## Want to keep learning?

This content is taken from the Pompeu Fabra University Barcelona's online course, Why Biology Matters: The Genome and You. Join the course to learn more.
3.16

## Pompeu Fabra University Barcelona

Skip to 0 minutes and 11 seconds But then you have the second part which is maybe even more complicated which is statistical analysis of that. That’s correct. Yes. And, well, our purpose is to establish or to find coincidences between genetic profiles from unknown samples with known samples from, as I said, from suspects or victims. In case of coincidence between genetic profiles, we cannot just report there’s a coincidence; we have to perform a statistical analysis of this coincidence. Why is that? That’s because we study a number, a low number of genetic markers, so this means that we do not study or analyse the whole genome.

Skip to 1 minute and 5 seconds So, a coincidence by using a number of markers has to be analysed from a statistical point of view just to give a strong evidence in this case. We cannot just say coincidence and that’s it, no, we have to perform this kind of analyses and with a, I would say, probabilistic meaning, you know. At the very end, you end up or you try to end up with probabilities close to zero or close to one. That’s it. This is it. So, our results mainly, if we find a coincidence, although we use a low number of genetic markers, the high polymorphic degree of this genetic markers gives us a probability results quite near to 100%.

Skip to 2 minutes and 1 second But we do not reach this 100% and this is why we have to perform this kind of study. Beyond these two parts, the genetic analysis and statistical analysis, then there is… all the related issues that goes from your results into court. First there is what is called “the chain of custody” which seems from outside that it is very, very, very important. This chain of custody is one of the most important issues in our work. This is, I would say, a kind of document that is used mainly… is such a kind of… How would I say?

Skip to 2 minutes and 48 seconds When the evidence of the sample is collected from the crime scene or by the forensic doctor, this has been carried to de laboratory to perform the analyses. Well, all the steps between the collecting of samples until it reaches the lab, this has to be fully documented. So that we know in each step where has been the sample, in what conditions, who was responsible about it. If just one or two of these steps is not completely documented this could be risky because this evidence could be not considered to… as the correct one or maybe this is not the evidence.

Skip to 3 minutes and 42 seconds Well, there would be a lot of doubts and this is what usually the defence in the courts, (defend) the lawyers of the defence, use to try to put some doubt… well, in your analyses. Because people may try to exchange them or… This is it. Maybe one of the most famous cases in the early 90’s it was the O. J. Simpson trial and, well, the lawyers of the defence they were able to put a lot of doubts in the chain of custody because there were some samples in the lab that were lost and so they could even say that maybe it was the police who took both samples of the suspect and put them in the scene of crime.

Skip to 4 minutes and 26 seconds So… And this is it. This is because he became not guilty and… okay… He became free.

# Conversation with Àlex Pifarré. Part 2

Àlex Pifarré, officer at the National Institute of Toxicology and Forensics of the Ministry of Justice in Barcelona.

The use of DNA in forensics is a subject that spans several disciplines: genetics, because it concerns various regions of the genome; statistical genetics, which are used to calculate the probability of a given hypothesis; and law, as the evidence obtained from the DNA is presented in court.