How long does it take to make a DNA profile?
Roughly speaking, once ESR has received a biological sample, there are five steps that need to be performed to generate a DNA profile: screening and sampling of the sample, extraction of the DNA, DNA quantitation, PCR amplification and electrophoresis. Each of these processes can take anywhere from a few hours to a few days to perform. Once the DNA profile has been generated it needs to be analysed and the results interpreted. Depending on the complexity of the profiles and the case involved, it may take anywhere from a few days to several weeks before the results for all the profiles in a case are analysed and the results incorporated into a statement.
Can you say that a DNA sample has definitely come from a certain individual?
It is not possible to test all of an individual's DNA, only small portions of it. As a result, it is not possible to definitively state that an unknown DNA profile came from a given individual, only whether or not it is possible. When performing a DNA analysis, ESR assesses how common each allele seen is in the general population, and makes a statistical assessment of the likelihood of finding this combination of alleles in a person other than the suspect. Using these methods, a matching DNA profile generally indicates a very high probability that an unknown sample came from the individual that it matches, but because of the small chance that another person shares that profile with the person in question it is never possible to definitively state this.
What is the chance of DNA identifying the wrong person?
With the DNA profiling system that ESR currently uses, the chance of a person being incorrectly linked to a crime because their DNA just happens to match DNA found at the crime scene is very small. The exact chance of this happening depends on the specific alleles seen in the DNA profiles in question, as some alleles can be more or less common than others. Forensic scientists can calculate the chance of a specific DNA profile being randomly found in the population and the probability of a given person having this profile is typically very low - in the order of one in several billion. Despite this low probability, there still exists a possibility that a person can be linked to a crime scene purely by chance and has had nothing to do with the crime at all. Because of this risk, a forensic scientist must always take into account all possible explanations for the presence of a piece of evidence when preparing testimony to be presented in court.
Can DNA evidence tell if a person is innocent or guilty?
No. Forensic DNA analysts only consider the question of whether or not an unknown DNA profile from a crime scene matches ndividual profiles. While this can link a person to a crime with a high degree of probability, it says nothing regarding the question of a person's guilt or innocence. A person considered to be a suspect in a case could have been present at the crime scene for entirely legitimate reasons, for example. Whether or not a suspect is guilty normally depends on a large number of factors, and is a question that should be considered only by members of a jury, who are presented with all of the relevant information in a case.
Isn't DNA the same between members of the same family?
All humans have 23 pairs of chromosomes, one set inherited from the father, the other from the mother. Exactly which chromosomes are passed from parent to child is random however, and there are literally billions and billions of combinations of chromosomes that are possible from just one set of parents. As a result, although people will have a tendency to share alleles with people they are related to, individuals will in general have DNA that can be distinguished from that of their close relatives. The only exception to this is the case of identical twins who result from the splitting of a single fertilised egg, and as a result share the same set of chromosomes and the same DNA profile.
Can ESR tell anything about a person's appearance or health by looking at their DNA?
Whilst medical research has identified many genetic abnormalities in DNA that can result in a person being afflicted with various diseases and disorders, the sections of DNA that can contain these abnormalities are entirely different to those focused on by forensic DNA analysts. Forensic scientists make use of the vast areas of 'non-coding' DNA that humans have. This is DNA that does not contain any genes (genetic code that is used to programme proteins) but is useful to the forensic scientists because it tends to vary between people more than the coding areas of DNA do. Non-coding DNA allows people to be told apart from each other, but tells a scientist nothing about the functionality of a person's body.
DNA tests currently employed can detect gross chromosomal abnormalities such as Down's Syndrome, but not anything about health issues caused by changes to specific genes. In some cases the likely ethnic ancestry of a person can be partially indicated although this feature may not be evident from physical characteristics and is therefore of limited value.
Can you see DNA under a microscope?
No, at least not in a way that is useful to forensic scientists. It is possible to see chromosomes under a microscope and as chromosomes are the carrier of genetic information in a cell, and are comprised of large coils of DNA, it is possible in this sense to see DNA under a microscope. However, even at the most powerful levels of magnification possible, scientists are still unable to see a DNA strand clearly enough for any of the details that allow individuals to be told apart by their DNA to be seen.
Is it possible to tell the ethnic origin of a person from their DNA profile?
Particular DNA alleles may be more common in one ethnic group than another which may give some indication of a person's ethnic group, however there is considerable overlap between races so it is not yet possible to say that a DNA profile must have come from a member of a particular ethnic group. Further complicating this is the fact that a person will not necessarily look as if they are a member of the ethnic group that is indicated by their heritage and their DNA. Attempts are being made by some international laboratories to develop methods to accurately determine a person's race from their DNA but these techniques are not currently used by ESR.
Do blood, saliva, semen. etc from the same person give the same DNA profile?
Yes. A person's DNA is the same throughout their entire body, and so will be the same in all body fluids. If, for example, the DNA profile gained from a blood sample does not match the DNA profile obtained from semen, these two samples must have come from different individuals.
What is a likelihood ratio?
For a piece of DNA evidence that matches a known suspect, forensic scientists consider two hypotheses, known as the prosecution hypothesis and the defence hypothesis. The prosecution hypothesis is that the DNA evidence came from the suspect, who left it at the crime scene; while the defence hypothesis is that the evidence didn't come from the suspect, but from another unrelated person in the wider population. The likelihood ratio is a statistical measure that considers the relative probabilities of these two hypotheses being true, and in doing so gives an indication of the value of a piece of evidence. The likelihood ratio is equal to the probability of seeing a piece of evidence given that the prosecution hypothesis is true, divided by the probability of seeing the same evidence given that the defence hypothesis is true. As a result, the larger the likelihood ratio the stronger the evidence is. DNA evidence can often result in likelihood ratios of one billion or more, which means that the evidence seen is a billion times more likely given the prosecution hypothesis than the defence hypothesis.
Does the likelihood ratio change if further tests are carried out?
The likelihood ratio can increase if further DNA tests are carried out on the same samples. This is because each additional test gives new information that can then be multiplied by the original results. The more tests that match a given individual, the rarer the combined DNA profile will be and the higher the likelihood ratio. It is also possible that extra DNA tests may exclude a suspect who has matched on earlier tests. This is always possible, even though it may be unlikely.
Is it possible to say if DNA has come from a man or a woman?
Yes. The SGM Plus multiplex that is used by ESR to generate DNA profiles contains a DNA sex test based on a gene that is found in slightly different forms in males and females. This gene is called Amelogenin and is found on the sex determining X and Y chromosomes. The version of the Amelogenin gene on the X chromosome is slightly smaller than the version on the Y, and because men have an X and a Y chromosome and women have two X chromosomes, the two sexes can be told apart by their DNA.
How long are profiles left on the DNA Databank? Why would you destroy a Databank sample?
All National DNA Databank samples, including the biological material provided by the donor and any sub-samples made during its analysis, are destroyed after being held by ESR for three months. This is done after a DNA profile has been generated from each sample however, and the DNA profiles are stored indefinitely in electronic form on the National DNA Database and are not typically removed. Databank samples can be destroyed and profiles removed from the NDD however, if the sample was taken with voluntary consent from the donor and that consent is withdrawn.
Is the blood taken from newborn babies added to the DNA Databank?
No. Only profiles from samples taken under the provisions of the Criminal Investigations (Bodily Samples) Act 1995 can be added to the DNA Databank. This Act does not in any way allow for the blood samples taken from newborn babies to be added to the DNA Databank.