All language subtitles for 37031x_MH_Tetrad_Analysis_02_Linked_Genes_v2-en

MICHAEL HEMANN: OK, so let's think about two linked genes.

In this case, two very tightly linked genes.

One of them is A, and one of them

is B. So if we have a yeast haploid strain that is AB--

big A, big B-- we can cross it with another strain that's

a, little b.

We can generate a diploid that is AB over ab.

Here we're using, again, our linkage designation.

So this line says they're linked together

on the same chromosome.

This, of course, is a diploid.

And we can induce it to undergo meiosis,

and we'll get a tetrad out of this.

So let's think about two events.

One is where we have a meiosis where

we have basically no crossovers between A and B

on the same chromosome, and another

where we have a meiosis where we have one crossover.

So with a meiosis with no crossovers, we have AB, AB,

and we have little a, little b; little a, little b,

no crossovers.

So the four haploid cells that are

going to be produced have just the unrecombined chromosomes

from the parents, so AB, AB, little a,

little b; little a, little b.

These we call parental, because they

look like these original haploid cells that went into the cross.

So just like in the fly experiments

we were talking about, we have parental alleles.

So not only are they parental, but we actually

have two kinds of haploid cells.

One kind is big A, big B. The other kind is little a, little

b.

So we're going to call these PDs.

And what PD stands for is Parental--

because they're both parental--

and Ditypes, so ditypes because there are two of them.

There are two types of offspring,

and they're both parental, so parental ditypes.

So let's think about an event that has one crossover.

So with one crossover, so here again we

start with the same big A, big B; big A, big B

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

But we have one crossover event that's between A and B.

And so the result of this is four, again, haploid cells.

One is big A, big B. The other is big A, little

b because this little b here has gone over to the top.

The third is little a, big B, and the bottom

is little a, little b.

So how many different kinds of haploid cells do we have?

Four, right.

They're all different from one another.

Unlike a ditype before, we have what we call a tetratype.

And we'll indicate that with a T.

So there are four different kinds of haploid cells.

And so tetra, four.

So we here have two different kinds

of recognizable conformations of haploid cells.

We're looking, essentially, not only not at individual cells,

because there are individual haploid cells like big A,

big B, and big A, big B that are common in both types,

but we're looking at the aggregate, what

all four of them look like together.

So in one case, one tetrad is a parental ditype tetrad

and the other is a tetratype tetrad.

So if genes are really close together,

we're only going to get single crossovers and no crossovers

as an output.

So genes that are tightly linked don't have a lot

of recombination between them.

So we really only have two possibilities.

One is that we have a parental ditype here,

and the other is that we have a tetratype here,

because we're assuming that there

aren't multiple crossovers between multiple alleles.

So if genes are close together, we only

have parental ditypes and tetratypes.

All right, so let's think about distance here.

And so our formula, our recombination formula

for genetic distance in all organisms is centimorgans,

or the distance and centimorgans equals 100 times the number

of crossover gametes over the number of total gametes.

This is true in yeast, it's true Drosophila,

it's true in people.

So what is the number of crossover gametes in a T

tetrad?

So in this kind of tetrad here at the bottom,

how many crossover gametes do we have?

Right, we have two of them.

One of them is there, big A, little b, and the other

is little a, big B. So in a T tetrad,

we have two crossover gametes.

So we're going to come up with a term, epsilon,

which is essentially the number of total tetrads.

And 4E equals the number of total gametes,

because there are four gametes, there

are four haploid cells per tetrad.

So it's the number of tetrads times four.

So here we can say that centimorgans equals

100 times 2T, so two crossover gametes, times the number of T

tetrads over 4E, which is the number of total gametes,

equals 100 times T over 2E.

So this is the genetic distance for tightly linked genes

in yeast.

And so again, here we're looking at genetic distance

as a function of tetrad type.

So above this formula, everything

is referred to individual gametes.

Down here, we're looking at it in terms

of the kinds of tetrads that you're actually seeing.

So we're now thinking about recombination

in terms of a total tetrad, as opposed to individual gametes.

And that will become important when we're looking now

at double crossovers.