All language subtitles for 97031x_MH_DNA_Markers_02_SSRs_v3-en

MICHAEL HEMANN: So there are a couple kinds of main DNA

markers that we'll talk about, and the first of them

are called SSRs.

And SSRs are simple sequence repeats.

So as you know, much of our genome

consists of repetitive elements.

Some of these are different viruses or transposons that

have popped into our genome.

We have lots of repeat sequences at centromeres

and the telomeres, and we also have simple repeats, so two,

or three, or four nucleotide repeats that are actually

found throughout our genome.

And again, they likely have no functional relevance at all,

or very few actually do.

But we have a genome that is much

larger than our actual total sum gene size, or gene content.

And so we have a simple sequence repeats.

And so what does a simple sequence repeat look like?

Well, it could be just two nucleotides, like c and A.

And they'll be present at some length.

There could be 20 repeats, there could be 50 repeats,

there could be hundreds repeats of just

this dinucleotide sequence.

And they're, again, spread out through our entire genome.

And they are variable between individuals and variable

between alleles.

So in some cases, a person may have

a 40 repeat stretch, so 40 copies of CA on one chromosome,

and 80 on the homolog, the one that you got from mom and dad.

So they are variable.

They're polymorphic.

And so we can use these changes and distinctions in size

to identify which allele that we're talking about

and which parent that we're talking about.

So how do we actually mark a particular CA repeat?

Well, you can think of them as just sitting

in a place in the genome.

So you have at some point CACACA.

Maybe this is on chromosome 10, 10p, the short arm.

At some precise location, you have a repeat length

of some n, some number of CA repeats.

And we define their location essentially

by using PCR primers.

So you'll have PCR primer here and a PCR primer here.

And these target sites for the PCR primers

are invariant between people.

So they're exactly the same.

It's just the repeat length between these PCR primers

is different.

So the PCR primers define the precise place in the genome.

So we know these sequences are on 10p

in a particular location, and we can amplify them using PCR.

And we'll get a certain length product

that corresponds with the length,

or the number of CA repeats.

So we can visualize these things on, say, an agarose gel.

So I'm going to draw a gel here with a few wells.

And say we have different lengths.

So we're to run the gel this way,

so bigger things are at the top and smaller things

are the bottom.

And so say that this corresponds to a CA repeat of 80 bases,

and this corresponds to CA repeat of 40 bases.

And so you have maybe a mom, and mom has one of each.

So one allele has 80 repeats and the other has 40 repeats,

and maybe you have dad, and dad has one that's even bigger,

and one that's sort of in the middle.

And you can have a child, and the child

will have, say, one there and one there.

So we can look at inheritance, we

can look at the segregation of both of these alleles

based on this variation in repeat length.

Now I actually remember, when I was starting in science,

I was working as a lab technician over at Harvard

and I was doing this kind of analysis.

And I ran a gel like this and I got this result.

How is this possible?

One of the parents isn't the parent.

This is an example of non-paternity.

It's not the father.

And in fact, this happens, and this is generally

how we do a paternity test.

We don't look at genes, we actually

look at markers that are variable in populations.

And you say, oh, did the child inherit

a marker from the proposed mother

and from the proposed father?

And if they didn't, then it's a possibility

that it's not the father and that it's

a case of non-paternity.

So this is a map of a chromosome.

In this case, it's chromosome 21 where

all of these things that are listed like D21S1410, these

are all simple repeat markers.

And so you imagine that if you are looking

at a bunch of different markers and all these markers

will have a different allele frequency,

which means how common is this length in the population.

So say you have 10 of these markers that you're looking at,

and they have different frequencies.

So maybe one is there at 10%, and 30%, and 5%,

and just putting in different numbers here.

So the different alleles that you're

looking at in an individual are going

to be present at variable amounts.

Sometimes they're in half the population, sometimes 70%,

sometimes they're low frequency.

But you can imagine that looking at 10 of these, if you actually

multiply these out, you're looking at less than 1

in 250 million chance, if you had these markers, of randomly

having these in an individual.

So you can get a really high resolution picture

of somebody's identity based on the identity

of these alleles within them.

And so a lot of forensic testing, paternity testing,

is based entirely on this, what are the repeat markers

that this person has.