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,