Building a case

The use of DNA in Forensic Science is widely acknowledged – but Forensic Science is a very broad field. Here we look more closely at how cases are built up within the Criminal Justice System following sexual offences

There are a range of forensic techniques employed in the investigation of crime, including the interpretation of bloodstain patterns (BPA), analysis of textile fibres (from clothing - one-to-one taping techniques enables textile fibres and other tiny evidential particles to be mapped on surfaces), damage to clothing, forensic ecology (such as insects and plant debris, wood fragments, leaves, seeds, pollen grains, diatoms), mechanical fit. However, perhaps the most widely-known analysis is that of DNA.

DNA is a valuable ‘weapon’ as it is thought to be unique to an individual (except identical twins) and can help police link offenders to crime scenes by matching DNA profiles that have been stored on the National DNA Database (NDNAD) to DNA samples taken from crime scenes or suspects. It can, of course, also be used to eliminate suspects from inquiries.

So what part can Forensic Science (in particular DNA) play in helping the Criminal Justice System, in the investigation of sexual offences?

Forensic Science makes use of evidence recovered by police forces and medical professionals, often within specially designed Sexual Assault Referral Centres (SARCs). Forensic scientists are able to provide advice, support and forensic services from the outset of a case, during the investigation and through to successful outcomes at court.

As is the case in all crimes, it is particularly important to ‘think’ Forensic Science early on in the investigation of sexual offences.

The Faculty of Forensic and Legal Medicine champions victim support as central to the investigation, having developed national guidelines, with input from forensic science providers, the medical community, the police, and scientific kit manufacturers, etc. The guidelines ensure that appropriate evidence is recovered by the doctor / medical examiner. It is vital for forensic scientists to have the full details of the case, to enable them to develop their examination strategy to assist in the investigation of the crime alleged to have occurred. For example, forensic scientists can examine swabs and clothing taken from the victim for the presence of body fluids such as blood or semen, assisting the police in selecting the most evidentially useful samples for DNA analysis and suggesting additional or alternative lines of scientific enquiry.

The latest figures from the Crime Survey for England and Wales (CSEW), for the year ending September 2014, show that the numbers of rapes (24,043) and other sexual offences (48,934) are the highest recorded by the police since 2002/03.

Although these figures must take in to account improvements in recording, it is also thought to reflect a greater willingness of victims to come forward to report such crimes.

Females in the survey, who had reported being victims of the most serious sexual offences in the last year, were asked, regarding the most recent incident, whether or not they had reported the incident to the police. Only 15% of victims of such offences said that they had done so. Frequently cited reasons for not reporting the crime were that it was ‘embarrassing’, they ‘didn’t think the police could do much to help’, that the incident was ‘too trivial or not worth reporting’, or that they saw it as a ‘private/family matter and not police business’. Therefore, given the numbers of sexual offences being reported, Forensic Science has a significant part to play in many of those cases.

DNA evidence is pertinent in many scenarios (see the cases of Richard III, soldiers lost at the Battle of Fromelles and the Grimsby Chums, as well as its increasing use in paternity disputes) and has become a particularly compelling part of the presentation of evidence in criminal trials.

Blood and other body fluids (such as semen and saliva), hairs, nails, skin flakes and dandruff collected at a crime scene, or from exhibits such as clothing, can all potentially be examined (depending on the circumstances) for cellular material to produce a DNA profile. These DNA profiles can then be loaded on to the NDNAD to attempt to identify a suspect or can be used for comparison purposes, in cases involving a known suspect. In addition, non-visible traces of DNA, such as from sweat and skin flakes on the inside of garments and footwear where the wearer may be in doubt, from tools, drinking vessels and discarded cigarette ends, or from anything else that may have been touched during the course of an incident and could be relevant to an investigation can be examined and processed. It is this profile which would be used to help put the suspect at the scene of the crime.

DNA labs use automated platforms which can provide rapid, cost-effective DNA profiling of mouth swabs and other personal reference samples, with results loaded to the NDNAD (or provided directly to individual forces and other customers) for cross-checking.

Using the latest techniques, even old - and somewhat degraded - samples can be analysed, which is particularly useful in enabling the re-opening of historic, or ‘cold’, cases.

There are several specialist DNA techniques which, used together, give forensic scientists the best chance of discovering information from samples of DNA, however small, mixed or degraded.

Y-STR (short tandem repeat) profiling is a specialist DNA technique that targets male DNA and has been in use in the UK for a number of years. Given that the Y-STR profile is inherited generally unchanged through the paternal line it is not as discriminating as standard DNA profiling. However, as the technique targets male DNA, it is especially useful in allegations of sexual assault as mixed male/female DNA is often encountered. Examples include cases of sexual intercourse when no semen is found. The vaginal swabs of the victim may contain small amounts of DNA from the suspect in addition to lots of DNA from the victim herself. Forensic service providers are always looking to improve processes and Y-STRs are one example of where they can embrace new technologies.

Another specialist technique, mitochondrial DNA profiling, is particularly useful for old or degraded samples or to trace family connections down the female line.

Typically, in sexual offence cases, the most compelling evidence will often be provided if bodily fluids – such as semen – are discovered.

If semen is recovered during examination of the victim, this can be extracted from the swabs taken and / or clothing before DNA analysis is undertaken. Water-based extraction techniques are still widely used in the forensic community though, more recently, extraction techniques have developed to involve the use of different ‘buffers’ – chemical solutions.

Recovery rates seem to be increased using such buffers. That is, buffer-based extractions yield more sperm which increases the chances of obtaining a DNA profile from the sperm. Hence this technique is starting to be used more widely than water-based techniques.

Why is this so? Buffers normally include detergent and an enzyme, resulting in the removal of the semen and cells from the swab and the breakdown of the non-sperm cells. By releasing an increased level of sperm into solution, this increases the likelihood of obtaining a profile from the sperm with less interference from non-sperm DNA.

In cases of vaginal intercourse in which no (or minute) traces of semen are observed, positive samples from vaginal swabs are more likely to see successful rape convictions; it may be the case that if semen is only found outside the vagina, the charge may be downgraded to sexual assault. An example of the benefits of buffer-based extractions is where, if a rape has been committed by a vasectomised male, or one with low-or no-sperm count, or in cases with no ejaculation; where previously it was thought that 12/24 hours was the optimum timeframe to retrieve cellular evidence, recent research indicates that positive results can be obtained up to 48 hrs.

For a number of reasons, sexual offences are sometimes not reported immediately, and often when they are there are still logistical issues posed in getting victims to hospital or Sexual Assault Referral Centres. Now, with the recovery time for extracting evidence having potentially been doubled, the victim and the criminal justice system are supported further.

What are the next steps for DNA profiling? FSPs are working continuously to develop new DNA technologies for forensics. One area of advancement is likely to come from the next generation of YSTR, which potentially will provide increased sensitivity and increased discrimination between males.

DNA developments offer increased discrimination and sensitivity as there are more markers and improved chemistry – for example, in July 2014, the NDNAD was upgraded to allow profiles obtained via the improved DNA-17 technology to be loaded from across England and Wales. DNA-17 is based on the 10 STR loci and gender identifier used previously, plus a further six STR loci. Any matches between a full DNA-17 profile obtained from a crime scene sample and a full DNA-17 subject profile will provide a match probability estimated in the region of one in a billion.

However, even taking into account this increase in sensitivity, and despite being the European standard, DNA-17 is – in some ways – already being outstripped by DNA24/Global Filer technology, which has recently been implemented in Scotland.

Other advancements will likely come in the shape of new, improved, DNA quantification and extractions systems which are currently being developed. Given the increasing sensitivities of DNA techniques we are encountering an increasing level of mixed DNA profiles. DNA transfer studies are increasing our ability to examine and understand the levels of DNA transfer we might expect from a primary transfer (that is, the first touch of an item), secondary transfer and tertiary transfer. Significantly, this will allow a more thorough understanding of timelines of who touched specific pieces of evidence.

Forensic service providers have developed DNA mixture interpretation software that uses continuous probability models to determine the likelihood of whether someone has contributed DNA. This is a more robust way of interpreting mixed DNA results.

It is very apparent that Forensic Science continues to play an important role in the investigation of crime in general. In particular, with the continued investment into research and development to develop new and improved techniques such as those detailed above, we should expect Forensic Science to continue to play a significant part in the future.

The author:

Finlay Kennedy, Sexual Offences Lead Officer, LGC

Contact: 

www.lgcgroup.com