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Ask Penny

Please post your questions for this week in the comments section below. Penny will select the most liked/interesting questions and publish her response to these on this step by Wednesday of Week 4.

Please ‘like’ questions posted by other learners if you are also interested in having these answered.

Thank you very much for all of your great comments and questions this week, it’s fantastic to see that you are all still enjoying our case, and that you’re thinking so carefully about the details. You put forward some really great questions about the DNA and BPA material, it’s great to see that you’re understanding these topics so well. I have answered as many of the questions as I can, but I just want to explain that there are some questions about the case that I’m not going to give you answer to just now, either because the answers will be revealed over the coming weeks, or because we want your knowledge and understanding of the case to develop alongside the evidence being presented to you. I hope you find the answers below useful, and remember there will be another ‘Ask Penny’ next week. I also wanted to remind you that the answers to the ‘Ask Penny’ questions in Week 1 were posted on the ‘Ask Penny’ article in step 1.19.

Lisa Goddard asked whether different forensic labs use different STRs, and how this works for comparison and sharing information. Within countries, all labs will usually use the same set of STRs, although they may use different kits from different manufacturers. However, different countries have historically used different sets of STR loci, or sets that only partially overlap, but over the last decade countries have reviewed these and there is now a much greater consensus over which STRs are analysed. Until recently, the standard DNA profile in the UK consisted of ten STR regions plus a marker to determine the sex chromosome complement of the donor of the sample, i.e. whether they have two X chromosomes or and X and Y chromosome. This was known as the SGM Plus system, and was developed by the UK Forensic Science Service in 1999. However, in the last few years labs in the UK have moved to new systems. In England and Wales, since 2014 a DNA profile now consists of 16 STR loci plus the sex marker, and in Scotland we moved to a system with 21 STR loci and three sex markers in 2015. Part of the reason for implementing these new kits, as well as increasing discriminatory power by increasing the number of loci, was to bring the UK in line with the recommendations of the European Network of Forensic Science Institutes and the European DNA Profiling Group, who specified a European Standard Set of loci. In terms of sharing DNA profile information more widely among countries, this is a subject of much debate, and many people have concerns about the sharing of DNA profile data between countries, for example due to the potentially sensitive information stored within an individual’s DNA profile, and the wide variation in scientific standards among different countries. Sharing of DNA data is not done automatically, but depends on legal agreements between different countries. For example, several EU member countries have signed bilateral agreements with the United States to share DNA profiling information. Countries who are members of INTERPOL are able to submit DNA profiles to the DNA Gateway, which automatically searches against DNA profiles contributed from 73 member states and returns a result within 15 minutes. Within the European Union, the sharing of DNA profiling information is also carried out through the Prüm Convention, a treaty signed in 2005 allowing EU member states to make direct searches of DNA profiles in national DNA databases of other EU member states. Currently, not all member states are signed up to this agreement, and the UK initially opted out of the treaty due to concerns over the increased chance of adventitious or false matches due to the unusually large size of the UK database. This is exacerbated by the fact that until recently there was only limited overlap between the UK and countries in Europe in terms of the regions of the genome that were tested. This meant that a search could be made under the Prüm Convention with data for only six regions (whereas at the time typically ten regions were tested in the UK), which makes the likelihood of a coincidental match much higher. The newly adopted DNA-17 system in England/Wales and DNA-24 system in Scotland means that there is now much greater overlap with DNA profiles produced in Europe, and so the chance of adventitious matches should be lower. As a result of this and advice from law enforcement agencies that Prüm offers significant benefits for the investigation and prevention of crime in the UK, in December 2015 the UK parliament voted to join the Prüm Convention. This is considered to be much more efficient than current methods of cross-border cooperation through INTERPOL, and will include safeguards against the issue of potential adventitious matches, with the National Crime Agency acting as a gatekeeper for the process. In late 2016, the UK government confirmed that this would go ahead despite the UK EU referendum result in June 2016, although it remains to be seen whether this will be the case.

Lisa Goddard also asked whether anyone has ever hacked into a DNA database to change their recorded profile, and I’m not aware of this ever having happened, and access to DNA databases is extremely tightly regulated. Only a small number of laboratories in the UK are permitted to load DNA profiles on the UK National DNA Databases and the Scottish DNA Database, and only a very small number of people are permitted to search it. The National DNA Database Strategy Board monitor all searches and have to give permission for any non-standard searches such as ‘familial’ searching, where the database is searched for individuals who may be related to a suspect.

Related to this, Dorothy Grimshaw asked about whether DNA databases hold information about genes or only about STRs, and whether this would be of any interest to insurance companies. The answer is that national DNA databases of crime scene and offender profiles only contain information about STR genotypes and not about any other aspect of someone’s genetic code. This would therefore not be of huge interest to insurance companies in terms of predicting the types of diseases that someone might be predisposed to. Concerns around keeping an individual’s DNA profile relate more to the fact that their profile can be used to identify them, than the potential for the profile to reveal anything else about them. This is not the case with databases of other types of genetic tests, for example online DNA testing kits and ancestry/genealogy tests, as many of these contain information that is medically relevant—the results of these types of test may be of much more interest to insurance companies.

Terry Davies asked a question related to this, about how DNA databases gathered by genealogy companies compares to those kept as part of the criminal justice system. The DNA testing carried out by genealogy/online DNA testing companies is actually more extensive than forensic DNA testing, and looks at a larger number of sites in the genome, and sometimes different types of DNA marker. Whilst forensic DNA testing typically involves analysing up to 24 STRs around the genome, genealogy sites now usually examine more than 100 STRs. In addition, these sites are starting to use single nucleotide polymorphism markers, and these can be assessed in very large numbers. These DNA test results can therefore contain a lot of genetic information about an individual, and this has recently been used in a forensic context, in the ‘Golden State Killer’ case, which received a great deal of high profile publicity earlier this year. This raises some ethical concerns, but could potentially be an effective means of narrowing down a suspect list when no match to standard DNA databases exists. If you want to read some more about this case there are some interesting articles and discussions about this in the Washington Post,, Technology Review and, as well as a list compiling a lot of the articles relating to the case here.

Lisa Goddard also asked a technical question about PCR fragments that are elongated beyond the site of interest and how these are eliminated from the amplicon mix. These are not specifically removed, but they should not cause any problems. To give a slightly technical answer, during the first round of amplification from the genomic DNA template, the first copies can elongate from the forward/reverse primer beyond the target region that is bound by the other primer, and so these will be longer than the target region. However, after the second round of amplification from these initial copies, the binding of the other primer delimits the other end of the copies, so the copies that are produced from these initial copies only include the target region. Thus, it is only the copies that are made directly from the genomic DNA that have this issue, and they will represent only a tiny fraction of the final pool of amplicon. At 100% efficiency, a PCR will produce billions of copies of a target region, and the vast majority of these will only include the target region. I hope that makes sense!

Jessica Murr asked about whether hands can be tested for gunshot residue, and under what circumstances this might be useful, and Moira Bolt also asked about gunshot residue. The answer is that hands can be tested for gunshot residue, and they would usually be swabbed and then the swabs tested for the presence of gunshot residue. However, there are some important limitations of gunshot residue evidence, particularly in this specific case. The first is that in any shooting, particles of gunshot residue are transferred onto surfaces near to where the gun was fired, including skin, clothing, nearby surfaces etc., but they are also lost very quickly. Many forensic laboratories will not accept items for examination for gunshot residue if they are collected more than a few hours after the incident took place, as it would be unlikely to produce any useful information. By the time the police got to Mr Ward it is possible any gunshot residue on his hands would have been lost. The second limitation, specific to this case, is that here we have a situation where two people were shot inside/around a vehicle, and so it is likely that there will be a lot of gunshot residue all over those people and the inside of the car. Mr Ward’s hands could be tested for gunshot residue, but if this is found then it could be explained by him touching his own arm where he was shot, touching Mrs Ward or touching the inside of the car. So, whilst it might be possible to identify gunshot residue, we have to think about whether this would actually provide any useful investigative information.

Tom Wilson asked an interesting question about how common it is for people in the UK to have handguns, and how this compares to the US. Jay H, Christine S and Patricia Wormald were also involved in this discussion, and correctly answered that handguns are banned in the UK, mainly as a result of two mass shooting incidents, in Hungerford in 1987 and in Dunblane in 1996, although there has been one mass shooting in the UK since, in Cumbria in 2010. After the Dunblane massacre, two new firearms laws were introduced that meant that all types of handguns were banned (excepting some antique/historic guns and guns that don’t fall under the definition of handguns). The UK now has some of the strictest gun laws in the world, and the incidence of gun crime is very low, particularly compared to the US. If you want to read more then there is a very detailed Wikipedia page on UK firearms policy, and a couple of interesting articles about the history of gun laws in the UK and also in Australia, and from

Kay Schwink asked a question about the difficulty in interpreting DNA profiling results from the scene when more than one individual’s DNA is likely to be present. Generally, all individuals involved in a case will have a DNA sample taken, so that they can either be linked to the DNA profile of interest from the case, or eliminated as a potential donor. If a sample comes back with a single DNA profile then this is a straightforward procedure as the two profiles can just be compared. However, this gets much more complicated when you get a DNA profile result when more than one person has contributed DNA to the sample, and this is known as DNA mixture. Mixed DNA profiles are extremely common, and as DNA profiling techniques become more and more sensitive, this issue will become even more widespread. DNA mixtures are particularly common in sexual offences, because samples will often contain biological material from an individual who has been attacked, and from their attacker. This type of result can be very difficult to interpret and it can be very challenging to determine which components in the profile come from which individual. Many forensic laboratories will only interpret mixed DNA profiles with relatively small numbers of contributions (e.g. from two or perhaps three individuals), and this is a major challenge in the field. Some very complex statistical analyses are being introduced into DNA profile interpretation, which can assist in separating out the most likely contributions from different individuals in the mixture, but this is not at all straightforward.

Malcom Turff asked whether it is possible to get DNA from urine or faeces, and Jay H, Kay Schwink, and Hilary C were involved in a discussion about this. As Kay suggested, for urine, the presence of epithelial cells from the urinary tract mean that DNA is present, and in addition the presence of sperm cells in urine may also add to the cellular content of the urine. Vaginal cells (which are also epithelial cells) can also contribute significantly to the amount of cellular material in urine, and this will result in a higher amount of DNA in a urine sample that contains vaginal cells. Faeces can also contain high levels of DNA, as they will contain DNA in the form of epithelial cells from the digestive tract, as well as bacterial DNA from digestive bacteria, and DNA from any food that has been consumed. It is therefore possible to extract DNA from these types of material, for example DNA testing from faeces can be carried out to identify dogs whose owners have not picked up after them (for example read this article from the BBC, and for those outside of the UK, yes Barking is a real place!). Faeces are actually quite commonly found at crime scenes, and a crime scene manager colleague of mine tells me that she was frequently asked to swab faeces at crime scenes for DNA testing. However, one issue with these types of samples is that they can contain higher levels of inhibitor chemicals, which can interfere with the DNA profiling processes. Specifically, inhibitor chemicals found in a variety of different materials can inhibit PCR reactions, which can prevent a profile from being produced. For example, haem in blood, indigo in denim, humic acid in soil, calcium in bone and melanin/eumelanin in hair are known inhibitors that can be found in DNA samples.

Jay H raised a question about blood pattern analysis and whether the spatter seen in a shooting incident can tell you anything about where a shot was fired from, and there was an extensive discussion about this, including Dorothy Grimshaw, bee kay, and Lisa McCarthy. I’m not sure I completely understand the issue so I’m sorry if this doesn’t answer your queries fully, but the directionality of any blood spot can allow you to identify the point of convergence for a given pattern, but this only indicates the point at which force was applied to wet blood. With a gunshot wound, there will be two elements to the blood patterns seen, back spatter from the entrance wound and forward spatter from the exit wound (if the bullet exits). The point of convergence that the forward spatter would track back to would be the exit wound itself, as this is where the blood was subjected to the force that caused the spatter. Similarly, any directionality in the back spatter from the entrance wound would track back to the wound itself. The nature of the spatter will depend on a number of things, including the calibre of the gun, how/where the bullet strikes, whether the shot is short- or long-range, whether the bullet exits the body etc. Spatter caused by gunshots tends to be made up of very small droplets, often more of a fine mist, as there is a very large amount of energy involved. The forward and back spatter will differ to some extent, and the forward spatter will tend to be a fine mist, whereas the back spatter may have larger droplets in it. Blood spatter from a gunshot would therefore is unlikely to tell you anything about where the shot was fired from, although looking for blood spatter in the barrel of the firearm may tell you something about the range at which the shot was fired—with a short range shot the back spatter can be sucked back into the barrel. One way in which scientists can get some information about where a shot was fired from is by consulting with the forensic pathologist who can look at the path of the bullet through the body, and if the shot travels straight through the body then depending on the context of the case, this may tell you something about where the shot was fired from. However, it is important to remember that with longer range shots there can be curvature of the bullet’s trajectory, and if a bullet ricochets inside the body then very little can be said about directionality. Tom Wilson also asked why the blood spatter on the wind screen and the general pattern of blood dispersal on the carpeted transmission tunnel had not been presented to you—I’m not sure exactly what you mean here but the blood pattern evidence was all presented to you, I don’t think there was any more blood evidence detected on the windscreen/transmission tunnel.

Alicia Botham also asked a question about BPA about what might affect its accuracy. There are a variety of things that can affect BPA, and Alicia identified one of the most important issues, which is when there is blood from multiple individuals present as blood spatter from them can form overlapping patterns. This can be an issue even with a single victim, as there may be many different blood pattern types overlapping, and these can have completely different directionality, especially if the victim has been moving around a lot during the incident. This can become extremely complex to interpret, with multiple points of origin and multiple different types of pattern. Another issue can relate to blood spatter resulting from gunshots, and with closer range shots the gases from the muzzle of the firearm may influence the travel of the blood spots, so the directionality of the spots could be less reliable in determining the origin of the force. The reliability of BPA can also be influenced by things like the type of surface that the blood is deposited on, and it is also worth remembering that there is a certain level of subjectivity in BPA, which can impact on its accuracy. One interesting factor that can affect the interpretation of blood patterns is the presence of insects at crime scenes, and if insects land at crime scenes and walk around in blood then they can deposit tiny stains at the scene that may be mis-identified as blood spatter. You can read more about this issue here.

Finally, I also spotted a question from Kay Schwink that I had missed from last time, relating to the use of dogs in forensic science, which I wanted to answer because I am a dog lover! Dogs are used in many aspects of forensic science because they have incredibly sensitive senses of smell—their noses contain about 200 million olfactory cells, compared to 20 million in humans. As a result, dogs can be trained to sniff out a variety of different substances, such as illicit drugs, explosives, and accelerants at the scene of a suspicious fire. In addition, dogs are often used to track and search for missing people or fugitives. One fascinating way in which dogs are used in criminal investigations is to detect the scent of decomposing flesh in order to find the bodies of deceased human beings. Cadaver dogs can locate human remains, and also determine where bodies or body parts may have been stored. Even if a body is buried underground or is underwater, a cadaver dog’s sense of smell is powerful enough to detect it. One really interesting example of a case in which cadaver dogs were used is the murder of Suzanne Pilley, a bookkeeper from Edinburgh who went missing in 2010 and whose body has never been found. Despite this, David Gilroy was convicted of the murder in 2012, with evidence being put forward by the prosecution that two cadaver dogs had identified several areas of interest in the office building where it is believed Gilroy carried out the murder, and in the back of his car. If anyone is interested in reading more about the involvement of the cadaver dogs in this case, you can read this article from the, and this Wikipedia page, and if you are interested in reading more about cadaver dogs in general, you can read this article.

The last thing I wanted to do was answer Megan C’s question about my research, now that you have a good understanding of the DNA material. In general, I am interested in the development of nucleic acid (DNA and RNA) assays to determine the source, type, age and time of deposition of forensic samples, such as single or mixed body fluid stains and touch DNA. For example, we are working on an assay that can be used to test a DNA sample and determine the age of the donor by analysing a type of chemical modification in the DNA. This chemical modification, known as DNA methylation, changes as an individual gets older so can be used to estimate the age of the donor of an unknown DNA sample, for example a sample recovered at a crime scene. Some of these types of tests are very accurate, and can estimate an individual’s age to within a few years. We are also using RNA, which is a molecule similar to DNA but whereas your DNA is the same in almost every cell of your body, RNA differs between different cell types depending on their function. There are different types of RNA molecule that differ principally in their size, and this means they also differ in their rate of degradation. We are analysing the degradation rates of these different types of molecule in different types of body fluid stains, to see if we can estimate how long ago a body fluid stain was deposited. This can be useful to identify when a crime was committed, or to exclude a body fluid as being relevant to a specific crime. We are also collecting population genetic data in a number of under-represented populations, for example in the Middle East, and in Africa. It is important to have reliable genetic data from different populations around the world so that we are using relevant data for calculating DNA profiling statistics. We are analysing STRs in these populations, but also testing new genetic marker types such as insertion-deletion markers, single nucleotide polymorphisms and sequencing data using next-generation sequencing techniques.

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Introduction to Forensic Science

University of Strathclyde