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OK.  Let's get started.
We're going to complete our

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discussion about mitosis
and meiosis today.

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And then get into a topic called
Mendelian Genetics,

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understanding genetic principles.
And we'll give you as examples the

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work of the famous Austrian monk
Gregor Mendel who sort of set down

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the rules that we still follow today
regarding simple genetic inheritance.

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So I mentioned to you last time,
and I think you all know that your

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genes are carried on chromosomes.
And chromosomes come in different

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sizes and shapes.
You have 46 total chromosomes in

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all of your cells.
Those chromosomes come from your

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parents.  You get one set from one
parent and another set from another

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parent, so you have 22 pairs plus
the two sex chromosomes.

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And the chromosomes are made of DNA,
and along those DNA sequences are

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your genes scattered along the
length of the chromosomes.

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And there are about 30,000 genes in
total.

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Now, I've also told you that the
genome, the whole sequence of your

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DNA has been determined over the
last couple of years.

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We now know that in great detail.
We know every single nucleotide of

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the three times ten to the ninth
base pairs of DNA,

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and we can actually go to a website
and look at it.

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You can do this.
It's publicly available,

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sponsored by the government,
but it's a little like hacking into

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the database of humanity.
So if you go to this website that I

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just showed you,
you'll find access to the genomes of

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many organisms.
This is an evolutionary tree which

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shows the relationship of these
organisms from an evolutionary

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perspective.  There are nine
mammalian genomes that are present

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within this database.
If you clink on that link to

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homosapiens, that's us,
assuming this is functioning.

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I had this working.
I'm not sure why it's not working.

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I'm getting a good signal here.  Oh
well.  Had this worked,

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you would have seen an alignment,
a diagram of all the human

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chromosomes, one through 22,
plus X plus Y.  You could then click

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on chromosome whatever you're
interested, let's say chromosome one.

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It would show a diagram of
chromosome one from end to end.

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And you could then click on the
sequence of chromosome one.

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All two point four times ten to the
eighth base pairs of chromosome one.

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It would show you all of the genes
on chromosomes one.

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You would know exactly where they
were.  And you could then click on a

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particular gene and look at its
sequence.  And it would actually

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give you information about that gene,
as much information as is known.

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So, we know the nucleotide sequence
of all the genes.

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And the question is,
is that sequence that's in that

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database applicable to all of you?
Is that the sequence of your DNA?

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What do you say?  Why do you say no?
Right.  So it's somebody's DNA.

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Actually, the genome sequence
that's present in a publicly

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available database is a collection
of a small number of people's DNA,

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but it's a representation of the
human genome.  And you all have

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subtle differences.
In between genes quite a few,

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and within your genes some.  And we
call those differences within genes

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allelic differences,
you have different alleles,

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and that's what makes you all
different, which alleles you carry,

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and importantly which combinations
of alleles of genes that you carry.

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OK.  I apologize for that not
working.  I don't know why it didn't.

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All right.  Well,
last time we left off talking about

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the fact that you inherit your
chromosomes through the process

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of fertilization.
Fusion of sperm and egg produces a

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diploid cell from two haploid cells.
This diploid cell has two copies of

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all the chromosomes,
two copies of all the genes,

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and then through the process of
development there is a great deal of

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mitosis.  We talked about mitosis in
some detail last time.

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The process of chromosome
duplication followed by chromosome

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separation, chromatid separation
really, which allows daughter cells,

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during this process of mitosis to
faithfully inherit all of the DNA.

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And this process continues through
development, and it also continues

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in adults.  It's not over when the
baby is born.  There's a lot of

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mitosis that takes place in many of
your organs, and your intestines for

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example, your blood.
Not in all of your organs.

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In the brain, for example, there is
relatively little mitosis.

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Once the brain forms, cells don't
divide anymore.

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It's true of the cardiac muscle as
well in the heart.

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But many tissues do continue to
divide.  So this is happening inside

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of you as we speak.
Now, what I want to turn to for

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today, though,
is to talk about this process.

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How do you go back to the beginning?
How do you generate haploid cells

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from diploid cells?
And this occurs through a related

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process called meiosis.
Now, meiosis takes place at

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different times,
depending on whether you're male or

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female.  As I mentioned last time in
males meiosis takes place

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in puberty.
So diploid germ cells are set aside

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in embryogenesis,
but they don't undergo this process

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of meiosis until males reach puberty
at which point it happens very

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abundantly, as I said,
about a million or a couple of

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million meiotic cells produced per
hour.  In females meiosis actually

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begins in embryogenesis,
quite amazing.  The cells are set

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aside and they start to undergo
meiosis.  And they don't make it all

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the way through.
You don't make haploid germ cells

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in embryogenesis.
They get stuck partway through.

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Then in puberty,
during ovulation a subset of those

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cells continue through the
subsequent stages of meiosis.

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Then they get stuck again.  And
only at fertilization does meiosis

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get completed,
a haploid cell is produced which is

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then fused with the sperm cell,
a diploid cell is generated and

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development proceeds.
So what is meiosis and how does it

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compare to mitosis?

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Well, they're related in the sense
that both of them follow S phase.

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If you remember during S phase
chromosomes get duplicated.

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And during chromosome duplication
two chromatids are produced.

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You're not going to start singing,

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are you?  No. Just kidding.  When a
chromosome gets produced two

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chromatids are generated and then
they stick together at the

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centromere.  Remember that?
Now, in mitosis, as we talked about

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before, this is followed by one
round of chromatid separation.

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That's all it is.
The chromosomes with their two

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chromatids line up along the
metaphase plate,

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and they get pulled apart during
anaphase and telophase.

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And importantly the homologs don't
pay any attention to one another.

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The maternal copy of chromosome one
and the paternal copy of chromosome

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two ignore each other in mitosis.
They could be anywhere along that

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metaphase plate.
It doesn't matter.

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They ignore one another.
OK?  That's relevant because it's

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different in meiosis.

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In meiosis, also following one round
of chromosome duplication and the

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generation of two chromatids per
chromosome, this involves one round

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of chromosome separation.
And when I say chromosome here,

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I'm talking about the chromosome
which is at this point composed of

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two chromatids.
In this case the homologs do pay

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attention to one another and they
separate from one another.

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Followed by one round of chromatid
separation.

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Which is very similar to what
happens over here.

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OK?  So the way you go from having
46 chromosomes to 23 chromosomes is

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you go through one round of
duplication but two rounds of

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separation.  The way you maintain 46
chromosomes in mitosis is to go

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through one round of duplication and
only one round of separation.

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Make sense?  OK.  So let's look at
that in detail.

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And I won't draw these pictures on
the board for you because,

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actually, I want to review it in
writing one more time.

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I'm not going to draw the details
of meiosis for you on the board

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because it just takes too long,
and the book does a perfectly good

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job diagramming it for you.
And I'll show you those diagrams in

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a second.  The terms that we use in
meiosis are the same terms that we

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use in mitosis.
But there are two rounds,

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as I mentioned, and they're broken
down by meiosis one and meiosis two.

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In meiosis one you have a prophase,
and it's called prophase one.

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You have a prometaphase,
prometaphase one.

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A metaphase where the chromosomes
align on the metaphase plate.

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And anaphase where the chromosomes
get pulled apart by the mitotic

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spindle, actually,
in this case the meiotic spindle.

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And finally a telophase where they
get pulled all the way to the poles.

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And then there's a second round,
meiosis two, where the same terms

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are applied but they're
denoted with twos.

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So there's a second phase of
prophase, of prometaphase,

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metaphase, anaphase and telophase.
And this is where the chromatids

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align on the chromosome plate and
get separated.

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OK?  Now, there's a very key event,
and I'm going to draw it for you

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because it's so important,
that takes place during prophase of

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meiosis one.  And this is the
distinguishing feature between

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meiosis and mitosis.
As I said in mitosis,

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the homologs, the two copies of
chromosome one,

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the maternal one and the paternal
one for example ignore one another.

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That's not true in mitosis, in
meiosis.  In meiosis they bind to

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one another.  So imagine again our,
I think I called this the paternal

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copy of chromosome whatever,
one previously.  It had four genes

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that we showed previously.
It's represented as two chromatids,

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which are associated at the
centromere.  And then you have

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another homolog,
the maternal homolog.

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It likewise has the same four genes.
It's been duplicated so it's

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represented as two chromatids held
together at the centromere.

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In prophase of meiosis one,
these homologs pair.

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And it's the pairing of the homologs

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that allows the two to separate
faithfully during meiosis one.

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They also interact in an
interesting way that we'll come to

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in a moment.  They don't just pair.
They actually undergo interactions

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between each other which allows them
to exchange sequences which is

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critical for generating diversity
amongst our chromosomes.

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So now I'm going to take you through
these steps as shown in your book.

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This is figure 9.14 in your book.
We're going to go through meiosis

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one first.  So in prophase of
meiosis one, we've undergone

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chromosome duplication already into
the chromatids.

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Now the chromosomes are going to
condense, and you can see that here.

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And importantly during prophase the
two chromosomes,

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represented by the four chromatids
align.

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In this picture you're seeing two
different chromosomes,

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a larger one and a smaller one.
And you can see that the maternal

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and paternal copies have paired with
one another.  OK?

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So we get pairs of homologs.
Still later in prophase they

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interact in this other fashion that
I just alluded to where they

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actually exchange information.
They exchange genetic information.

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And we'll go through this in a
little bit more detail in a bit.

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At metaphase of meiosis one,
metaphase one,

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the homologs line up on the
metaphase plate with one homolog to

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the left, the other homolog to the
right, one homolog to the left,

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the other homolog to the right.
This is one of two possible

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configurations that could take place
here.  In fact,

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in another cell undergoing metaphase
one, the red one might be on the

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left, the blue one on the right,
but here the blue one might be one

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the left, the red one might
be on the right.

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And that's important.
You'll see why later.

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So the homologs line up along the
metaphase plate.

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And now during anaphase one the
homologs separate from one another.

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The two chromatids of each homolog
separate from one another,

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and that's completed in telophase.
Now, without an intervening round

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of DNA duplication,
this does not involve another round

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of S phase, we go straight into
meiosis two where again the DNA

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condenses, a mitotic
spindle is built.

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And now, just like in mitosis,
the two chromatids line up along the

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metaphase plate with one chromatid
to the top, the other chromatid to

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the bottom, one chromatid to the top,
the other to the bottom.

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And then they get separated during
anaphase and telophase

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of meiosis two.
The end product of that are germ

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cells which are now haploid.
They have only one copy of each of

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the chromosomes,
one of the big ones and one of the

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small ones.  So that's how you go
from a cell that has two copies of

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each to a cell that has one copy of
each.  One round of DNA duplication,

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two rounds of chromosome or
chromatid separation.

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Now, importantly you'll see if you
can squint a little bit,

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that the chromosomes that come out
of this process of meiosis don't

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always look, in fact,
never look like the chromosomes that

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come in.  They undergo this process
of exchange of genetic information.

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This process of exchange of genetic

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information is called meiotic
recombination.

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And I'll show it to you in a figure

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in a second.  But it generates first
recombinant chromatids,

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chromatids that look different from
the chromatids that were generated

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initially.  That's why they're
called recombinant.

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And these, when resolved during
meiosis two, generate recombinant

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chromosomes.

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So, again, if we imagine our two
homologs of a given chromosome with

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the four genes that we talked about
previously lined up along the length

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of those chromosomes,
during this process of meiotic

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00:18:04,000 --> 00:18:20,000
recombination --

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00:18:20,000 --> 00:18:29,000
-- new versions of the chromosomes
are produced.  We can generate a

223
00:18:29,000 --> 00:18:39,000
chromosome which has a little bit of
Dad's DNA and a little

224
00:18:39,000 --> 00:18:46,000
bit of Mom's DNA.
A chromosome that was generated

225
00:18:46,000 --> 00:18:51,000
specifically in you.
It's your unique contribution to

226
00:18:51,000 --> 00:18:56,000
the offspring.
It's not solely what you got from

227
00:18:56,000 --> 00:19:01,000
Mom or Dad.  It's your unique
version of that chromosome that's a

228
00:19:01,000 --> 00:19:07,000
hybrid between what you got from Mom
and what you go from Dad.

229
00:19:07,000 --> 00:19:11,000
And this process of meiotic
recombination allows for the

230
00:19:11,000 --> 00:19:15,000
increased diversity in the
population.  You don't just pass on

231
00:19:15,000 --> 00:19:19,000
what you inherited.
You actually mix it up a little bit

232
00:19:19,000 --> 00:19:23,000
and pass on new combinations of the
alleles that you got from your

233
00:19:23,000 --> 00:19:27,000
parents.  So this is shown in
greater detail here.

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00:19:27,000 --> 00:19:32,000
And, again, gone through in much
detail in your book.

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00:19:32,000 --> 00:19:36,000
This is figure 9.
6.  Here we are in prophase of

236
00:19:36,000 --> 00:19:41,000
meiosis one.  The homologs have
paired and they've undergone a

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00:19:41,000 --> 00:19:45,000
genetic interaction.
The DNA of the paternal and the

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00:19:45,000 --> 00:19:50,000
maternal chromatids have literally
exchanged information.

239
00:19:50,000 --> 00:19:55,000
That's what this crossover is
called.  It's referred to as a

240
00:19:55,000 --> 00:20:00,000
chiasma.  And you can literally see
it in the electron microscope.

241
00:20:00,000 --> 00:20:04,000
When this gets resolved,
like a cut and paste reaction.

242
00:20:04,000 --> 00:20:08,000
When this gets resolved, you
generate new chromatids.

243
00:20:08,000 --> 00:20:12,000
This chromatid has mostly red
sequence but a little bit of blue

244
00:20:12,000 --> 00:20:16,000
sequence.  And this chromatid has
mostly blue sequence and a little

245
00:20:16,000 --> 00:20:20,000
bit of red sequence.
If you think about that with

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00:20:20,000 --> 00:20:24,000
respect to genes in our analogy over
here, the red chromosome had alleles

247
00:20:24,000 --> 00:20:28,000
that we'll refer to as the white
versions of gene one,

248
00:20:28,000 --> 00:20:32,000
two, three and four.
The blue homolog had the black

249
00:20:32,000 --> 00:20:36,000
versions of genes one,
two, three and four.  The products

250
00:20:36,000 --> 00:20:41,000
of this reaction include chromatids
that look just like the parental

251
00:20:41,000 --> 00:20:45,000
ones, a red chromosome with white
one, two, three and four,

252
00:20:45,000 --> 00:20:49,000
a black chromosome with, a blue
chromosome with black one,

253
00:20:49,000 --> 00:20:54,000
two, three and four, but also new
chromatids that didn't exist

254
00:20:54,000 --> 00:20:58,000
previously, a red one with white one,
two, three but a black four,

255
00:20:58,000 --> 00:21:03,000
and a blue one with black one two
and three and a white four.

256
00:21:03,000 --> 00:21:13,000
New versions of the chromosomes.
And this is important.  It's

257
00:21:13,000 --> 00:21:23,000
important for you to understand how
genetics works,

258
00:21:23,000 --> 00:21:34,000
but it's also important for the
species to generate increased

259
00:21:34,000 --> 00:21:44,000
diversity which when acted upon by
natural selection forces might

260
00:21:44,000 --> 00:21:54,000
select out more fit organisms.
So that is mitosis, meiosis and

261
00:21:54,000 --> 00:22:02,000
meiotic recombination.
And we'll be drawing on your

262
00:22:02,000 --> 00:22:06,000
knowledge of meiosis and mitotic
recombination to understand the

263
00:22:06,000 --> 00:22:11,000
principles of genetics.
And that will be the next topic

264
00:22:11,000 --> 00:22:16,000
that we turn to.
Before I do so I want to just

265
00:22:16,000 --> 00:22:20,000
mention that this process of meiosis
doesn't always work perfectly.

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00:22:20,000 --> 00:22:25,000
No biological system works
perfectly.  There's always some

267
00:22:25,000 --> 00:22:29,000
error rate.  And that's true in
meiosis in germ cell development,

268
00:22:29,000 --> 00:22:34,000
although it's supposed to be the
case that during metaphase one the

269
00:22:34,000 --> 00:22:39,000
homologs, which are paired together,
separate to the two daughters.

270
00:22:39,000 --> 00:22:44,000
That doesn't always happen.
Occasionally errors take place such

271
00:22:44,000 --> 00:22:50,000
that one daughter gets nothing and
the other daughter gets both copies.

272
00:22:50,000 --> 00:22:55,000
This is a problem because now when
these guys go through meiosis two

273
00:22:55,000 --> 00:23:01,000
they have two copies of a given
chromosome instead of just one.

274
00:23:01,000 --> 00:23:05,000
And if those germ cells,
in this case they're eggs,

275
00:23:05,000 --> 00:23:10,000
and that's usually the problem in
humans where it occurs,

276
00:23:10,000 --> 00:23:14,000
if those eggs get fused by a sperm
that carries its own copy of that

277
00:23:14,000 --> 00:23:19,000
chromosome the zygote,
instead of having just two copies,

278
00:23:19,000 --> 00:23:24,000
has three.  And this is a situation
called trisomy,

279
00:23:24,000 --> 00:23:29,000
an extra copy of a given chromosome.
The opposite can happen as well.

280
00:23:29,000 --> 00:23:32,000
This guy, this egg,
if it were to be fertilized would

281
00:23:32,000 --> 00:23:36,000
have only a single copy of that
given chromosome.

282
00:23:36,000 --> 00:23:40,000
That would be monosomy,
single instead of two.  This does

283
00:23:40,000 --> 00:23:44,000
happen during human development.
In the vast majority of cases when

284
00:23:44,000 --> 00:23:48,000
it happens it's incompatible with
early development and the fetus

285
00:23:48,000 --> 00:23:52,000
aborts.  There are a few examples
where the fetus can actually make it

286
00:23:52,000 --> 00:23:56,000
quite late in gestation and even be
born but then doesn't

287
00:23:56,000 --> 00:24:00,000
thrive thereafter.
But there's one example where the

288
00:24:00,000 --> 00:24:04,000
fetus comes to term,
is born and the individual can

289
00:24:04,000 --> 00:24:08,000
survive, and that's chromosome 21
trisomy or Down syndrome.

290
00:24:08,000 --> 00:24:12,000
This happens when during meiosis
one nondisjunction takes place such

291
00:24:12,000 --> 00:24:17,000
that two copies of chromosome 21
wind up in an egg.

292
00:24:17,000 --> 00:24:21,000
Sperm comes along and delivers
another copy of chromosome 21.

293
00:24:21,000 --> 00:24:25,000
Now that zygote is trisomic for
chromosome 21.

294
00:24:25,000 --> 00:24:30,000
And you can see the karyotype here.

295
00:24:30,000 --> 00:24:34,000
Everything else is diploid but this
has three copies of chromosome 21.

296
00:24:34,000 --> 00:24:38,000
And this leads to a characteristic
defect in development which results

297
00:24:38,000 --> 00:24:42,000
in an individual with very
characteristic features.

298
00:24:42,000 --> 00:24:46,000
And this extra copy of 21 is
responsible for this too much gene

299
00:24:46,000 --> 00:24:50,000
product from the genes on chromosome
21.  And I don't remember how many

300
00:24:50,000 --> 00:24:54,000
there are.  There are 600 or so
genes on chromosome 21.

301
00:24:54,000 --> 00:24:58,000
Having too much of some or some of
them, some combination of them,

302
00:24:58,000 --> 00:25:02,000
too much product from those leads to
defects in development,

303
00:25:02,000 --> 00:25:06,000
which results in this very clear
phenotype.

304
00:25:06,000 --> 00:25:10,000
So meiosis usually insures that you
get just one copy of each

305
00:25:10,000 --> 00:25:15,000
chromosomes, but it doesn't always
work.  And this can be one

306
00:25:15,000 --> 00:25:19,000
consequence.  OK.
Let's take a breath and change

307
00:25:19,000 --> 00:25:24,000
gears.  So we're going to move now
from mitosis and meiosis to genetics

308
00:25:24,000 --> 00:25:29,000
and genetic principles,
and specifically Mendelian genetics.

309
00:25:29,000 --> 00:25:32,000
So what we want to now begin to
understand is besides the mechanics,

310
00:25:32,000 --> 00:25:36,000
what are the consequences?  What
happens when you inherit alleles of

311
00:25:36,000 --> 00:25:40,000
given genes?  What is the effect of
the inheritance of those alleles and

312
00:25:40,000 --> 00:25:44,000
what are the principles that guide
that?  Why is it that when a child

313
00:25:44,000 --> 00:25:47,000
is born of two blue-eyed people that
child has blue eyes?

314
00:25:47,000 --> 00:25:51,000
When a child is born of a
brown-eyed person and blue-eyes

315
00:25:51,000 --> 00:25:55,000
person that child is likely to have
brown eyes, but not always.

316
00:25:55,000 --> 00:25:59,000
Maybe they'd have blue eyes.
What governs the presentation of a

317
00:25:59,000 --> 00:26:03,000
given trait based on the genes that
that individual inherits?

318
00:26:03,000 --> 00:26:06,000
Or more precisely the alleles of the
genes that individual inherits.

319
00:26:06,000 --> 00:26:10,000
You might actually be able to see
that the murderer can be seen in the

320
00:26:10,000 --> 00:26:13,000
reflection of this person's eye
right here.  You've got to look

321
00:26:13,000 --> 00:26:17,000
carefully.  We can learn a lot about
this by looking at inheritance

322
00:26:17,000 --> 00:26:20,000
patterns in humans.
And that's increasingly powerful

323
00:26:20,000 --> 00:26:24,000
given our knowledge of the human
genome today.  There's a lot more we

324
00:26:24,000 --> 00:26:28,000
can do in human genetics today than
we could have done ten years ago.

325
00:26:28,000 --> 00:26:32,000
But the field has actually been
brought along to this point using

326
00:26:32,000 --> 00:26:36,000
genetics in model organisms.
And this is a fly, a fruit fly,

327
00:26:36,000 --> 00:26:40,000
drosophila melanogaster, a major
genetic organism in biology.

328
00:26:40,000 --> 00:26:44,000
And you can see again that you can
isolate flies with different colored

329
00:26:44,000 --> 00:26:48,000
eyes, red eyes,
white eyes, dark eyes.

330
00:26:48,000 --> 00:26:52,000
And you can understand the
principles of genetics by crossing,

331
00:26:52,000 --> 00:26:56,000
mating flies together and looking at
what happens in the eye color of the

332
00:26:56,000 --> 00:27:00,000
offspring of flies that start off
with a given eye color.

333
00:27:00,000 --> 00:27:05,000
Now, the principles that guide us in
our thinking about genetics derive

334
00:27:05,000 --> 00:27:10,000
from this individual here whose name
is Gregor Mendel.

335
00:27:10,000 --> 00:27:15,000
And the principles that he laid
down are referred to as Mendelian

336
00:27:15,000 --> 00:27:21,000
inheritance.  And he,
through work that I'll briefly

337
00:27:21,000 --> 00:27:26,000
summarize, developed certain laws of
inheritance which turned

338
00:27:26,000 --> 00:27:31,000
out to be true.
And we use them even now as we think

339
00:27:31,000 --> 00:27:35,000
about how genes and alleles get
passed on through the generations.

340
00:27:35,000 --> 00:27:40,000
Mendel lived in the 1800s.  He did
his work around 1850s,

341
00:27:40,000 --> 00:27:44,000
1860s.  His organism was the pea
plant and the peas that give rise to

342
00:27:44,000 --> 00:27:48,000
the pea plant.
And he spent a lot of time

343
00:27:48,000 --> 00:27:53,000
observing peas and pea plants,
breading or crossing pea plants

344
00:27:53,000 --> 00:27:57,000
together, looking at the products of
those crosses and coming

345
00:27:57,000 --> 00:28:01,000
up with these theories.
He published a paper on that work in,

346
00:28:01,000 --> 00:28:04,000
I think, 1865,
and it was roundly ignored.

347
00:28:04,000 --> 00:28:07,000
Nobody paid any attention.  I
should have mentioned that he was an

348
00:28:07,000 --> 00:28:10,000
Austrian monk.
He lived up in the hills in Austria

349
00:28:10,000 --> 00:28:13,000
and did his work largely in
seclusion.  But he did,

350
00:28:13,000 --> 00:28:16,000
in fact, publish a paper in 1865
reporting his principles of

351
00:28:16,000 --> 00:28:19,000
inheritance.  And he was largely
ignored until about 50 years later

352
00:28:19,000 --> 00:28:22,000
when other workers started to do
similar things and basically

353
00:28:22,000 --> 00:28:25,000
rediscovered these principles that
he had laid down so many years

354
00:28:25,000 --> 00:28:28,000
before.  And we now give him credit
for coming up with these principles

355
00:28:28,000 --> 00:28:32,000
in the first place.
So Mendel focused on traits,

356
00:28:32,000 --> 00:28:37,000
traits that he could observe in the
pea plant or in the peas themselves.

357
00:28:37,000 --> 00:28:42,000
And this is an example of the
traits that he might look at.

358
00:28:42,000 --> 00:28:47,000
You may know that peas can come in
different shapes and textures.

359
00:28:47,000 --> 00:28:52,000
There are peas that are smooth.
There are peas that are wrinkled.

360
00:28:52,000 --> 00:28:57,000
And Mendel wondered what controlled
whether a pea was smooth

361
00:28:57,000 --> 00:29:01,000
or wrinkled.
If you crossed a pea plant that

362
00:29:01,000 --> 00:29:04,000
would have produced smooth peas with
a pea plant that would have produced

363
00:29:04,000 --> 00:29:08,000
wrinkled peas,
do you get smooth or wrinkled peas?

364
00:29:08,000 --> 00:29:11,000
The principles that guided the
thinking previously were that this

365
00:29:11,000 --> 00:29:14,000
was some sort of a mixture.
If you had one type and another

366
00:29:14,000 --> 00:29:18,000
type, the offspring would have some
intermediate type.

367
00:29:18,000 --> 00:29:21,000
There was sort of a mixing of
genetic information,

368
00:29:21,000 --> 00:29:24,000
not really referred to as genetic
information at the time,

369
00:29:24,000 --> 00:29:28,000
but heritable information.
And Mendel wondered whether that

370
00:29:28,000 --> 00:29:32,000
was true.
And then he actually determined that

371
00:29:32,000 --> 00:29:36,000
for most things that was not true.
One type determined the appearance

372
00:29:36,000 --> 00:29:41,000
of the trait in the offspring.
In order for you to understand what

373
00:29:41,000 --> 00:29:46,000
Mendel did, you have to understand a
little bit about pea plants and peas.

374
00:29:46,000 --> 00:29:50,000
One thing you need to know is that
peas are the embryo.

375
00:29:50,000 --> 00:29:55,000
They are the product of
fertilization.

376
00:29:55,000 --> 00:30:00,000
And when you plant a pea it will
develop into a pea plant.

377
00:30:00,000 --> 00:30:04,000
And you can score traits either in
the pea itself,

378
00:30:04,000 --> 00:30:09,000
in the embryo, smooth,
wrinkled, dark green, light green,

379
00:30:09,000 --> 00:30:13,000
or in the pea plant that results
from that pea.

380
00:30:13,000 --> 00:30:18,000
Does it have red flowers or white
flowers or pink flowers?

381
00:30:18,000 --> 00:30:23,000
And importantly you also can
fertilize one pea plant from another.

382
00:30:23,000 --> 00:30:27,000
You can take the germ cells,
the male germ cells or male gametes

383
00:30:27,000 --> 00:30:32,000
from one pea plant,
and purposely fertilize a female

384
00:30:32,000 --> 00:30:37,000
gamete of another pea plant.
You can carry out these very precise

385
00:30:37,000 --> 00:30:41,000
crosses.  Or you can do it within
the same pea plant.

386
00:30:41,000 --> 00:30:45,000
You can take the male germ cells or
gametes and fertilize the female

387
00:30:45,000 --> 00:30:49,000
gametes from that same plant,
so-called self-pollination.  OK?

388
00:30:49,000 --> 00:30:53,000
And through this methodology,
Mendel was able to very precisely

389
00:30:53,000 --> 00:30:57,000
control what the pea plants looked
like and how he could

390
00:30:57,000 --> 00:31:07,000
manipulate them.

391
00:31:07,000 --> 00:31:12,000
Through this process of
self-pollination,

392
00:31:12,000 --> 00:31:18,000
Mendel was able to generate what he
called ìpure breading strainsî.

393
00:31:18,000 --> 00:31:23,000
That is they always produced the
same trait.  If you cross them

394
00:31:23,000 --> 00:31:29,000
together, if you fertilize,
self-fertilize them, you always got

395
00:31:29,000 --> 00:31:34,000
the same result,
always smooth peas or always

396
00:31:34,000 --> 00:31:40,000
wrinkled peas or always red flowers
or always white flowers.

397
00:31:40,000 --> 00:31:43,000
They were pure.
There wasn't any heterogeneity.

398
00:31:43,000 --> 00:31:46,000
And we're going to refer to these
in our discussion as

399
00:31:46,000 --> 00:31:55,000
parental strains.

400
00:31:55,000 --> 00:32:02,000
And kind of an example of a question
that Mendel would want to know is if

401
00:32:02,000 --> 00:32:09,000
you took two parental strains and he
crossed pollinated from one that

402
00:32:09,000 --> 00:32:16,000
always produced smooth peas,
and here we're talking about this X

403
00:32:16,000 --> 00:32:23,000
represents crossing fertilization,
with a plant that always produced

404
00:32:23,000 --> 00:32:30,000
wrinkled peas, what did the
offspring look like?

405
00:32:30,000 --> 00:32:36,000
What did the pea look like in the
product of that cross?

406
00:32:36,000 --> 00:32:43,000
These parental strains are referred
to as P.  The product of a cross

407
00:32:43,000 --> 00:32:49,000
between two parental strains is
called the F1 or first

408
00:32:49,000 --> 00:33:02,000
filial generation.

409
00:33:02,000 --> 00:33:08,000
And the question Mendel wanted to
know was what is the P type in the

410
00:33:08,000 --> 00:33:14,000
F1?  The answer turns out,
for this particular example,

411
00:33:14,000 --> 00:33:20,000
to be smooth peas.  And the question
is why?  Based on this kind of

412
00:33:20,000 --> 00:33:27,000
observation, Mendel generated
a hypothesis.

413
00:33:27,000 --> 00:33:47,000
He suggested that traits in the peas,

414
00:33:47,000 --> 00:34:02,000
as well as in the subsequent plants,
arise from the inheritance of two

415
00:34:02,000 --> 00:34:12,000
units.
These units we would think of now as

416
00:34:12,000 --> 00:34:18,000
two alleles of a given gene.
He wasn't thinking of genes or

417
00:34:18,000 --> 00:34:23,000
alleles.  He was just thinking of
what number of things was

418
00:34:23,000 --> 00:34:29,000
contributing to this particular
trait.  And that these two units

419
00:34:29,000 --> 00:34:35,000
were delivered to the embryo
from each parent.

420
00:34:35,000 --> 00:34:49,000
And importantly the parent then has

421
00:34:49,000 --> 00:35:03,000
two units so that each parent
delivers one of its two alleles to

422
00:35:03,000 --> 00:35:17,000
the offspring via the production
of germ cells.

423
00:35:17,000 --> 00:35:20,000
So everybody has two.
They pass on one of those two to

424
00:35:20,000 --> 00:35:24,000
their germ cells.
And then the embryo is the product

425
00:35:24,000 --> 00:35:28,000
of two germ cells coming together
generating, once again,

426
00:35:28,000 --> 00:35:36,000
something with two units.  OK?

427
00:35:36,000 --> 00:35:41,000
So let's think about this with
respect to pea plants,

428
00:35:41,000 --> 00:35:46,000
and specifically the shape of the
peas.  So the trait that we're

429
00:35:46,000 --> 00:35:52,000
interested in is smooth
or wrinkled peas.

430
00:35:52,000 --> 00:36:03,000
The gene, in our language,

431
00:36:03,000 --> 00:36:10,000
is the S gene.  It is going to
determine whether or not the pea is

432
00:36:10,000 --> 00:36:18,000
smooth or wrinkled.
And this gene comes in two

433
00:36:18,000 --> 00:36:25,000
varieties, two alleles.
One is called big S and the other

434
00:36:25,000 --> 00:36:32,000
is called little S.
In the cross that was done,

435
00:36:32,000 --> 00:36:39,000
the plant that always generated
smooth peas, which was a pure

436
00:36:39,000 --> 00:36:46,000
breeding strain,
had the same allele in both copies.

437
00:36:46,000 --> 00:36:53,000
It was diploid for the big S allele.
Both of its chromosomes that carry

438
00:36:53,000 --> 00:37:00,000
the S gene had the big S
version of that gene.

439
00:37:00,000 --> 00:37:07,000
And the wrinkled plant,
the plant that produced wrinkled

440
00:37:07,000 --> 00:37:15,000
peas carried two copies of the other
allele, little S.

441
00:37:15,000 --> 00:37:23,000
These plants produced smooth peas.
These plants produced wrinkled peas.

442
00:37:23,000 --> 00:37:31,000
When these plants produced germ
cells, what allele of this gene gets

443
00:37:31,000 --> 00:37:38,000
put into those germ cells?
Big S.  It's the only one there.

444
00:37:38,000 --> 00:37:45,000
So when these plants undergo
meiosis they're going to produce

445
00:37:45,000 --> 00:37:53,000
germ cells that carry in them the
big S gene.  When these plants

446
00:37:53,000 --> 00:38:00,000
undergo meiosis what allele of the S
gene did they put in their

447
00:38:00,000 --> 00:38:07,000
germ cells?  Little S.
These haploid germ cells have the

448
00:38:07,000 --> 00:38:13,000
little S gene.
Now, when I cross,

449
00:38:13,000 --> 00:38:19,000
I take one of these germ cells,
mix it with one of those germ cells

450
00:38:19,000 --> 00:38:25,000
in the cross, what gene,
what genes, what alleles does that

451
00:38:25,000 --> 00:38:33,000
offspring inherit?
It gets a big S from here and it

452
00:38:33,000 --> 00:38:41,000
gets a little S from here.
OK?  And what do I observe when I

453
00:38:41,000 --> 00:38:49,000
make an organism,
a pea, that looks like that?

454
00:38:49,000 --> 00:38:58,000
What does the trait look like?
It's smooth.  OK.

455
00:38:58,000 --> 00:39:04,000
Now, I've drawn details without
given you some of the relevant

456
00:39:04,000 --> 00:39:11,000
nomenclature.  The parental strain
over here had big S,

457
00:39:11,000 --> 00:39:18,000
big S.  We call that the genotype.
A genotype refers to which alleles

458
00:39:18,000 --> 00:39:25,000
you have; your genotype.
The genotype of the other parental

459
00:39:25,000 --> 00:39:32,000
strain is S, S.
And the genotype of the offspring is

460
00:39:32,000 --> 00:39:39,000
big S, little S.
That's the genotype.

461
00:39:39,000 --> 00:39:46,000
If you have two of the same alleles
it is called being homozygous.

462
00:39:46,000 --> 00:39:53,000
This also has two of the same
alleles so it is also homozygous for

463
00:39:53,000 --> 00:40:00,000
the other allele but
still homozygous.

464
00:40:00,000 --> 00:40:08,000
If you have one allele of one type
and the other allele you are called

465
00:40:08,000 --> 00:40:16,000
heterozygous.  OK.
So that's some relevant terminology.

466
00:40:16,000 --> 00:40:24,000
Now, in contrast to the genotype,
we also refer to the phenotype.  And

467
00:40:24,000 --> 00:40:33,000
the phenotype refers to the
manifestation of the trait.

468
00:40:33,000 --> 00:40:40,000
What you look like.
Regardless of what the gene

469
00:40:40,000 --> 00:40:47,000
combination or allele combination
you have, what do you actually look

470
00:40:47,000 --> 00:40:54,000
like?  So what is the phenotype of
these peas?  Smooth.

471
00:40:54,000 --> 00:41:02,000
And what is the phenotype of these
peas?  They're wrinkled.

472
00:41:02,000 --> 00:41:09,000
And what is the phenotype of these
peas?  They're smooth.

473
00:41:09,000 --> 00:41:17,000
Based on the patterns that he
observed when he crossed two pure

474
00:41:17,000 --> 00:41:25,000
breading strains and generated
offspring that resembled one of the

475
00:41:25,000 --> 00:41:33,000
two pure breading strains,
Mendel suggested that in this

476
00:41:33,000 --> 00:41:41,000
particular cross the big S unit,
we would call it allele, is dominant

477
00:41:41,000 --> 00:41:48,000
over little S.
If you have a big S allele,

478
00:41:48,000 --> 00:41:54,000
regardless of what you have as the
other allele, you're going to have

479
00:41:54,000 --> 00:42:00,000
the big S trait which is smooth.
Big S is dominant over little S.

480
00:42:00,000 --> 00:42:06,000
And a related term,
little S is recessive to big S.

481
00:42:06,000 --> 00:42:13,000
The only way you observe the
phenotype associated with the little

482
00:42:13,000 --> 00:42:20,000
S allele is if you're homozygous for
the little S.  If you're

483
00:42:20,000 --> 00:42:27,000
heterozygous, as in the case in the
middle, you manifest the phenotype

484
00:42:27,000 --> 00:42:34,000
associated with the other allele.
Dominance and recessive.

485
00:42:34,000 --> 00:42:41,000
OK.  Now, based on these ideas of
inheritance of a single unit or

486
00:42:41,000 --> 00:42:47,000
allele from the parents who carry
two and the concept of dominance and

487
00:42:47,000 --> 00:42:54,000
recessiveness,
you can come up with methods for

488
00:42:54,000 --> 00:43:01,000
determining the frequency of
observing genotypes and phenotypes

489
00:43:01,000 --> 00:43:08,000
in crosses such as this.
So I want to consider now not a

490
00:43:08,000 --> 00:43:16,000
parental cross giving rise to an F1,
but rather a cross between F1.  We

491
00:43:16,000 --> 00:43:24,000
call this a hybrid cross.
And because we're focusing on one

492
00:43:24,000 --> 00:43:32,000
gene we actually call it
a monohybrid cross.

493
00:43:32,000 --> 00:43:44,000
The genotypes of the plants that

494
00:43:44,000 --> 00:43:49,000
we're going to start with in the F1
generation are as we described

495
00:43:49,000 --> 00:43:54,000
previously.  They're heterozygous
big S, little S.

496
00:43:54,000 --> 00:44:00,000
And that's true for one plant and
the other.

497
00:44:00,000 --> 00:44:07,000
They're both F1s.
They're both heterozygous for big S,

498
00:44:07,000 --> 00:44:14,000
little S.  If we now cross these
together, we can think about what

499
00:44:14,000 --> 00:44:22,000
germ cells these plants produce.
So what are the four products of

500
00:44:22,000 --> 00:44:29,000
meiosis look like from this?
What do you get in the meiotic

501
00:44:29,000 --> 00:44:37,000
products starting with
these genotypes?

502
00:44:37,000 --> 00:44:43,000
Two of them get big S,
two of them get little S.

503
00:44:43,000 --> 00:44:49,000
And likewise over here.  Two of the
germ cells get big S,

504
00:44:49,000 --> 00:44:55,000
two of the germ cells get little S.
Therefore, half of the germ cells

505
00:44:55,000 --> 00:45:01,000
are big S, half are little S.
If I think about the frequency of

506
00:45:01,000 --> 00:45:06,000
big S germ cells it's one-half.
If I think of the frequency of

507
00:45:06,000 --> 00:45:12,000
little S germ cells it's one-half.
And likewise over here, half of the

508
00:45:12,000 --> 00:45:17,000
germ cells are big S,
half of the germ cells are little S.

509
00:45:17,000 --> 00:45:23,000
Based on this idea I can generate a
grid --

510
00:45:23,000 --> 00:45:32,000
-- which will allow me to calculate

511
00:45:32,000 --> 00:45:37,000
the frequency of offspring that have
four different combinations.

512
00:45:37,000 --> 00:45:43,000
If this germ cell combines with
this germ cell I'll be big S,

513
00:45:43,000 --> 00:45:48,000
big S.  If this germ cell combines
with this germ cell the genotype

514
00:45:48,000 --> 00:45:53,000
will be big S,
little S.  If this germ cell

515
00:45:53,000 --> 00:45:59,000
combines with this germ cell the
genotype will be little

516
00:45:59,000 --> 00:46:04,000
S, little S.
And if this germ cell combines with

517
00:46:04,000 --> 00:46:08,000
this germ cell the genotype will be
big S, little S.

518
00:46:08,000 --> 00:46:13,000
This box, which is a useful device
for calculating the frequency of the

519
00:46:13,000 --> 00:46:17,000
genotypes that you observe in
different crosses,

520
00:46:17,000 --> 00:46:31,000
is referred to as a Punnett Square.

521
00:46:31,000 --> 00:46:36,000
So let's think about what the
numbers would be in such a

522
00:46:36,000 --> 00:46:42,000
monohybrid cross.
There are three possible genotypes.

523
00:46:42,000 --> 00:46:48,000
If you look at the Punnett Square
there are three possible genotypes.

524
00:46:48,000 --> 00:46:54,000
You can be homozygous big S, big S.
You can be homozygous little S,

525
00:46:54,000 --> 00:47:00,000
little S.  Or you can be
heterozygous big S, little S.

526
00:47:00,000 --> 00:47:07,000
Those are the three possible
genotypes from a cross such as this.

527
00:47:07,000 --> 00:47:15,000
The ratio of those genotypes, if
you look in the Punnett Square is

528
00:47:15,000 --> 00:47:23,000
there is one of these for every one
of these and two of these.

529
00:47:23,000 --> 00:47:31,000
The ratio of the three genotypes is
1:2:1 as determined by this

530
00:47:31,000 --> 00:47:37,000
kind of calculation.
The reason that you get that ratio

531
00:47:37,000 --> 00:47:43,000
is that a quarter of these,
sorry.  Half of these germ cells are

532
00:47:43,000 --> 00:47:48,000
big S, half of these germ cells are
big S, so a quarter plus,

533
00:47:48,000 --> 00:47:54,000
times a quarter, sorry, sorry,
sorry, I said that wrong.  Half of

534
00:47:54,000 --> 00:48:00,000
these germ cells are big S.
Half of those germ cells are big S.

535
00:48:00,000 --> 00:48:05,000
So a half times a half is a quarter.
Half of these germ cells are little

536
00:48:05,000 --> 00:48:10,000
S.  Half of these germ cells are
little S.  The probability of

537
00:48:10,000 --> 00:48:15,000
getting one of each is a half times
a half or a quarter.

538
00:48:15,000 --> 00:48:20,000
For big S, little S you can do it
two different ways,

539
00:48:20,000 --> 00:48:26,000
so it's a quarter plus a quarter or
a half.

540
00:48:26,000 --> 00:48:35,000
The ratio of these fractions is

541
00:48:35,000 --> 00:48:42,000
1:2:1 which determines the products
that you see.  Now,

542
00:48:42,000 --> 00:48:49,000
before I let you go,
I want you think about the

543
00:48:49,000 --> 00:48:56,000
phenotypes.  The phenotypes that you
observe in these peas is smooth,

544
00:48:56,000 --> 00:49:03,000
in these peas is smooth and in these
peas wrinkled.

545
00:49:03,000 --> 00:49:07,000
So the ratio of phenotypes is 2:1.
You'll need to think about why

546
00:49:07,000 --> 00:49:12,000
those, sorry, sorry,
two plus one is three,

547
00:49:12,000 --> 00:49:16,000
3:1.  You'll need to think about why
those principles give rise to these

548
00:49:16,000 --> 00:49:19,000
calculations and these frequencies.