All language subtitles for 67031x_MH_DNA_Markers_08_Making_Maps_v2-en

MICHAEL HEMMAN: So let's go back to our reciprocal pairs, right?

So which is the class that is the rarest class?

It's a little bit difficult to navigate here

with going from slide to slide.

But here we had the rarest class as wingless

minus M1B M2B, wingless plus M1A M2A, right?

And that's precisely the class that we

see in the middle, wingless minus M1B M2B,

wingless plus M1A, M2A.

So let me go back and forth again, right?

So here at the bottom, our rarest class,

wingless minus M1B M2B.

Here we have wingless minus M1B M2B.

And then wingless plus M1A M2A.

Does that make sense to everybody?

So we've just identified the rarest class.

And so we can actually now look at some of these numbers,

right?

So we know now that the right order is M1A or M1 wingless M2.

So M1 wingless M2, right?

So we can think about the recombination distances

actually between each of these components, right?

So we can figure out what is the distance based

on this order between M1 and wingless.

So here we're going to look at the total number

of recombinants that separates wingless and M1 wingless minus

and M1A and wingless plus and M1B, right?

So here at the top, there are no recombinants

So we're looking for something that is separating essentially

wingless minus and M1A.

And so is there any recombination

here between wingless minus and M1A?

No these are the parental classes, right?

Similarly, if we look here, wingless minus and M1A

are still together.

And wingless plus and M1B are still together, right?

So there's no recombination between these two.

However, when we look at this class,

we suddenly see wingless minus with M1B and wingless

plus with M1A.

So we know that these two are recombinants, right?

So if we look at the frequency of recombination,

we have 100 times 12 plus 10, right?

And we can also see that our double recombinants of course

are recombinants, right?

So we have separation of wingless minus and M1A here

and wingless plus and M1B there, right?

So we have plus 3 plus 5 over a total of 200 gametes.

And so this is 100 times 30 over 200.

So this equals 15 centimorgans.

We can similarly look at recombination between wingless

and M2, so things that are separating essentially

wingless minus and M2A, which we don't

have in the nonrecombinants but we do have in this group.

So wingless minus is now with M2B.

And wingless plus is with M2A.

So we have 100 times 20 plus 22.

And of course these double nonrecombinants also

have a separation of wingless minus and M2A.

So it's now wingless minus and M2B and wingless plus and M2A.

So it's plus 3 plus 5 over 200, which is 50/200,

which is 25 centimorgans.

So we can put together a map here.

And the map has M1, wingless.

M2, M1, and wingless are 15 centimorgans apart.

And then wingless and M2 are 25 centimorgans apart.

So the total distance that we actually have in this context

is 40 centimorgans.

So M1 and M2 are separated from one another

by 40 total centimorgans when we take into account wingless.

Now, this is somewhat at odds to our earlier statement, which

is that when we mapped M1 and M2,

we saw that they're 30 centimorgans apart.

And so why would that be the case?

Exactly.

Exactly right, double crossovers, right?

So we're underestimating the double crossovers here, right?

So basically we're losing this entire class

of double recombinants because there we're

looking at what is separating M1A and M2A, right?

And so here M1A and M2A at the top and M1B and M2B

are together.

They're separate here.

So now we have M1A and M2B.

But in the double crossovers, they

realign back together, right?

And so we're excluding this entire class of 8.

So what would our calculation be there,

where our calculation would just include these numbers, 42

and 22.

And there would be 100 times 42 plus 22

over 200, which equals 64/200, or 32 centimorgans.

So without that middle wingless gene,

without being able to see the double crossovers,

we will systematically underestimate

genetic distances.

So the more markers that we have,

the more genes that we have, the more fine mapping we can do.

And again, this is going to be really essential when we

start to fine map human genes.