Skip to 0 minutes and 4 secondsSo we've done all the process of DNA profiling. Could you explain the result we've got here? OK, so after all the DNA went through the capillary, because you've just seen the capillary electrophoresis, what we're left with is this pattern of peaks. So, the further to the right means that it was later, and as this little bit of DNA was going through the capillary, so it's further to the right. The different colours are all to do with the different fluorescent dyes we added during PCR. So what we're left with is this unique pattern. Yes. So we've got blue, we've got green, we've got yellow and we've got red.

Skip to 0 minutes and 45 secondsSo, between the four of them, what it means is that this particular pattern of peaks we see is, on average between unrelated people, around about one in a billion - one in a billion rarity. OK. So, it's quite good for us to be able to match. Yeah. Specific. So, what exactly does it all mean? Well, if you remember when we were talking about DNA to start with, I said that from your parents, you get two copies of all your DNA in every cell. One from your mother, one from your father, and what we can see is that we've got different DNA markers you're looking at. And yep, we have two peaks at each one.

Skip to 1 minute and 25 secondsSo it means that in all of these cases - From mother and father? -- yep. One from the mother, one from the father. If we look down here, we can see that this particular marker - which is called D1, so its on chromosome one - we only have one peak. So, what that means is that for this particular DNA sample, the DNA that came from the mother and from the father was identical. OK. So, we have heterozygous alleles and we have homozygous alleles.

Skip to 1 minute and 55 secondsNow, everything we're looking at here are STR markers - these are these short tandem repeat markers, so they're bits of DNA that repeat multiple times - and all we've done is we've just counted how many repeats are in there, and that, as far as this analysis goes, is what we see below. So, we can see that these grey bars behind tell us where these peaks should appear for normal sets of repeats that we'd see within a population. And we can see that there's a peak here, that is saying that there's 14 repeats, and another peak here that's saying that there's 15 repeats.

Skip to 2 minutes and 31 secondsIf we look at a different marker, we can see here we've got a peak that lines up for where you would expect to see something with 10 repeats, and there, with 23. So it depends on the individual. The place and the number is different. Yep. OK. So depending on the place would depend on the number, and it's the collection of all of these numbers. So, we've got 16 STR markers we're looking at. So for every marker, we'll have either one or two numbers depending on whether it's the same from both parents or different. It's that collection of anything up to 32 numbers that is this unique signature of an individual. OK.

Skip to 3 minutes and 11 secondsThat's what we'll upload on the DNA database, and from the suspect profile, we would hope to find a match. I can see this XX mark there. It means it's female. Yep, so on the left-hand side, what we've got is we've got an extra marker, which is a sex marker. Obviously, women have two X chromosomes, the man has an X and a Y, and you can see here that it's says that we have 2X chromosome. So, this profile is definitely from a woman. So, what we have to do is to find someone matched to this same pattern, and then that's going to be a suspect. That is correct.

Skip to 3 minutes and 52 secondsSo this particular sequence that we are looking for, or this particular profile we're looking at, is the positive control sample. So it's just a synthetic DNA sample we run with every run. And we can see that this is all typed correctly, so the thing you were doing in the lab is correct. What we now want to do is we want to look at the samples from the crime scene. We want to see whether any of them match the victim. We would expect that probably there's some blood that would match to the victim, so we want to see if any of those profiles matched.

Skip to 4 minutes and 22 secondsWe also want to check to see whether any of those profiles are going to match to any of the people that have done the laboratory work, say, for example, your profile. OK. Because we wouldn't want to have it that actually you're just amplifying your own DNA. So hopefully, if we can find out one of the profiles from the crime scene belong to the victim, we can put them to one side. We can then check that the other profiles don't match to anyone in the lab. Then probably, that is our suspect, and we can then put that in the national DNA database, that profile, and we can see if we can find someone. OK.

Skip to 4 minutes and 55 secondsOK, so I'm going to leave you now to do this analysis. And we'll see what we get. Thank you very much. Thank you.

Profile Analysis

Once DNA samples have been profiled, analysis needs to be carried out to determine what the results mean. Watch David Ballard explain more.

Share this video:

This video is from the free online course:

The Science Behind Forensic Science

King's College London

Get a taste of this course

Find out what this course is like by previewing some of the course steps before you join: