December 12 2024
To mark 45 years in business, we asked Mike Appleby, a forensic biologist with as many years’ experience in the industry, to guide us through the development of blood, body fluids and DNA analysis during that time.
Mike started working for the Forensic Science Service (FSS) in 1980 in the biology section, specifically the ‘blood room’. That description might conjure up a mental image more akin to a crime scene, but we’re assured it was a relatively clean space!
Blood grouping, both serological and biochemical, was a mainstay of the investigation of cases involving ‘Offences Against the Person’, as they used to be called. Blood grouping could be used in cases where body fluids such as blood, semen and saliva had been shed or deposited during a crime.
In retrospect, the discriminatory power of blood grouping was limited – a statistical figures of 1 in 1000 was considered a notable result back then. The sensitivity of blood grouping was also, judged by current standards, poor, and contamination issues were sometimes problematic. If mixtures of body fluids were encountered, an interpretation of the findings was often impossible.
DNA analysis started to be investigated by the FSS in 1986, with a gradual introduction across the 6 FSS laboratories and the Metropolitan Police laboratory in 1987/88. The initial techniques used did not involve the DNA copying technique we refer to as Polymerase Chain Reaction (PCR). Hence, they required relatively large amounts of DNA/cellular material and relied on any degradation of DNA being limited – what was in the stain of interest was all you had to play with.
A quantum shift in DNA analysis occurred with the introduction of PCR-based DNA profiling, targeting what are called ‘Short Tandem Repeats’ (STRs) of DNA spread throughout the human genome. The PCR approach allows specific areas of DNA that contain STRs to be targeted and copied. This has opened up an ever-increasing number of applications for DNA analysis in criminal investigations. Whereas a relatively rich source of DNA, such as blood or semen, was required to obtain a DNA profile using the early techniques, PCR use enables DNA/cellular deposits containing very low levels of DNA to be analysed. ‘Touch DNA’ analysis was born: if a person handled a surface with bare hands, it became entirely possible to obtain a DNA profile that provided evidence to support the assertion that handling took place.
STR-containing areas of DNA do not ‘code’ for anything – i.e. they are not used to make proteins used in the body. They appear to be effectively ‘spacers’ between areas of the DNA molecules that carry the code for all the chemicals required to make a human. Because of this, they can alter slightly, i.e. change in length due to random mutations, and not have a deleterious effect on the body. Due to a build up over time of these length variations, each individual’s DNA profile, depending on how many areas of DNA are analysed, is highly likely to be unique.
Due to the possibility that separate individuals could, theoretically, share the same profile, the probability of a chance match has to be assessed. Hence the use of the phrases ‘1 in a billion’, or ‘a billion times more likely’, when matching profiles are obtained. The billion figure is an upper bound used in the UK as a catch-all safety approach. In the US and elsewhere, the ‘true’ figures generated during a statistical assessment are given – these often being in the quadrillions (10 to the power 15).
Another major change in DNA profiling over the years is the interpretation of profiles. Early DNA techniques analysed a relatively limited number of DNA areas and the profiles obtained could be statistically assessed, to place an objective ‘level of support’ on the significance of a ‘match’, using simple statistical approaches. Even if this was not possible, the interpreting scientist was allowed to use their experience in DNA profiling to subjectively interpret indicated matches, even in instances where mixed DNA profiles were obtained. (A ‘mixed’ profile indicates the presence of DNA from more than one person.) It should be noted that, due to the relative insensitivity of early DNA techniques, and the requirement for what nowadays would be called very high levels of DNA, mixed profiles were less commonly obtained.
Current PCR-based DNA profiling techniques are many orders of magnitude more sensitive and hence require less DNA many orders of magnitude less. Due to this, mixed profiles are now largely expected unless analysing an identifiable body fluid, such as blood, which contains a raised level of DNA.
Consider now this increase in sensitivity, the proliferation of mixed DNA profiles, and ‘touch DNA’… Touching a surface, especially if only on a single occasion, often results in very limited amounts of DNA being deposited. If the surface of interest has been touched by a number of people, it may bear limited deposits of DNA from all those people who have touched it. Any resultant profile is likely to be low level and mixed – what is its significance? Attempting to interpret such profiles using only one’s experience can be extremely difficult and prone to error. For example, in many low level, mixed profiles, the contributors’ full (as opposed to incomplete) profiles may not be detected. Does that mean they have not contributed, or just that such a low level of their DNA is present that a complete profile cannot be obtained?
In recent years, a number of statistical software packages have been developed to address the issue of mixed DNA profiles. The complexity of the assessments carried out by the various software packages is, from the perspective of a DNA scientist rather than a statistician, truly astounding. However, the requirement for them became essential (in England and Wales, mandatory), not only due to the increasing complexity and ubiquity of mixed DNA profiles, but also as a result of guidelines published by the Forensic Science Regulator in which carrying out a statistical assessment of the significance of matching DNA profiles was stipulated. The reason was because accurate assessments, based on experience of the interpreting scientist, became extremely problematic. Often, the outcome could be “I just don’t know”.
Another consequence of the increased sensitivity of DNA profiling techniques is that compelling evidence can now be obtained to place vanishingly small amounts of DNA from a named individual on a surface or object. This raises the question, how did the DNA get there?
Laboratory-based experimentation has shown that DNA can be transferred to one surface directly from a person, then it can be re-transferred by another person or from the surface on which it was initially deposited, to another surface. This is referred to as indirect or secondary DNA transfer and means that the specific location of any recovered DNA, especially if present at a very low level, must be considered in the context of the circumstances of the specific case being investigated.
The need for DNA evidence assessments to be case-specific has led to the introduction, or at least the formalisation of, a structured approach to the overall interpretation of DNA evidence. This involves the use of opposing propositions, often termed the Prosecution and Defence hypotheses. The simplest example of this is when calculating the chance of two people sharing the same DNA profile. If a blood stain yields a complete, non-mixed profile matching one person, the propositions are:
The first proposition has a probability of 1 as we know the profile matches that of Mr A. The second proposition has a probability based on how likely another person in the general population would be to share the same profile. Given the variable nature of DNA profiles, this probability is vanishingly small and hence the outcome is commonly “the chance of obtaining a matching profile, if the blood did not originate from Mr A, would be in excess of 1 billion”. This is often termed a ‘source level’ interpretation as it addresses only the likely source, i.e. which person the DNA is likely to have originated from.
The next level of interpretation relates to whether the DNA can be attributed to a specific cellular source, such as semen or blood. This level is mostly informed by chemical testing and visual examination, in respect of blood, and chemical testing and microscopic identification of the presence of sperm cells (spermatozoa). If these body fluids are shown to be present, a DNA profile can be attributed to a specific cellular source.
The next stage of interpretation involves assessing whether the activity that led to the deposition of DNA/cellular material can be inferred, and this again often uses opposing propositions as outlined above.
As an example of this, take the case of a very low level of DNA, not attributable to any specific cellular source (there are currently no tests that identify cells that originate from the surface of the skin), found on an internal door handle at a burglary, which is linked to the person accused of the burglary. Again, we’ll refer to him as Mr A. The use of opposing propositions can be employed to hopefully derive an objective level of support for or against one or other of the propositions, these being again the Prosecution and Defence. For example:
The outcome of this simple comparison of probabilities would provide support for the prosecution. However, consider that Mr A says that he is a builder and carried out work at the burgled property a couple of weeks before the burglary. He has a legitimate explanation for the presence of a low level of his DNA on a door handle at the property. Based on this proposition, the outcome of the assessment of the probability of each proposition being accurate would most probably be balanced, providing no particular support for either.
Such assessments of opposing propositions are often carried out in more complex cases, addressing specific activities that would be likely to occur if a specific crime had been committed, and can involve a series of propositions assessed in sequence. These, given their appearance when written down, are often referred to as ‘probability trees’, which consist of a series of opposing probabilities, each ultimately derived from what is the basic question in a case: the defendant did it, against the defendant didn’t do it.
Following on from this, amongst the main forensic providers currently operating in England and Wales, to an extent driven by the Forensic Science Regulator, the use of opposing propositions in assessing the overall significance of any findings in the context of a case is growing. Continuing with Mr A, he gave an explanation for the presence of his DNA at the burgled property and, by taking this into account, the interpreting scientist reached a conclusion that the finding of a low level of his DNA at the scene was effectively neutral, or inconclusive with respect to whether Mr A burgled the property or just worked there some time before.
In the absence of a version of events from the defendant in a case, it is becoming more common for the scientist to say, “Without a version of events from the defendant, the evidence cannot be fully interpreted.”
The provision of a version of events by the defendant obviously allows a structured approach to the overall interpretation of forensic evidence. However, given the extremely limited amounts of DNA now required to obtain an evidentially significant link to a person, and the various possibilities associated with the transfer and persistence of DNA, expecting an informed explanation for the presence of a trace of DNA from a lay person may be a tall order.
In the time I have spent practicing forensic biology, the approach to criminal investigations has changed almost beyond recognition. The use of blood grouping has now effectively disappeared. The restriction of ‘Biology’ to only offences against the person is a thing of the past. Any crime involves people and all people shed DNA. Even in the field of DNA analysis, with improvements in sensitivity over the years making current techniques at least three orders of magnitude more sensitive, the changes are significant.
There are also new areas where variations on DNA analysis have been found to be useful. The use of Y-STR (male-specific) analysis has become common in sexual assault cases. Mitochondrial DNA analysis, which has been possible for many years, has been refined and found uses, not only in forensic science, but in areas where very old human remains may be of interest.
A relatively new technique (in reality, a combination of different techniques) called MPS – Massive Parallel Processing – has become available, albeit not widely. By incorporating several DNA analysis approaches, science is approaching the point where it may be possible to say, ‘your DNA’ rather than ‘highly likely to be your DNA’. This technique also highlights another issue in forensic science which is cost. The cost of the MPS approach is significantly higher than current techniques which, in themselves, are not cheap. Given that most forensic work is funded by the Police, widespread introduction of this new technique would, put bluntly, very rapidly exhaust the money available.
Regarding cost, an adage that seems to fit is that there is no money to be made in forensic science. This issue is evidenced by the relatively frequent change in ownership of the three main private companies that carry out the majority of the Police forensic work in England and Wales. In 2012, the FSS was wound up and, as such, the government was no longer involved in the direct provision of forensic science in England and Wales (notwithstanding the appointment of a Forensic Science Regulator with statutory powers). The three largest providers that stepped in after the closure of the FSS have each changed ownership on a number of occasions since. It is hoped that the current model of private sector forensic science provision in England and Wales will improve stability.
DNA and body fluid evidence is constantly changing; what applied to yesterday’s case may not apply to tomorrow’s. The Forensic Biologists at Keith Borer Consultants have a combined experience of over 180 years and actively pursue Continuous Professional Development to ensure knowledge and practices are current.
If you have a case involving disputed blood, body fluid or DNA profiling evidence, or if you want to understand the value and limitations of evidence in greater depth, please call 0191 3324999 to speak to a member of the team.
Author
Mike Appleby
BSc(Hons)
