is enough DNA in the samples, and is only useful in comparing samples. Crime samples can be compared to a sample from a known suspect or compared to a database of DNA profiling results from other scenes or from convicted offenders. If there is a “match”, the two samples may have originated from the same person.
A recent development discovered in Victoria is that a DNA profile can be obtained from objects touched by skin, providing a powerful new tool for crime scene investigation.
However, the high sensitivity of this method means that extra caution must be taken when exhibits are handled. If a particular exhibit is handled by a number of people the DNA profiling results indicate a mixture; so interpretation is not always straightforward.
DNA is usually collected from suspects using mouth swabs. DNA may be collected from a crime scene from traces of blood, semen, saliva, hair roots or bones. Trace DNA may even be picked up from objects touched by the suspect, such as the inside of gloves. Once the sample is collected the DNA must be processed.
DNA analysis is a laboratory procedure that requires a number of steps. There are a number of techniques used by different laboratories. At the VFSC a process based on STR is used. The steps are:
- 1: Extract and purify DNA.
DNA must be recovered from the cells or tissues of the body. For instance the sample may be boiled with a detergent breaks down the proteins and other cellular material but not the DNA. Mechanical methods may also be used. The method varies slightly depending on the tissue involved. Hair and spermatozoa have particularly tough cell membranes.
- The DNA is then separated from the remains of the proteins and other cellular material. Only a small amount of tissue – like blood, hair, or skin – is needed. For example, the amount of DNA found at the root of one hair is usually sufficient.
- 2: Target and Amplify.
In the early methods of DNA profiling special enzymes called restriction enzymes were used to cut the DNA at specific places. For example, an enzyme called EcoR1, found in bacteria, will cut DNA only when the sequence GAATTC occurs. This required significant amounts of DNA.
- A new technique became available in 1991 using PCR. This allowed the copying of particular sequences of DNA. By adapting this process to copy particular areas of DNA called short tandem repeats (STR), DNA profiling could now work with very small samples or damaged samples to produce very sensitive and discriminating results.
- 3: Tagging.
In the PCR technique fluorescent tag are added to each sample to help with profiling in latter steps.
- 4: Sizing and sorting.
- The DNA pieces are sorted according to size by a sieving technique called electrophoresis. The DNA is poured into a gel, such as agarose, or onto a sheet that has been coated with this special gel. An electrical charge is applied to the gel, with the positive charge at the bottom and the negative charge at the top. Because DNA has a slightly negative charge, all the DNA will be attracted towards the bottom of the gel. However, the smaller pieces will be able to move more quickly and thus farther towards the bottom than the larger pieces. The different-sized pieces of DNA will therefore be separated by size, with the smaller pieces towards the bottom and the larger pieces towards the top.
- 5: DNA profile.
The final DNA profile is built by using several STR loci (5-10 or more) simultaneously. As the DNA moves through the gel, a laser excites the fluorescent tags and a picture is made of the pattern of bands representing the lengths of the STR is built up. The pattern resembles the bar codes used by grocery store scanners, though the use of coloured probes can make a quite pretty pattern.
How are results interpreted?
For each locus a database containing the frequency each type which occurs in the population. This is used to determine at what frequency a particular combination of types is expected to occur in the population.
When the results of all the loci are known, they could be something like this.
|LOCUS||DNA TYPE AT SCENE||DNA TYPE OF B. SMITH||WHAT % OF GENERAL POPULATION WOULD HAVE THESE RESULTS|
The percentages can be multiplied together and statistically corrected.
The results could then be written as-
“The donor of the blood sample in the bag labelled “B. SMITH” could not be excluded as the source of the biological material in the blood found at the scene.
This DNA evidence is 1 in 1 million (1,000,000) times more likely to have arisen if the scene sample came from “B. SMITH” than if it came from a random member of the Caucasian population.
In my opinion, in the absence of evidence to the contrary, this provides strong support to the proposition that the samples have the same source.”
Not every case will give a result of 1 in 1 million. The number will vary, depending on how much scene sample there is, and therefore, how many typings can be done and on how common are the DNA types that were found: the number may be smaller or larger. The reliability of matches will also change depending upon the sample size you match the DNA against.
The results of a DNA profiling with the current technology will never give results that claim that a sample could only have come from one particular person.
What do the DNA profiling results mean for a case?
DNA profiling does not claim to be absolute identification, but may be very strong evidence, and generally forms just one part of a case. It is really a question of looking at all the evidence in the case such as; who had the opportunity to commit the crime, eye-witness descriptions, fingerprints, the transfer of glass fragments, paint flakes or fibres linking a person to a crime and the DNA profiling results. DNA profiling is presented to the court as in the example above and the jury or magistrate can draw their own conclusions, as they do about all the evidence.
DNA profiling can be a very powerful investigative tool. Of the cases carried out so far, approximately fifty percent of the profiling results have established that the suspect was not the source of the sample associated with the crime – i.e. he/she was excluded as being the perpetrator of the crime.
However, prior to DNA profiling commencing, the case must first be processed. It must be submitted, prioritised and the individual articles must be examined by a scientist. This examination may include, depending on the circumstances of the case, analysis of hairs and fibres, identification of body fluids and analysis of blood stain patterns. DNA profiling may be just one component of the case.
When the laboratory work is completed a “Statement” suitable for presentation in Courts of Law is written. All the work on the case then undergoes a rigorous checking procedure before it is authorised as being completed. All of these processes add to the time involved in obtaining a profile from an individual sample. There is a lot more involved than just the extraction and profiling of the DNA.
Admissibility in Court
It is important to remember that in many cases DNA is not the only evidence, but some of the consequences of the DNA evidence range from charges against suspects being withdrawn, to defendants pleading guilty.
There have been no successful major challenges to DNA as being a valid technique in Victoria/Australia, though in some cases evidence has been challenged for procedural reasons. The reliability of the results is maintained by the stringent quality management program, which includes proficiency testing, validation studies and quality control procedures.
As part of the Australian CrimTrac system a national DNA database was developed and became operational in mid- 2001. Major issues in this system becoming operational included agreement to common testing and analysis procedures and privacy issues for the DNA profiles.
DNA profiling has attracted more attention, hype and speculation than any other forensic science tool of recent time. In many respects it reflects the impact fingerprinting had on crime investigation and some of the press for and against it is of a similar ilk. It is also linked to the threat people feel to their life styles and privacy of what is so fundamental to our nature – our genetic make-up.
|Base pairs||The building blocks of the DNA molecule|
|Deoxyribonucleic acid (DNA)||The genetic material that contains all the information that determines our inheritable characteristics|
|DNA profiling||The identification of variable characteristics at one or more loci in an individual’s DNA, and the comparison of those characteristics with other DNA samples to determine whether they could have a common origin|
|Locus||The site on a chromosome where a gene or other feature of the DNA is located|
|Multiplexing||The process of amplifying using the PCR reaction a number of different loci in the one tube|
|Polymerase chain reaction||The process of copying repeatedly a small piece of DNA until that area is present in larger, more detectable amounts|
|Probe||A fragment of DNA that will hybridise to another complementary sequence of DNA|
|Restriction fragment length polymorphism RFLP||The first method of DNA profiling used in casework in Victoria. It involves cutting up the DNA into small fragments, running them on a gel and applying a probe which binds to the DNA|
|Short tandem repeats STRs||A VNTR in which the repeated sequence is from 1 to 5 base pairs in length|
|Variable number tandem repeats VNTR||A defined region of DNA containing multiple copies of a short sequences of bases which are repeated a number of times, the number of repeats varying among individuals in the population|