All language subtitles for 47031x_MH_Tetrad_Analysis_03_Unlinked_Genes_v2-en

MICHAEL HEMANN: So let's think about the random segregation

of these alleles if they were completely unlinked.

So for random segregation we've got, basically, four alleles.

We've got big A, two big A's, two little a's, two big

B's, and two little b's.

So let's think about all of the ways that a's can segregate

with b's.

So here we have our A alleles and there are basically

six different combinations or ways

they can interact just randomly with a set of b alleles.

So you can have big B, big B, little b, little b.

You could have little b, little b,

big B, big B. You could have big B, little b, big B, little b.

You could have little b, big B, little b,

big B. You could have little b, big B, big B, little b and--

what am I missing?

OK.

You can have a big B, little b, little b, big B.

So this is essentially all of the possible interactions that

you can have with a's and b's.

So let's think about this first case here.

So in this case, you would have big A, big B; big A, big B;

little a, little b; little a, little b.

So if we had that in a tetrad, what kind of tetrad

would that be?

So of the two I introduced before, PD versus T,

which kind would it be?

Right.

So it's a PD.

Again, we have big A, big B. That's

one of the parental types, big A, big B. So we-- little a,

little b, little a, little b.

So there are two types.

It's a ditype.

They're both parental, so we're called a parental ditype.

All right, so let's think about this third case

here in the third case--

1, 2, 3.

The third case, we have big A, big B;

big A, little b; little a, big B; little a, little b.

What kind is that?

Looking at this one there.

Exactly.

So this is a tetratype because we have four different kinds.

We have big A, big B. We have big A, little b.

That's different.

Little a, big B. That's different.

And a little a little b.

There are two parentals and two nonparentals in that.

And so if you actually look, then this

is the same for all of these other cases for four, five,

and six as well.

In all these cases, if you work it out,

you're going to come up with four different types.

For the fifth case, big A, little b; big A, big B;

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

But it is different if we look at this second class here.

So for the second class we get big A, little b; big A, little

b; little a, big B; little a, big B. All right?

So that's different.

We haven't seen that one before.

So how many kinds of alleles do we have,

or how many kinds of haploid cells we have in the end here?

How many kinds of spores?

We have two, so it's a ditype.

And are either of these parental types?

No, neither of them are parental.

There are two types that are nonparental,

and so we're going to call that a nonparental ditype, so NPD.

And this ratio of having one parental to one nonparental

to four tetratypes is what you expect for unlinked loci.

So you get one to four to one of PD to T to NPD.

It's the ratio that you would expect

if they're on completely different chromosomes

or they're at 50 centimorgans or greater on the same chromosome.