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Image of Dr Penny Haddrill

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 so many great questions; I have answered as many of them as possible below. I hope you find the answers below useful, and remember there will be another ‘Ask Penny’ in Week 5.

David Parker asked how long DNA stays viable outside the body, and how quickly it will break down, particularly when exposed to the elements. As I said in my answers to the questions in week 1, there are two factors to consider here. Firstly, the length of time that evidence would remain viable for analysis would depend on the conditions in which the sample had been stored. If a sample was at a crime scene that was outdoors, and there was a lot of rain, or very high temperatures or humidity, then samples of a biological nature such as DNA, body fluids and fingerprints could deteriorate and degrade very quickly, and this is a major challenge of managing a crime scene of this nature. However, if samples were indoors, in dry environments that were not subject to major fluctuations in temperature etc., then samples could still be analysed after months or even years. David also asked about whether this would need to be addressed in court and whether the time between the samples being deposited and collected should be addressed. As always, the answer depends to some extent on the specific circumstances of the case, but the scientist would certainly need to consider the possibility that contamination had occurred if a long period of time elapsed between the deposition and collection of samples; this may also allow a defence team to introduce an element of uncertainty and doubt into a case. However, if enough DNA could be recovered from the sample to produce a good quality DNA profile then, if it could be shown that no contamination had occurred, this should be a reliable result.

Jo Simmonds asked why identical twins share exactly the same DNA but have different fingerprints. The reason why identical twins share the same DNA is that they originate from a single fertilised egg that splits into two during development, so they originate from the same genome. Fingerprints develop whilst a foetus develops in the womb and it is thought that friction ridge development is completed by 24 weeks of pregnancy. Whilst the patterns that arise in an individual’s fingerprint are determined by genetics, they are also influenced by the environment in the womb. Thus, any variation in conditions in the womb, such as fluctuations in hormone levels, variation in pressure in different parts of the amniotic sack or if the foetus touches anything with their developing fingers, can lead to changes in the fingerprint patterns. Given that even identical twins developing together at the same time will not experience exactly the same conditions as they develop, their fingerprint patterns will not be exactly the same. An interesting related issue is that until recently, one of the limitations of forensic DNA analysis was that it could not be used to tell identical twins apart. However, in 2014 a German company called Eurofins published a paper in which they sequenced the whole genome of a set of identical twins, as well as the child of one of the twins. Out of the three billion letters in the human genome, they were able to identify five genetic mutations that were present in the child and the twin who was his father. This has the potential to allow investigators to distinguish between identical twins in paternity cases, or criminal cases where one of a set of twins is the alleged source of DNA found at a crime scene. The study used a technique called next-generation sequencing, which is a method for very quickly and accurately sequencing a large amount of DNA. Although such methods are currently too expensive to be routinely implemented in forensic casework, the use of these new technologies looks set to revolutionise the way forensic DNA analysis is carried out in future.

This is also related to Dana Murray’s question about whether there have been any cases of two people, other than identical twins, who have been found to have matching DNA. The answer is that it depends how much of the DNA you examine. In DNA profiling, we only look at a small number of regions in the human genome, we don’t analyse the whole genome. So even if two people have unique genome sequences, that doesn’t mean they will necessarily have unique DNA profiles. The smaller the number of regions of the genome we examine, the more likely it is that two people will have the same profile. It is therefore likely that there have been cases where only parts of a person’s profile could be generated, perhaps because of a low amount of DNA in a sample, and then there may be quite a large number of people who would share the same genetic types as that individual has at those small number of regions. In the UK now, forensic labs examine between 17 (in England and Wales) and 24 (in Scotland) regions of the genome and so the chances of two individuals sharing the same profile is vanishingly small. One other point worth thinking about is that people who are related will share more of their DNA profile than unrelated individuals, and so there is a higher chance of two relatives having matching elements in their DNA profiles.

That leads me on to the questions from K S, who asked whether DNA profiling will render fingerprinting obsolete in criminal investigation, and the answer is probably not, it is likely that there will always be a place for fingerprinting in forensic investigations. One example is above, in the case of identical twins, where fingerprints could quickly and cheaply distinguish identical twins, in comparison to very expensive specialised DNA analyses. Another reason why fingerprints are likely to continue to be useful relates to new technologies being used to analyse them - some interesting work being done by a team led by Dr Simone Francese at Sheffield Hallam University allows fingerprints to be analysed for much more than just identifying someone, by identifying substances on or within the ridges of the fingerprint. If you are interested you can find out more in this BBC news article.

K S also asked about whether sweat is a source of DNA, and the answer is that if it contains any cellular material, such as skin cells sloughed from the surface of the skin then it will contain DNA. In addition, it is also thought that cell-free DNA may be found in many biological materials, such as saliva, blood and sweat. There is therefore a reasonable chance of being able to recover enough DNA from sweat to generate a DNA profile. Finally, K S also asked about the effect of blood thinners such as Warfarin on blood pattern analysis - medication such at this will change the viscosity of the blood, as will the level of hydration of the person bleeding, and whether they have consumed alcohol or not - and this may affect the formation of blood stains at a crime scene, so care should be taken to consider these issues.

Guido Dallmann asked about the minimum amount of DNA needed to produce a useful and reliable DNA profile. Most commercial kits currently on the market are optimised for use with around 0.2-1 nanograms of DNA, or 200-1000 picograms. 200 picograms represents the DNA from approximately 30 cells, and this quantity of DNA can reliably and robustly produce a DNA profile, as long as there are no inhibitory chemicals in the sample, which can be the case in some specific types of samples - for example the haem present in blood, indigo dye in denim, calcium in bone or melanin in hair can inhibit the reactions used in the DNA profiling process. Many kits will work with smaller quantities of DNA than this, and various adjustments can be made to DNA profiling techniques to increase the chances of generating a useable profile from samples containing small quantities of DNA, although this can lead to increased chances of artefacts in the profile. Useable, full DNA profiles have been generated from only 10-20 pigograms of DNA, representing only around 2-4 cells.

Several of you asked about the testing of Mr and Mrs Ward’s hands for gunshot residue, and why this was not done. Cynthia N, Christine Schoop, Stephen T, and Elizabeth Darby all asked about this. In general terms analysing samples for gunshot residue could be useful, and in some case contexts could give you some useful information. 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.

Elisa Bird asked a question about whether it is possible to get DNA from teeth, and the answer is that yes, it is. Often in cases of degraded or decomposed human remains, teeth and bones are the only sources of DNA. Teeth can also be particularly useful because they are very well protected within the jaw and so may be less likely to be contaminated by external sources of DNA, or fragmented/degraded. Specialised techniques are required to extract DNA from bones and teeth, usually involving grinding the material into a fine powder, and this is often a technique that has to be done in a specialised laboratory.

Sue FitzHugh asked about why someone would want to hack into a DNA database, and I’m not sure of the answer to that, other than the answers suggested by others, including Joyce Reid, Aminat Adeyemo, Beverley Robinson and Francis Mitchell. However, I’m not aware of a DNA database having been hacked into, and they are 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.

Finally, there were quite a number of questions relating to the blood in the car. For example, Ian Bryson, Carol M, Jillian Jackson, Elaine Belton and Vicki Eipper were involved in a discussion about whether Mrs Ward may have been shot elsewhere and moved to the car because more blood should be expected inside the car. However, as I mentioned in my answers to the questions in week 1, the amount of blood that would be expected at a scene depends on a lot of factors, including the mechanism by which the blood was shed. If someone is beaten repeatedly, for example with an iron bar, the blood vessels at the surface of their skin will be damaged and their heart will pump blood out through the damaged areas. If someone is stabbed and an artery is damaged, there will be a very forceful spray of blood from the arterial pressure up until the point that the heart stops and a victim dies. A gunshot to the head with no exit wound will result in instant death and so does not produce an injury that will bleed dramatically. We would therefore expect to see a relatively small amount of blood dripping from the wound, as is observed in the bloodstaining on Mrs Ward’s arm and the car seat. In addition to this, as Vicki Eipper pointed out, this would mean that the blood spatter in the car had been created by the perpetrator, and this would be quite difficult. Also note that the car seat covers were dark coloured so it may be difficult to say for certain how much blood is on the seats. For that reason, in answer to Graham Murray’s question about whether there would be a void area on Mr Ward’s seat cover, there might be but it would probably be difficult to tell.

There were also a couple of questions about blood spatter. Janet Alcántara asked about the fine blood spatter with no directionality in the car. The size of the spatter produced when force is applied to wet blood depends on how much force is applied. The larger the force, the smaller the blood spots, so a gunshot wound will produce very fine blood spatter. The directionality of the blood spatter relates to the angle at which the blood came into contact with the surface - a blood spot hitting a surface at 90 degrees will form a round blood spot, whereas as the angle moves away from 90 degrees the spots will become more elliptical, and the angle at which a blood spot hit a surface can be estimated from the width and length of the resulting blood spot.

Patience Usman asked about the blood spatter that would be expected from a close-range gunshot versus a long-range gunshot. Both would produce very large forces impacting into blood and so would produce very small blood spatter, less than 1-2mm in diameter. With longer range shots the force may be somewhat reduced and so the spatter may be slightly larger than with closer range shots, 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. However, in general, the blood spatter would be fairly similar.

Fiona James asked about the difference between blood spatter produced from different levels of force, resulting in low, medium and high velocity impact spatter. Sometimes this is defined in terms of the force used to produce the spatter, sometimes by the size of the spots produced - higher forces produce smaller spatter. The meaning and utility of these terms has been debated over the years, and classification based on velocity can be misleading, so spatter tends to be more often categorised on the basis of spot size. However, in general, low velocity spatter is produced by blood that is not moved by any force other than gravity, and is generally travelling at less than ~5 feet per second, for example blood dripping from a bleeding wound - this produces blood spots of more than 6mm in diameter. Medium velocity spatter is produced by some level of force impacting into blood, generally up to ~25 feet per second, resulting in blood spots around 2-6mm in diameter. For example, this might be caused by a blunt object or knife being used to attack somebody, blood being coughed out of the mouth, or cast off from an item soaked in blood being swung. Finally, high impact velocity spatter is produced when very large forces are impacted into blood, causing blood to travel at very high speeds, usually more than 100 feet per second, for example as a result of a gunshot - this results in blood spots of less than 2mm in diameter, and can including mist patterns, with spots smaller than 0.1mm in diameter. These sizes can be used to categorise blood spatter on the basis of the force that must have caused it. It is my understanding that there is a gap between ~25 feet per second and >100 feet per second simply because velocities greater than 25 feet per second cannot really be generated by the types of mechanism that cause medium velocity spatter, whereas high velocity spatter caused by things like gunshots or explosions generate blood travelling at very high velocities. However, these distinctions are not rigid and there will to some extent be a continuum of forces and resulting blood spot sizes.

Finally, a couple of minor comments in relation to Kim Rouse’s confusion over the comment I made about mobile phones last week. This case relates to a real case, which occurred in the 1980s, but is re-set in 2013. However, to reflect the real case, Mr Ward did not have a mobile phone, which may seem a bit strange as nowadays everyone would likely have a mobile phone with them. Mr Dougan, on the other hand, does have a mobile phone, reflecting the modern world better. As I said last week - we have used a bit of artistic licence in places!

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This article is from the free online course:

Introduction to Forensic Science

University of Strathclyde