All language subtitles for 17031x_MH_DNA_Markers_03_SNPs_v1-en

MICHAEL HEMANN: So with these markers,

these dinucleotide repeat markers, maybe there

are around 1,000 informative markers in the genome.

And so our genome is haploid, about 3 billion bases.

And if you divide this by 1,000, then it

means that they're spaced about 3 million bases apart.

So 3 million base spacing between markers.

And so this is pretty informative.

It puts us in the neighborhood if we're

looking to map a particular gene to a particular location,

but not really--

it gives us the block, but not the house number.

And so we need to find a way to do finer mapping.

And the way that we do that is by looking at single nucleotide

polymorphisms, or SNPs.

And we'll talk a lot more about this later in the class

when we're talking about genome-wide association

studies.

But SNPs, as the name suggests, are just single nucleotide

polymorphisms.

They are distinctions.

You have the same base in some place in the genome.

One allele is an A and the other is a T. So just a single base

difference between alleles.

And again, they can have different frequencies.

Sometimes having a T is very, very rare.

And so we call something a SNP if it's

present in greater than 1% of the population.

So it must be present in, say, at least 1% of the population.

But there are tons of these.

So there are estimated maybe 10 million SNPs in the genome.

And so given a 3 billion base genome,

they're spaced around maybe 330 base pairs apart.

Now, this is an average.

Some are very close to one another,

some are actually quite far away from each other.

And there are spaces that are highly

repetitive in the genome where it's difficult to look at SNPs.

There are some places that are highly conserved within genes,

for example, where that frequency is quite low.

But again, we can have a really high resolution

map of the genome.

So our chromosomes are essentially

covered in these SNPs.

And so this allows us to do linkage studies

and really map genes to very, very, very, very

specific locations if we have enough people

and if we have enough informative SNPs.