Statistical genetics in forensic science

Published on November 30, 2016   51 min

Other Talks in the Series: Statistical Genetics

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Hi there. My name's James Curran. I'm a Professor of Statistics at the University of Auckland in New Zealand. And I'm also an expert in the field of forensic interpretation of DNA evidence. And today, I'll be talking to you about some of the issues involved in statistical genetics in forensic science.
So the first question that I usually ask people when I start to talk about DNA evidence is, "What do you actually think DNA evidence looks like? Do you think it's like a barcode? Do you think it's like a telephone number?" The picture that you see in front of you is called an electropherogram, and this is what a modern DNA profile looks like. This, in fact, is my own DNA profile. It's what comes out of a modern genetic analyzer that's employed in a forensic laboratory. So going across the page on the horizontal axis is a scale that basically represents molecular weight and that's measured in the terms of number of bases. If you look at the top of each panel, you'll see gray rectangles that are framed in green, and these represents locations along the chromosome which we call, in singular we call them a locus, and in plural, we call them loci. Now if you look at each panel in turn, you can see that there are, at each locus, there is either one or two peaks, okay. So if there is one peak, this means that I'm homozygous at that locus and I got the same allele from my mom and from my dad. And if I see two peaks, then that means that I'm heterozygous at that locus. So let's look for example at the first locus there which is D3S1358, or just D3 for short, and you can see that there's one very large peak at 16. There is a little bump next to it. That's called a stutter, and I may talk about that later on, but that's not actually out of my genetic profile. That's an artifact of the amplification and measuring process. So I'm a 16, 16 at that locus, at locus D3. So I'm homozygous at D3, the genotype 16, 16. If I take the next locus along, however, which is TH01, you can see that I have two alleles. So 6 and 8, so I'm heterozygous at this locus. I got 6 from one of my parents and then 8 from another of my parents. I don't know which one I got from which parent unless I have their genotypes. The different colors in the electropherogram represent the different dyes that are bound to the DNA. So what happens is that the DNA is exposed to enzymes, which cuts the DNA at specific locations at the loci and the fragments that are left are genes or alleles and they vary in length, so the number of times a certain pattern is repeated. So if I see an allele which is designated as 6, that means that pattern of bases that makes up the variance at the locus TH01 has been repeated six times. If I see 8, that means it's been repeated 8 times and so on. The other thing that you'll notice here on the electropherogram is that at some loci. In fact, in my profile I had only locus in particular, there is a fractional repeat. So you'll see that at D21S11, I have the genotype 29 and my other allele is 32.2. The 0.2 means that the motif or the pattern has been repeated 2 out of the 4 bases that make up that pattern. So when we're talking about forensic DNA evidence,