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 so much, and that you’re thinking so carefully about the details. You put forward so many great questions; unfortunately, it isn’t possible to answer them all, as much as I would like to, but I have answered as many of them as possible. I hope you find the answers below useful, and remember there will be another ‘Ask Penny’ next week.

What is DNA profiling?

Firstly, there seemed to be a bit of confusion over what DNA profiling involves in terms of the genetic information that it is targeting. Doaa Banaama asked about what DNA typing relates to, if not to genes, and Kayla Wildman correctly commented that the procedure indeed does not target genes, but STR regions. The human genome contains 3 billion bases, or letters of code, and trying to analyse all of these would be extremely time-consuming, expensive and also excessive, as we can get a lot of information about the identity of an individual by looking at a small subset of the genome. Some regions of the genome, known as genes, encode proteins that need to be made in order to build a human being, and keep that human alive. These are known as coding regions, and make up only about 1.5% of the human genome. The rest of the DNA is non-coding, some of which has no function, and some of which functions as regulatory regions controlling when genes are turned on, where they are turned on, and for how long etc.

In DNA profiling, we analyse non-coding regions called short tandem repeats (STRs), which are made up of short core DNA sequences that are repeated many times. The number of times the core sequence is repeated varies among individuals, which means these regions can be used to discriminate among different people. We examine multiple STR regions around the genome, to get more information about the person whose sample we are analysing. Each of these regions is known as a locus, and this simply defines the position of the STR region in the genome, i.e. which chromosome it is on, and where on that chromosome it is. At each locus, every individual has two copies of the STR region, because everybody has two copies of each of their chromosomes, one of which they inherited from their mother, and one from their father. The two copies of the STR region are known as alleles, and a DNA profile is just a list of allele codes found at each locus, which define the number of core repeats an individual has in their two alleles at each of the STR loci examined.

DNA databases that contain DNA profiling data therefore do not contain any information that would reveal anything about an individual’s predisposition to genetic diseases or other health-related issues. These databases contain only information about the alleles that an individual carries at different STR loci. However, DNA sequencing technologies are increasingly being used in the forensic research community, and it is anticipated that these technologies may eventually be implemented by forensic service providers. These technologies have the capability to extract a great deal more information about an individual from their DNA sample, and so the ethical issues that have been raised in your discussions this week, for example by Cheralyn Owens and Kayla Wildman, will need to be carefully considered.

DNA Databases

There were also a few questions that related to DNA databases and the sharing of information between investigators in different countries in the world. For example, Martti Sarkia asked about how profiles from different jurisdictions can be used in international investigations, and what the technical and legal issues associated with this are. This relates to a wider issue about the sharing of investigative information between countries, which has been subject to much debate in recent years, and so I thought it would be useful to give you a bit more information about this.

There are now more than 60 countries worldwide who operate national DNA databases, with many more countries working to establish them. As I said above, a DNA profile consists of a series of codes that represent the genetic types that an individual carries at different regions in their genome. Different databases worldwide differ in terms of the number of regions that are analysed, and so this can cause challenges when trying to compare data between countries. The legislation surrounding when DNA samples can be taken and in what circumstances they can be stored, and for how long, also differs between countries.

In the UK there are two DNA databases, the UK National DNA Database, which was the first in the world to be established, and the Scottish DNA Database. The Scottish DNA database contains all DNA profiles produced in accredited forensic laboratories in Scotland, and these are automatically uploaded to and searched against the UK National DNA Database. As Scotland and England/Wales have different legal systems, these databases are subject to different legislation, and until recently the regulations surrounding the retention of DNA profiles differed. In Scotland, if someone is convicted of an offence then their DNA profile can be retained indefinitely, but if they are not convicted it must be deleted (except under some specific circumstances). In England and Wales, DNA profile and fingerprint information used to be kept indefinitely, until a ruling in 2008 by the European Court of Human Rights in relation to the case of S vs. Marper that the indefinite retention of innocent people’s DNA profiles breached the European Convention on Human Rights. In response to this, the Protection of Freedoms Act (2012) was passed in England and Wales, resulting in the deletion of over 1.7 million DNA profiles from the UK national database, and the destruction of 7.7 million DNA samples.

In terms of sharing DNA profile information more widely among countries, this is a subject of much debate. 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 (i.e. their identity), 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 chance 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 various other developments, in December 2015 the UK parliament voted to join the Prüm Convention.

One final question relating to DNA databases came from Rebekah Ilieva, who asked what happens when an unknown DNA profile or fingermark is recovered from a crime scene, but a search of the national and international DNA and fingerprint databases reveal no match. The answer is that there is very little that can be done at this stage, and often cases will remain unsolved for decades with no DNA match. However, these crime scene profiles will remain on the national DNA database and will be searched against any new profiles that are added. If an individual subsequently comes to the attention of the police, their DNA profile will be added to the database and searched against any unmatched crime scene profiles. This has sometimes resulted in crimes being solved many years after they were first committed; an excellent example of this is the case of Marion Crofts from the UK, and you can read more about this case in this BBC news article.

Interpretation of mixed DNA samples

There were also a number of questions about how DNA profiles can be interpreted when more than one individual has contributed to a sample; Valeria Soto, Tracy Harris and Elisa Martin all asked great questions about this. Mixed DNA profiles are extremely common, and as DNA profiling techniques become more and more sensitive, this issue will become even more widespread. 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.

This difficulty with DNA mixtures is often encountered in cases of a sexual nature, because samples will often contain biological material from an individual who has been attacked, and from their attacker. For example, in a case of multiple rape, this can be very challenging because if there is DNA present from more than two individuals in a sample, it becomes extremely difficult to separate out which components of the DNA mixture came from which individuals. One way that these types of samples can be analysed is to look at STR regions that are present on the Y chromosome. If a female alleges that she has been raped by multiple males, we can analyse the sample and look only at DNA that is found on the Y chromosome – this means that we exclude any DNA of the female from the analysis. As males only have a single copy of the Y chromosome, we generally expect to see a single allele at Y STR loci from each male individual who has contributed to the mixture. We can therefore look at how many different alleles we see in a mixture at these Y loci and this can give some information on the number of different males who have contributed to the mixture, and also some information about their Y chromosome profile. However, this information is quite limited so it can still be very difficult to determine whether a specific individual contributed to a mixture, but this can be useful in excluding someone as having contributed, if they have a Y chromosome allele that is not present in the mixture.

Viability of evidence over time

There were several questions about how time affects the reliability of evidence, and whether evidence remains viable if any significant period of time elapses between evidence being deposited and recovered. For example, Dimitra Pithara, Elaine Grrom, Sidny Tingley, Kay Jenks and Nick Tingley had all thought about this in some detail and put forward some excellent questions about this topic. 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. Secondly, over time after an incident has occurred, if a crime scene has not been secured then investigators would also have to consider the possibility that the scene may have become contaminated with material that is not related to the case. Depending on the circumstances of the case, it may still be valuable to search for and recover evidence, even after a considerable amount of time has passed, but it would be important to keep in mind that the crime scene could have been contaminated; this may also allow a defence team to introduce an element of uncertainty and doubt into a case.

Are fingerprints unique?

Finally, John Adams asked a really interesting question, asking how it can be the case that fingerprints are unique, and yet there may be multiple matches to a mark recovered at a crime scene, for example in the Brandon Mayfield case. There are a number of intersecting issues that relate to this, including some issues with the way in which fingerprint examination is carried out. One of the key issues is that even if fingerprints are unique – and no two individuals have ever been identified with the same fingerprints – this does not mean that the marks that are left at crime scenes are necessarily unique. Fingerprints taken from suspects in controlled conditions include all of the details that is present in that person’s prints, but partial finger marks (or partial latent prints are they are known in other jurisdictions) that are left at a crime scene may be of poor quality and have limited amounts of detail in them. Thus, it can be very challenging to match these partial marks to a fingerprint, and indeed these marks may be potential matches to fingerprints from a number of individuals. You can read some more about this fascinating topic in this article, which discusses some of the problems that the forensic community has been debating over the last couple of decades.

Share this article:

This article is from the free online course:

Introduction to Forensic Science

University of Strathclyde