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ERIC S. LANDER: Now, what happens to Mendel?
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Mendel makes this amazing discovery.
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He produces this great paper.
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Mendel never writes an amazing paper again.
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His entire scientific reputation hangs on this 1865 paper.
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He ends up working on another plant, a hawkweed that turns out to be a very
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unfortunate experimental choice.
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It has a lot of weird properties.
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And he can't ever make sense of it.
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And what Mendel does, because he's so successful, is Mendel gets promoted to
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be the abbot of the monastery, becomes consumed with administration and never
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really writes another great scientific paper, although he studies meteorology
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and other things like that.
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But that's pretty much it for Mendel's career.
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Mendel passes away.
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And this Mendel stuff is kind of largely forgotten.
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And the 1800s roll along.
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And an entirely different line of work appears.
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The entirely different line of work--
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section five now--
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cytology, cells in microscopes, people studying cells in microscopes.
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Well, microscopes had been invented long before.
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But when you looked under a microscope at cells, you could see all sorts of
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interesting structures, all sorts of interesting shapes
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and things like that.
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But what really began to make cytology interesting was the
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German chemical industry.
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The German chemical industry began inventing dyes, dyes
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you could stain things.
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And somebody thought to put dyes on cells.
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And they put dyes on cells.
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And what they found was that in the nucleus of a cell at certain times,
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dyes would stick to certain structures that look like these things, these
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some kind of bodies there.
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And these colored things were really interesting.
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And they had no idea what they were.
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And what do you do in science when you have no idea what something is?
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STUDENT: Poke at it.
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ERIC S. LANDER: You poke at it.
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But the problem is, you're going to poke in the microscope
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there and mess it up.
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Now, in order to make yourself feel better, you give it a name, because if
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you give a name, you're in control over it.
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The only thing they knew about these colored bodies were that they were
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colored bodies.
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And so they called them chromosomes, meaning colored things.
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That's what it means.
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Chromosome means colored thing.
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So they found these chromosomes.
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And they didn't know what these chromosomes were doing.
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But they made observations of cells at many points of their cell division.
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It turns out, in the 1800s, could not take movies.
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But they inferred by looking at cells at many times that these chromosomes
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underwent a remarkable choreography.
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They did a dance during cell division.
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And for you, let's show a movie of what that looks like.
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Let's get a black and white movie of a plant cell here-- fire away.
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These are these colored things.
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We don't have it in color.
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It's a black and white picture.
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Just look what they're doing as the cell is under--
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this is a lily of some sort, I think.
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And it's undergoing division, this cell.
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And look at this, they're moving around.
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They're moving around.
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They're arranging themselves.
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They're getting organized in some way.
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There we go.
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They're getting organized.
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Whoa.
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Check that out.
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Every one of them split, goes apart, and there you go.
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How interesting.
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Let's pull up another quick movie.
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Let's pull up a movie of a sea urchin.
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Check out this sea urchin here.
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Here we go.
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It'll go really quickly.
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Whoop--
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chromosomes going away.
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Let's pull up a frog embryo.
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Check out the frog embryo here.
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Check out those chromosomes.
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Tell me what you see here.
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And they're coming together.
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They're coming together.
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They're getting them all organized.
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Look at that.
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Oh, but there's a straggler.
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There's the straggler here.
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We're going to wait for the straggler.
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I'm waiting for the straggler.
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There we go.
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The straggler gets in there, and now it's ready to go.
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There we go, pulls back, divides, two cells.
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Not only do they organize themselves beautifully, but they even had the
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good courtesy to wait for the last chromosome to join.
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It is incredibly polite choreography.
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This was cell division, typical cell division called mitosis.
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What's going on here in mitosis?
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So in mitosis--
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now, from those gorgeous pictures, it's not so obvious how to follow
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everything.
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It took people a long time to figure out what was going on.
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And what the picture ends up looking like is something like this.
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That chromosomes, which we saw were going to split apart, really have two
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copies of something.
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And they're lining up along the middle of the cell.
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Take some interpretation to say this here.
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But it looks like they're lining up across the middle of the cell.
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When they've got themselves all perfectly lined up like that, they
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pull back and split apart.
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That's it.
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Yes?
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STUDENT: How accelerated was the video we were watching, or slowed down?
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ERIC S. LANDER: A fair amount accelerated.
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A fair amount accelerated, because we have a limited time in the class.
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So it's time lapse photography.
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That's right.
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So pretty easy.
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These things, somehow there were two of them.
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You saw how they split apart, right?
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When they split apart, there's singletons, but they must, at the time
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they appeared there look like a little x held together in the middle, because
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there were two copies of this thing.
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And by the way, after this movie, the chromosomes seem to disappear.
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And then they come back and re-condense, reappear.
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So they condense nicely into these shapes but then seem to go away.
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They're still there, but you don't see them.
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And they come back and they condense.
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And when they condense, the next time they'll have these two copies again.
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So somehow they've been duplicated while you're not watching.
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That was mitosis.
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But there was something else kind of interesting, which was when cells
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didn't just divide in their normal way, but cells divided to make
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gametes, say, like sperm or eggs, the cells that undergo fertilization.
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To make gametes, a very different picture emerged.
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The very different picture was that in meiosis, meiosis here, the chromosomes
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line up differently.
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Instead of the Xs, two copies of the same chromosome lining up like that
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all down the middle, it'll line up like this.
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I'll make a little chromosome here.
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And its little partner is over here.
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And the first meiotic division, meiosis number one, did not split
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apart the Xs.
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What it does is, it gives rise to two cells, each of which gets one
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copy of each pair.
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Then, in meiosis two--
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here's my little chromosome and my big chromosome.
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Just for the sake of argument, I'll call that one chromosome one and
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chromosome two.
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We've got chromosome one here, chromosome two.
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So we've got this.
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In the second meiotic division, meiosis two, now it undergoes a
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division that looks for all the world just like mitosis.
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And it gives rise now to that.
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That's it.
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So there's a big difference.
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What's the big difference here?
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Up there, there were four chromosomes or two pairs of chromosomes.
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They split.
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And you still have those four chromosomes.
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Here, I've only got two.
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They came in pairs and each offspring cell at the end of this process gets
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one of the two.
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Let's look over here.
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Mitosis--
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bring up my mitosis a second if you would.
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In mitosis, we have our homologs, meaning the two copies of a given
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chromosome.
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Each chromosome, they come in pairs here.
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They each split.
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Now, this picture here doesn't have them lined up nicely over each other
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like I did.
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But this is one chromosome, the other chromosome, and here are the Xs.
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I drew them lined up over on top of each other.
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But you can draw them any way you want to.
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And they get tugged.
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One of the two pieces gets tugged here.
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And the other piece gets tugged here.
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And you end up here with, the original chromosomes were
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duplicated, were separated.
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And it gets back to its initial state.
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That's mitosis.
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But in meiosis, we have an extra step.
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Chromosomes here--
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here, we're now just showing this pair.
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They get pulled over.
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And then secondarily, they undergo this separate division.
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So now, what does that tell us?
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Chromosomes come in pairs.
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This chromosome thing was found in 1889, 1890, actually, just about the
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time Buchner is working in his lab.
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Same time people are discovering chromosomes, and they're working out
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this choreography that says, when gametes are made, when you make sperm
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or eggs, something occurs in pairs.
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And one copy of each pair randomly goes to the daughter cell.
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And just about that time--
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1900 actually, January of 1900--
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people begin to redo the heredity experiments and rediscover and
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re-appreciate Mendel.
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Mendel is getting rediscovered.
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And the chromosome choreography is being worked out.
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And by 1902, someone says, wait a second, this idea that Mendel had of
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discrete particles of inheritance coming in pairs and going randomly,
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and these chromosomes coming in pairs and distributing randomly.
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Oh, we don't have to think abstractly anymore.
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That was a problem with Mendel.
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It was all so abstract.
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We didn't know what a gene was.
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It must be that these chromosomes are genes or carry genes or have something
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to do with genes because they have the same choreography.
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And so the idea starts floating around in 1902 that chromosomes must be the
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basis of heredity.
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It's controversial, but it's beginning to catch on.
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What a beautiful, beautiful idea--
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the Chromosome Theory of Inheritance.
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That here is the physical manifestation of Mendel's abstract
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genes, that when chromosomes line up, maybe this chromosome has the
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roundness gene.
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And let's see, this copy of the chromosome has been duplicated, but
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it's got the big Rs.
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And that guy there has got the little rs.
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And this chromosome over here, it's got the big G on it.
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That's the big G carrying chromosome.
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And over here, this chromosome, it's got the little g.
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And not only do the big Rs and little rs go randomly, one to each daughter
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cell, and the big Gs and little gs go randomly, one to each daughter cell,
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but there's no correlation between the big Rs and the big Gs, because they're
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on different chromosomes.
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So that explains Mendel's second law.
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Mendel's second law is perfectly explained by the chromosome theory,
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because if the genes are on different chromosomes, they're getting assorted
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independently.
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It's beautiful.
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But there's a problem.
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How many traits did Mendel study?
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STUDENT: Seven.
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ERIC S. LANDER: Seven.
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How many pairs of chromosomes do peas have?
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No reason you should know, but the answer is seven.
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What if Mendel had studied an eighth trait?
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Could it have been on a different chromosome?
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There aren't eight chromosomes.
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It would have to be that eighth trait on the same chromosome, if this theory
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is right, as one of the existing traits.
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Then how could it be assorting independently?
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Because it's going to be on the same chromosome.
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That's going to be correlated.
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So either Mendel's Law of Inheritance, his Second Law of Inheritance of
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independent assortment is wrong, or this chromosome theory is wrong.
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You can't have them both.
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So who is it?
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Mendel wrong, or the Chromosome Theory wrong?
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Unfortunately, we don't have enough time for it in today's lecture.
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So next time, we're going to find out who's right.
256
00:15:51,850 --> 00:15:54,150
Before we go on to, take a moment.
257
00:15:54,150 --> 00:15:56,905
We've got a question for you about the number of chromosomes that are found
258
00:15:56,905 --> 00:15:58,155
in gametes.
20105
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