All language subtitles for 87031x_MH_DNA_Markers_01_Intro_Problem_v1-en

MICHAEL HEMANN: Today, we're going

to continue our discussion on mapping genes in the genome.

We talked about mapping phenotypes relative to one

another, which we can do, and we can

identify a genetic distance, which correlates roughly

with the physical distance between two different genes.

But we noticed that this relationship breaks down

at a long distance, and that really the longest

distance that we can measure between two genes

is about 50 centimorgans.

It's actually less than 50 centimorgans,

because we're getting multiple recombination

events between these two genes.

And so we are frequently underestimating distances.

And we're not able to do fine mapping the way

that we need to, because the whole purpose of doing

this is we actually want to find a gene that we're

looking for that has some alteration that we care about.

So today we're going to talk about how we do fine mapping

and how we use DNA markers to actually do

that mapping in place of clear phenotypes.

And so again, an example of the problem that we have

is that we have intervals in the genome.

So we have an interval here that's about nine centimorgans.

And so that's roughly about three megabases in Drosophila.

So that's three million bases, and so that

would be about 30 genes.

So we're looking for an alteration,

and we have a choice of 30 different genes.

And that's really too many, again,

when we want to have a good sense of where to look.

And this is an even bigger problem

when we talk about people, because we

can't do crosses between people, and we

can't look at phenotypes and the segregation

of different phenotypes in clear ways.

There are some ways that we'll talk about later

on in the class where we can look

at some fine mapping in phenotypes,

but we need a strategy to do fine resolution mapping when

we do crosses that allows us to identify

very specific regions of the genome

where we want to have a deep dive

to look for the alterations, and then

do functional studies to try to identify

whether the alterations that we find account for the phenotypes

that we see.

So the real challenge is how do we do fine mapping?

So high resolution mapping in the genome.

And the approach that we take is to use DNA markers.

So what are DNA markers?

Well, DNA markers are essentially just signposts

in the genome.

They're basically mile markers.

They may actually have no functional relevance at all.

They just define specific places in the genome

that we can map next to.

So you can really think of DNA markers

as places in the genome.