All language subtitles for 57031x_MH_Tetrad_Analysis_04_Double_Crossovers_v2-en

MICHAEL HEMANN: So what we're going to do here

is we're going to actually use this kind of tetrad analysis

to identify double crossovers, which we know

are very difficult to identify in the Drosophila crosses,

the mammalian crosses, and are going

to be easier to identify using this tetrad analysis.

It's really the power of this tetrad analysis.

So we're going to use tetrad analysis to find

hidden double crossovers.

OK, so let's think about our initial event

again where we have big A, big B, big A, big B going

into meiosis, little a, little b, little a, little b.

And let's number of these chromosomes.

So the top is one, second is two, the third is three,

the bottom is four.

So let's say we have a crossover event that

occurs between chromosomes two and three.

So first crossover is between two and three.

And so if you get that crossover event,

you have AB; big A, little b; little a, big B;

little a, little b.

So again, what kind of tetrad is this

if this were the end of it?

T. It'd be a tetratype, so we have four different possible--

we have four different distinct haploid cells.

Now let's think about a second crossover.

So second.

And here we're going to have different possibilities.

We can have a second crossover being a two,

three crossover or it could be a one,

four crossover or it could be a one, three crossover

or it could be a two, four crossover.

So we're thinking about all of the other chromosomes

could recombine after this initial event.

Just sort of random.

So what if you actually had a crossover event that occurred

again between two and three?

What would the outcome of that crossover

be in terms of the kind of tetrad

that you would get in the end?

Exactly.

So this kind of tetrad or this kind of recombination event

would result in a PD because all you're doing

is flipping it back the way it came,

so you're just restoring the initial orientation.

So you're getting what you had to begin with,

which is that or this, which is a parental ditype.

All right, so let's think, OK, what

happens if you had a crossover event that

went between one and four?

So there you're putting a big B down to the bottom

and a little b up at the top.

Exactly.

So this kind is a nonparental ditype

because what you end up with is big a, little B;

big a, little B; little a, big B; little a, big B.

So two different types, they're either big A, little

b or little a, big B, and neither of them

are parental so they're nonparental ditypes.

OK, so what happens if you get a one to three a crossover event?

Yep, it's tetratype.

You notice you're not really changing much here.

You're actually moving a big B down to a big B and a big B

up to a big B, so you're not changing anything.

And this initial-- this second three cross

resulted in a tetratype, so you're just going

to maintain that tetratype.

You have four different kinds of haploid cells.

And you'll notice the same thing happens

if you recombined two and four.

So there you're just going to bring a little b up an exchange

with another little b.

So again, you're going to have a tetratype

if you do have a second two, four recombination.

And so you can go through this looking

at any of these different starting alleles

and if you have a crossover event between A and B,

regardless what chromosome they're on,

the first crossover always generates a tetratype.

And again, we're talking about crossovers between homologues

here, not crossovers between sister chromotids.

Crossover between homologues always yields a T.

And all of these second crossovers are equally likely,

and you expect them to occur at this ratio.

So one PD to one NPD to two different Ts.