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Image of Dr Bill Tilstone

Ask Bill

Please post your questions for this week in the comments section. At the beginning of next week, Dr Bill Tilstone will respond to the most interesting, useful and/or popular questions. Please ‘like’ questions posted by other learners if you are also interested in having these answered.

Thank you for all your questions. Here are Bill’s responses to your ‘most liked’ questions:

Let’s start with some unusual questions, two about the reality of something in a TV show and one about the reality of working to a budget.

“It was my twin that did it” as a defence. Yes, if the plot hinges on DNA testing and it was indeed an identical twin then it is technically correct that DNA profiling will not differentiate the 2. Fingerprints will though. I don’t recall ever working on a case where there was a twin, far less the proverbial “evil twin”, but it probably has happened.

The other TV question was about a man who ‘was a chimera and the DNA from his semen was different from the DNA from his saliva’. A chimera is defined as an organism that has genetically distinct cells due to the merger of multiple fertilised eggs. I was aware of the condition but have to confess I had never heard of it in humans and had to look it up. It would seem to be very, very rare so the writers were pushing things even further than the ‘evil twin’ plot line. However, having cells in the body that are genetically distinct from each other can occur from a much more likely (but still rare) source, namely a bone marrow transplant. The forensic science consequence of this is that the DNA profile obtained from blood (which arises from the DNA in the white cells) will be different form that in other tissues, including semen. (This is different from the semen - saliva cited in the question as being the difference in the TV show story line). There was a non-TV but related question asked about what would be the effect of a lymphoma on a DNA profile. There are several different kinds of lymphoma but the underlying cause is a defect in the DNA responsible for switching on and/or off the reproduction of lymphocytes. This should not alter the patient’s STRs.

BPA on a shoestring?
The very reasonable question was asked ‘given that we can use lasers, why would anyone want to conduct a BPA reconstruction with strings - fiddly, and a bit imprecise’. Yes, where budgets permit, lasers are used. However, strings can be easier to set up and photograph, and are readily available. In the real case, I was visiting the lab concerned and was asked (with no pre-warning) if I’d look at the car. The potential value of the BPA reconstruction was clear right away, and one of the detectives soon got me a ball of string and some adhesive tape.

How long did the real case take?
This could have been included in the “TV bunch”, where cases are solved between commercial breaks … It took about 3 years altogether. The BPA and blood typing was all over and done with in a couple of days, but the legal process was very drawn out, with preliminary hearings and then the trial accounting for by far the greater part of the time that elapsed between the crime and the trial.

Case study questions:
There were a few about gunshot residues and cartridge cases. Week 4 deals with firearms and these will probably be dealt with in the discussions during the week. If not, there is another “Ask Bill” in week 5.

Blood, general scene condition etc: This cropped up in Week 1, but the power of TV has left some of you still puzzled about the condition of Mrs Ward’s injury and general appearance! Our “Mrs Ward” (who is alive and well I’m happy to report) was expertly made up to look very like the real one, using the original crime scene photographs. Likewise how she appeared in the car, and the presence/absence of blood on clothing. What you saw in the video is what I saw at the scene too.

The trajectory reconstruction of the shot to Mr Ward’s arm is accurate, at this point it’s up to you what to make of it. (Same answer to the question “Did Mr W shoot himself”?).

DNA questions
Identification of bodies in a mass disaster from mtDNA. Yes, this is standard practice and does not need an entry in a database. Criminal databases do not include mtDNA information, only STRs. Some military databases (e.g. in the US) do contain mtDNA profiles. However, one of the good things about mtDNA is that it is very easy to obtain a reference sample - hair from clothing, bedding or a brush, and tissues from any relative who shares the maternal line will have the same mtDNA profile.

Is DNA checked by others to confirm that it is a match? This is a good question that has several threads to it. Everything is checked before a report is issued, but the DNA profile itself is generated by the analytical instrument. There is no human interpretation involved - quite different from what we saw last week with fingermarks and ACE-V. However, the analysis itself involves control samples at various stages (for example, one to show that there has been no sample contamination) and these are al confirmed before accepting the test results. Then there is the question of what we mean by “match”. This was discussed in Activity 3.11 and can be quite complex. For the sake of simplicity of illustration, let’s consider results from analysis of a vaginal swab from a rape victim, and let’s accept that there is semen there from the accused. There will also be traces of the DNA from the woman herself and there may be traces of male DNA from consensual sex with a partner. These have to be dealt with in the report and making sure that this is done correctly can be quite complex.

There was a similar question about whether a mixture of bloods could be analysed and show the component DNA or whether we would not be able to analyse the sample. Yes, we can technically identify all the STR alleles present, but interpretation can become very difficult.

There were lots and lots of questions and comments in the discussions on DNA databases, and the security of the information. Anything online can be hacked, but will it? And is it worth it? Tim Beale gave a measured response to this in the Ask Bill activity - the answer is that hacking is not a realistic problem. The other version of the “how secure” question is the misuse of the data, for example by granting access to requests related to disease risk - I’m going to “cop out” of that one and say your guess is as good as mine!

We can plant a fingermark, could we plant (or otherwise fake) a DNA trace? Well, if you review all that was in this week’s videos about contamination, it would lead to just one conclusion - yes, DNA could be planted. The actual question went quite a bit further and wondered about manufacturing DNA, and the answer to that one is a big NO! Almost impossible to do, and why bother - just get a glass that the person had taken a drink from, swab it, and there you go, all done.

Why 16 (STRs in the forensic profiles)? Because we can do it. In the beginning there were 4, then 6, then 8, then12, then 16, and now we are at 23. The limiting factors are that we must be able to separate the amplicons produced, that the distribution of the alleles in the target populations must show sufficient variation (e.g. an STR with 2 alleles one found in 95% of the population and the other in 5% won’t make it), and we have to know their frequencies in the target populations. The original work was largely carried out by government agencies such as the Forensic Science Service in the UK and the FBI in the US, but it is now almost all being driven by the commercial companies that make and sell the multiplex kits, with cooperation from forensic scientists in the validation studies.

Is the amelogenin sex test reliable? As the questioner says, the test does sometimes not work properly but it is not really an issue in forensic work. Let’s say you got a profile from a blood stain that matched that of the victim in 16 STRs, but came up with the “wrong” amelogenin. You’d repeat the test, certainly, but really the amelogenin is there for intelligence assistance not as a marker.

There were a few threads in the discussions regarding mtDNA and its inheritance. As Stijn Kiers correctly pointed out, sperm do have mtDNA but it is located in the tail region and not in the head. There is therefore no male mtDNA transferred in the fertilization process.

How do we conduct the search for DNA matches? Well, the computer does it for us, and therefore the process is thankfully free from subjective human problems.

And the last DNA question: What would be the effect mutations resulting from a nuclear disaster on the integrity of the DNA databases? If there was sufficient radiation to mean that the profiles in the database no longer matched the person from whom the samples were taken, that really would be the last DNA question! However, it is a good question on a less catastrophic level: Mutations occur all the time, but not at a rate that is an issue for database integrity.

Blood and bones
Those of you who live in the UK may be aware that there is a major urban rail project underway in London, and that the excavations uncovered some human remains. Some of these were identified as male, some as female, but gender was not assigned in seven of them. Laura Ellwood asks “Why not”? I don’t know Laura since I have no information on the investigation. However, assignment of gender from skeletal remains usually involves analysis of the physical properties of the pelvic bones - see here for more information.

Now back to blood, and in a way, back to the questions about the amount of blood in the car. The amount of blood at a scene depends on lots of things. It may help to think about the blood shed in our fatal shot to the head in the context of a stabbing or beating. If someone is repeatedly beaten with (for example) and iron bar, their surface blood vessels will be damaged and their heart will pump blood out through the damaged areas - many of use will have had first hand experience of this if we have cut ourselves and seen the blood drip out down our clothing and on to the ground. If they suffer an arterial stab wound there will be an forceful spray of blood from the arterial pressure - as long as the victim does not die … A gun shot to the head with no exit wound results in instant death and does not produce an injury that will bleed dramatically. Blood will ooze from the wound though - as seen by the blood staining on Mrs Ward’s arm and car seat.

How do I?
This cropped up in Week 1, and I gave a sort of qualified but generic answer. There was one specific question this week, from Adithya, who would like to become an investigative psychologist. This is similar to the answer to “I want to be a DNA analyst” sort of question. Anything that involves becoming a technical specialism (including pathology) has to include a grounding in the specialism, for example a medical degree, a chemistry degree, or in this case, as degree in psychology. The “forensic” aspects can be built on top of that through on the job training and courses, but there is no way to bypass the need for the required professional expertise that comes from the academic qualification. This does not in any way imply that the study required to become a crime scene examiner, or fingerprint expert, or firearms expert for example, is trivial - it is not.

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This article is from the free online course:

Introduction to Forensic Science

University of Strathclyde

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