All language subtitles for 47031x_MH_DNA_Markers_06_Three_Factor_Cross_v1-en

MICHAEL HEMANN: OK.

So let's think about two markers.

One of them is called M1 and the other one is called M2.

So not only can we look at the distance

between a marker and a phenotype,

but we could just look at the distance

between two markers using the same kind of analysis.

So in doing this, we determine that M1 and M2 are linked

on the same chromosome, and they're somewhere around 30

centimorgans apart.

So we have these two markers and we have our wingless gene.

And we want to simply ask, where is wingless

relative to these two markers?

Now we could do this by just looking

at pairwise interactions and pairwise crosses.

So wingless in M1, wingless in M2, and figure it out that way.

But we can do it another way that is somewhat faster,

an interesting way I think that tells us a little bit more

about meiosis, and that is by doing what's

called a three-factor cross.

Here, the three factors are going

to be M1 and M2 and wingless.

OK, so let's do this cross.

So we're going to start with two true-breeding flies

as we almost always do.

And this one is going to be wingless minus M1A and M2A.

And they're true-breeding, so M1A and M2A.

Now I've drawn them all on the same chromosome,

but we don't know what the order is.

It could be wingless M1A M2A.

It could be M1A wingless M2A.

That's what we're trying to figure out.

We're trying to figure out the order.

And we're going to cross with the male,

and the male is wingless plus M1B

So it's the B version of the M1 allele.

And M2B wingless plus M1B M2B.

Again, these are both true-breeding flies.

And I just want to remind you all that true-breeding does not

mean wildtype.

Breeding just means they are homozygous at all

of their loci.

So it essentially allows us to concoct an F1 constellation

of genotypes that is going to be informative in our cross.

So we're using this parental line

to generate an F1 where the meiosis is going

to be actually interesting.

OK.

So the F1 for this cross is going

to be heterozygous at all of these loci.

So wingless minus M1A M2A.

Wingless plus M1B M2B F1.

And so this is going to be the organism-- this

is going to be the female where all the action happens,

where we actually see meiosis and we

can look at the distances between all of these things.

OK.

So let's draw this female again at the top.

So we have female wingless minus M1A M2A, wingless plus M1B M2B.

And we'll cross with a male that is homozygous

for M1A M2A M1A and M2A.

And so we're going to look at just the meiotic of products

that are generated by the females.

So the gametes from the female.

I mean, importantly, we can see by phenotype

all of the offspring.

So just looking at the flies, we can

see whether the wingless are not,

and then we can look at the marker status

and see are they AA or AB.

So these are things that fall under the general constellation

of phenotypes that we can see, but in this case,

we're just going to look at the gametes.

So we can have wingless minus M1A M2A or wingless minus--

or wingless plus M1B M2B.

So these you'll recognize this the parental,

these are nonrecombinant.

And then we can get all of the possible recombinant.

So we can get wingless minus M1A M2B and wingless plus M1B M2A.

We can get wingless minus M1B M2A and wingless plus M1A M2B.

Or we can get wingless minus M1B M2B and wingless plus M1A M2A.

So there are eight total possible gametes that

are coming from this cross.

Now you'll notice the way that I've drawn them is in pairs.

And we actually call these pairs reciprocal pairs.

Because essentially, they are the two recombination products

you would get from crossing-over events.

So in each of these cases, I have

all of the markers represented.

In this case here, we have essentially M2A and M2B

that have flipped from one chromosome to the other.

And so we see the two recombination products

that would be generated from that recombination event.

And so we can put in numbers.

And let's say we have 60 flies, 68 flies, 20 flies, 22 flies,

12 flies, 10 flies, three flies, five flies.

These are experimental numbers.

You did a cross and you're just counting flies.

What you'd expect in theory is that in a given reciprocal

pair, the numbers would be identical.

And if you counted enough flies, they would be.

But you can already see here with the numbers

that we've generated that they're

very similar to one another.

60 and 68, 20 and 22, 12 and 10, 3 and 5.

OK.

So we have a total of 128 of these.

We have 42 of these, we have 22 of these,

and we have eight of those.

And we're going to call this pair number 1, pair number 2,

and pair number 3.

So the way that the three-factor cross works is we're

going to look at the rarest class of reciprocal pairs.

And we're going to say that the rarest class is the result

of a double-crossover.

Now we talked about double-crossovers last week.

And you'd expect that a single-crossover

is going to be more likely than a double-crossover.