All language subtitles for 17031x_MH_Chromosomes_Mitosis_Meiosis_01_Parental_R-en

MICHAEL HEMANN: Today, we're going

to talk about chromosomes.

We're going to talk about mitosis and meiosis.

And it's really an attempt to start

placing genes in particular places in the genome.

So in the past number of lectures,

we've been talking about the gene,

essentially, as a unit of function and phenotype,

looking at pedigrees and crosses,

and learning what we could learn without really

having any concept of what a gene is or where it is.

Now we're going to start placing genes within the genome.

And so why do we do that?

We do that because we actually want to be able to map the gene

and identify the alteration.

We want, in the end, to be able to sequence the gene

to understand, what is the etiology of the phenotype

that we're looking at?

That tells us about the molecular mechanisms

by which it's doing what it's doing

and also potential therapeutic options

for what we can do about it if we don't like the phenotype

that we're seeing.

So it really starts with the ability

to map a particular phenotype to a particular place

in the genome.

And so let's start by thinking about the kinds of crosses

that we've been working on recently.

So let's just say that we have a cross that's

between true-breeding wild-type organisms of some type

and a true-breeding homozygous recessive.

So let's say there are two different traits, so

two distinct phenotypes, that are both

caused by being recessive homozygous

for a particular allele.

So the little a, little a, little b, little b

have a different phenotype.

So we can cross these organisms together, and we get an F1.

And the F1's are all heterozygous at this locus.

And we can cross these heterozygotes together

and get an F2 generation.

And this F2 generation, as we've talked about before,

has this phenotypic ratio of 9 to 3 to 3 to 1,

where nine would be wild type, three

would show one of the mutant phenotypes.

If these are independent traits, three

would show the other trait.

And one of them, one out of 16, would

show both of these traits.

And so one of the key ways of examining this F1 generation,

these heterozygotes, and the gametes

that they're actually generating to go into the F2 generation

is to perform a test cross.

And so a test cross is generally a cross

to a homozygous recessive.

So if we take these F1 heterozygotes

and we cross them with homozygous recessives,

we can actually identify in this cross all of the gametes

that this F1 is generating.

And this is going to be really key in understanding

whether there's truly independent assortment of genes

or whether these genes are in close approximation with one

another on the same chromosome, whether they're linked.

So this F1 heterozygote--

double heterozygote-- can generate four different kinds

of gametes.

It can generate big A, big B. They can

generate little a, little b.

They can generate big A, little b.

Or they can generate little a, big B.

So these are the four kinds of gametes that this F1

heterozygote is generating.

And we can designate these, basically, in two groups.

One of them is parental.

And parental simply means that these were the alleles

that they inherited from one of these first-generation parents.

So they could have gotten--

the only alleles you can generate from this parent

is big A, big B. The only alleles

you can generate from this parent are little a, little b.

So these we call parental.

And the other two-- big A, little b and little a,

big B-- we're going to call recombinant.

Because it's some mixture of these first-generation alleles.

It means that they got a big A from one of the parents,

or they're passing on one big A from one

of the parents and a small b from the other parent.

So somehow, these are recombining

in their germ cells.

And they'll produce, when we cross them

with the homozygous recessive in the test cross,

they'll produce four kinds of progeny.

One is big A, little a, big B, little b.

It's just if you actually cross this with little a, little b.

Another will be little a, little a, little b, little b.

The other will be big A, little a, little b, little b.

And the final will be little a, little a, big B, little b.

The cool thing with a test cross is you can actually

see all of these things.

They all look different.

So little a, little a, little b, little b is homozygous

recessive.

They have two traits.

Big A, little b, big B, little b, they're totally wild type.

And in a test cross, we'd expect,

if they are independently segregating genes,

that we're going to have a ratio here of 1 to 1 to 1 to 1,

so equal representation for all of these.

And this is really the hallmark of the segregation

of genes that are on distinct chromosomes.

It suggests that the genes, in fact, are not linked together.

They're not close together.

But they're actually on distinct chromosomes.

So again, today, we're going to talk

about chromosome segregation and what that looks like

and what it means and perhaps what

happens when it goes wrong.