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PROFESSOR: Mendel's
second law--

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this thing over here about a
three to one ratio about a

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single trait being controlled
by a pair of alleles, and

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those alleles being distributed
independently of

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each other to the offspring, the
stuff you always learned

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about Mendel--

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that's often referred to
as Mendel's first law.

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Mendel, by the way, didn't call
it Mendel's first law.

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It's considered--

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you don't write that in your own
papers or something like

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that, right?

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So Mendel did actually observed
some other things

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beyond this independent
segregation of the alleles for

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a single trait.

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Mendel began to cross his peas
together and try to make

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combinations.

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He had rounds and wrinkles.

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He had greens and yellows,
talls and shorts.

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He started making
combinations.

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How about a plant that was
wrinkled and yellow?

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Green was the normal color.

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Round was the normal shape.

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But he had yellows.

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He had wrinkleds.

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How about making
a combination?

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So he begin to make plants that
bred true for different

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pairs of phenotypes.

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So for example, he had a pure
breeding line here that was

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both round and green.

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That was the stuff he could
pick up at the market.

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But he made a line here that was
both wrinkled and yellow.

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What is the genotype of this
round, green strain?

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At the round gene--

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the gene for roundness-
what is its genotype?

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Big R, big R. It's a pure
breeding strain.

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It was homozygous--

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we're just testing our words
here-- homozygous

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for the big R allele.

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Green is controlled by
a different gene.

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It has an allele big G. It's
pure breeding for this.

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And we call it big G, big G.

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Convention, when we use capital
letters, it tends to

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mean that the associated
phenotype is dominant.

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OK?

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It'll make you think that the
allele is dominant, but it's

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the associated phenotype
that's dominant.

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Now, wrinkled-- what
was wrinkled?

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Wrinkled was a homozygous
wrinkled-- little r, little r.

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And what was the genotype
at the yellow locus?

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Little g, a little g is
how we'll denote it.

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Geneticists use four or five
different kinds of notations.

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We're going to use this notation
today of big R's and

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little r's and little g's.

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But you'll get used to other
kinds of genetic notations.

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So when we cross these guys
together, this F0 generation

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here of these two pure breeding
parental strains, we

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get an F1 generation.

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The F1 generation--

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what is it phenotypically?

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What did it look like?

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Round and green, yep--
round and green.

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What was in genotypically?

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Big R, little R, big
G, little g.

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Now we could self
these plants.

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And our head would hurt with
the nine to three to

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three to one ratio.

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So instead, why don't we cross
these plants back to the

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wrinkled, yellow strain to
make our life easier?

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Little r, little r, little
g, little g--

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and when we cross that back,
what is the segregation that's

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going to happen?

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Well, what are the possible
gametes that could emerge from

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this parent?

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We could get a big R and a big
G. We could get a big R

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and a little g.

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We could get a little r and a
big G. We could get a little r

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and a little g.

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Those are the four possibilities
that could be

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contributed by this parent.

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What could be contributed
by this parent?

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Little r and little g--
that's it, right?

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No other options.

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So that's what our little
Punnett square looks like here

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of our options--

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parent number one, parent
number two.

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What will this be?

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This will be big R, big G over
little r, little g, big R,

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little g over little r little g,
little r, big G over little

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r little g, little r little
g over little r little g.

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In other words, this will
be round and green.

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This'll be round and yellow.

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This'll be wrinkled and green.

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And this'll be wrinkled
and yellow.

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And what will be the
ratio of these?

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One to one to one to one,
provided that those gametes

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were all equally frequent,
provided that that parental

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plant made each of those four
types in equal proportions.

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That one to one to one ratio
in the gametes will

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necessarily translate into a one
to one to one ratio in the

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phenotypes observed in
the next generation.

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When we cross back--
not by selfing--

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but when we cross back to the
parent that has the recessive

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phenotypes, we'll often
call this a

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backcross or a test cross.

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If I haven't done a backcross
or a test cross where I

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crossed back to the wrinkled,
yellow parent, I would instead

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have had to make a square here
that had 16 boxes in it, and I

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would have had to add up to 16
boxes to figure out how many

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were round and green--

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nine out of the 16.

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How many were round
and yellow?

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Three out of the 16.

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How many were wrinkled
and green?

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Three out of the 16.

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And how many were wrinkled
and yellow?

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One out of the 16.

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But for the purposes of using
the white board up here, I did

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the test cross or
the backcross,

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because it's simpler.

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But you can also do the four by
four matrix and figure out

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what it looks like.

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That was news.

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It didn't have to be
that way, right?

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Maybe it was something else.

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This is pretty cool.

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What it tells you is not just
is it the case that here the

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alleles segregate
independently.

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It's a random coin flip
which one you get.

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It tells you no correlation
between the two traits.

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They're independent.

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This is called independent
assortment.

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Mendel's second law is the law
of independent assortment.

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All right.

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So Mendel publishes the paper.

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1865, it comes out.

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Here's a copy of
Mendel's paper.

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It's translated into English
from the original German.

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So I got Mendel's paper here.

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There's no Punnett squares.

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It's kind of messy notation.

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Look at that.

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Look at all that--

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big A, little b.

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He's got A's, B's, C's.

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He's got three factor crosses
running around in here.

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Mendel really goes to town.

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It's a beautiful paper here.

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But it just goes on and on.

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It's incredibly hard to read.

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Look at this.

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It takes a lot to
read this thing.

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But in a way, it's simple.

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It's nothing so sophisticated.

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Oh yeah, here.

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Look at this-- long, green,
inflated, constricted--

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all this kind of stuff.

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It's pretty cool.

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You should look it up.

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It's online.

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You can find Mendel's paper.

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What happens to Mendel's
paper?

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Nobody reads it.

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It sinks like a stone.

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It's this cool paper and
nobody reads it.

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Nobody reads it for
a lot of reasons.

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Scientific communication wasn't
so big in those days.

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Oh, well.

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Mendel also ends up getting
promoted to become the abbot

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of the monastery, and that's
pretty much the end of his

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scientific career,
in my opinion.

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He got too many administrative
duties, doesn't do more

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science there.

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Also, he has some
poor choices.

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The next plant he works
on is hawk weed.

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Hawk weed turns out to have
really weird genetics that

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totally leads him astray.

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Basically, this is
the one important

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paper Mendel ever publishes.

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It's an incredibly
important paper.

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It's so important because it
contains the clue to what

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Darwin, living at exactly
the same time, wished he

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understood, which is what the
basis of genetic variation is.

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Wouldn't it be great if Darwin
had read Mendel's paper?

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Darwin actually owned a copy
of Mendel's paper.

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He received a copy of
Mendel's paper.

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In those days, the way they
printed books, there were

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folded pages and you had to
slit the page to read it.

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Darwin never slit the pages of
the copy of Mendel's paper.

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So we know he's never read
Mendel's paper, but he has one

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in his library.

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He had Mendel's paper.

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He had the answers sitting
there on the shelf,

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but never read it.

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The stuff sinks like a stone.

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Nobody really pays much
attention to it.

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And Mendel goes on,
dies that's it.

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[LAUGHTER]

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PROFESSOR: Until the end of
the 1800s, right at the

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beginning of the 20th century,
along comes cytology--

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looking at cells in
the microscope.

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Microscopes began to get
good in the late 1800s.

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And cytologists began to see in
their microscope that when

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cells divided, these funny
structures began to appear--

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these long, thread-like
things.

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And the German chemical industry
being developed at

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that time had invented
all sorts of dyes.

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And cytologists began
experimenting putting dyes on

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these cells.

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And the dyes let them see really
clearly these funny

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things that were condensing
out when cells divided.

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And they had no clue what these
funny things were other

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than that they took up dyes.

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And so in the absence of any
clue what were, they called

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them chromosomes, meaning
colored things.

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That is what chromosome means.

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They called them colored thing
Chromos colored bodies,

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colored things.

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That's all they knew.

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And they observed that these
chromosomes, these colored

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things, did really interesting
choreography.

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When cells divided, when they
underwent mitosis, what would

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happen is that the chromosomes
would line

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up along the midline.

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And they would have, at that
point, these funny X-like

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structures.

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And I'll draw four of
these chromosomes

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lining up like this.

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And what would happen
during mitosis?

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The cell would divide, and each
of the two cells would

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get one half of the X. So if you
started with four of these

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X's, you ended up with
four like this.

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There you go.

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That was the chromosome being
somehow tugged apart.

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Now, anything that gets tugged
apart when a cell divides--

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that's kind of interesting.

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Then those chromosomes
would disappear.

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You couldn't see them again for
a while until the cell was

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ready to divide again.

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And when the cell is ready to
divide again, darned if those

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single lines hadn't
turned into X's.

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Somehow the cell had turned the
single lines into these

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two pieces, these X's,
and they were

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ready to divide again.

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And that was mitosis--

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the process of ordinary
cellular division.

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But there was another process.

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Folks observed meiosis.

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That's what happens when
you make gametes.

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So when gametes get made, the

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choreography was a bit different.

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Instead of all the chromosomes
lining up on the midline as

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individuals, they lined
up as pairs.

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00:13:16,470 --> 00:13:17,720
They line up as pairs.

254
00:13:21,980 --> 00:13:33,180
When the cell divides, you
end up with now only two

255
00:13:33,180 --> 00:13:36,475
X's, not four X's.

256
00:13:39,990 --> 00:13:44,275
Then what happens is those
cells divide again.

257
00:13:51,120 --> 00:13:54,480
And you end up with those
straight lines--

258
00:13:57,450 --> 00:13:59,220
but not four of them,
only two of them.

259
00:14:03,400 --> 00:14:10,190
The first step gets called
meiosis number one--

260
00:14:10,190 --> 00:14:18,510
meiosis I. The second is
called meiosis II.

261
00:14:18,510 --> 00:14:21,710
The second step looks just
like mitosis, doesn't it?

262
00:14:21,710 --> 00:14:23,810
Chromosomes are lined up
along the midline.

263
00:14:23,810 --> 00:14:24,550
They separate it.

264
00:14:24,550 --> 00:14:27,010
It just looks like mitosis,
ordinary cell division.

265
00:14:27,010 --> 00:14:29,300
But that first step
is special.

266
00:14:29,300 --> 00:14:33,250
That first step says, somehow,
the chromosomes come in pairs,

267
00:14:33,250 --> 00:14:36,720
and the cell picks one from
each pair and gives to its

268
00:14:36,720 --> 00:14:38,720
gametes-- its sperm
or its egg--

269
00:14:38,720 --> 00:14:42,270
one from each pair.

270
00:14:42,270 --> 00:14:44,900
And what do you think happens
on fertilization?

271
00:14:44,900 --> 00:14:48,510
Well, you had one from each
pair, one from each pair, it

272
00:14:48,510 --> 00:14:51,560
comes together and it
restores a pair now.

273
00:14:54,200 --> 00:14:56,070
And you know what folks said?

274
00:14:56,070 --> 00:14:59,540
They said this sounds just like
what that dead monk was

275
00:14:59,540 --> 00:15:02,480
talking about.

276
00:15:02,480 --> 00:15:04,980
Pairs, particles of
inheritance--

277
00:15:04,980 --> 00:15:08,560
particles that come in pairs and
you give to your gametes

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00:15:08,560 --> 00:15:11,170
one of the two pairs.

279
00:15:11,170 --> 00:15:15,440
These colored things must be
the basis of inheritance or

280
00:15:15,440 --> 00:15:16,690
genes or something.

281
00:15:18,910 --> 00:15:23,630
Wow, because it fits Mendel's
model beautifully.

282
00:15:23,630 --> 00:15:25,090
It explains the first law.

283
00:15:30,680 --> 00:15:33,281
What about the second law?

284
00:15:33,281 --> 00:15:36,220
What about Mendel's
second law?

285
00:15:36,220 --> 00:15:42,240
How could it explain the big R
and the big G being inherited

286
00:15:42,240 --> 00:15:45,340
independently of each other
with no correlation?

287
00:15:45,340 --> 00:15:48,780
What would that have to mean?

288
00:15:48,780 --> 00:15:51,170
They're on different
chromosomes.

289
00:15:51,170 --> 00:15:53,710
The genes are on different
chromosomes.

290
00:15:53,710 --> 00:15:57,130
Because if big R is on one
chromosome and big G is on the

291
00:15:57,130 --> 00:16:02,230
other chromosome, then it's a
coin flip whether or not the

292
00:16:02,230 --> 00:16:05,830
big R might be here and a little
r might be there or the

293
00:16:05,830 --> 00:16:09,000
big G might be there or
maybe it's over there.

294
00:16:09,000 --> 00:16:11,290
It's a random draw which
way it's going to go.

295
00:16:11,290 --> 00:16:14,140
So it perfectly explains
Mendel's second law.

296
00:16:17,370 --> 00:16:18,620
Unless--

297
00:16:21,070 --> 00:16:23,400
what happens if big R and big G
are on the same chromosome?

298
00:16:26,430 --> 00:16:28,830
Then they're going
to go together.

299
00:16:28,830 --> 00:16:31,580
I'm not going to have
independent assortment.

300
00:16:31,580 --> 00:16:35,620
I may have totally dependent
correlated assortment.

301
00:16:35,620 --> 00:16:37,780
If big R and big G are on the
same chromosome, they're going

302
00:16:37,780 --> 00:16:38,530
to be inherited together.

303
00:16:38,530 --> 00:16:40,960
Mendel's second law is
going to be wrong.

304
00:16:40,960 --> 00:16:45,020
So why did Mendel find big R
and big G going together?

305
00:16:45,020 --> 00:16:46,910
Maybe was lucky and he picked
traits on different

306
00:16:46,910 --> 00:16:48,310
chromosomes.

307
00:16:48,310 --> 00:16:50,820
But what about his next trait?

308
00:16:50,820 --> 00:16:52,640
Lucky again?

309
00:16:52,640 --> 00:16:53,440
Lucky again?

310
00:16:53,440 --> 00:16:57,380
Mendel studied seven traits.

311
00:16:57,380 --> 00:17:00,575
How many chromosome pairs
do peas have?

312
00:17:03,100 --> 00:17:04,350
Turns out, seven.

313
00:17:08,619 --> 00:17:11,210
But anyway, what happens?

314
00:17:11,210 --> 00:17:12,190
What's going on?

315
00:17:12,190 --> 00:17:16,670
Mendel's second law can't be
right if the chromosome theory

316
00:17:16,670 --> 00:17:20,040
is right when the genes are
on the same chromosome.

317
00:17:20,040 --> 00:17:21,140
They would be dependent.

318
00:17:21,140 --> 00:17:24,530
They won't be independent one
to one to one to one.

319
00:17:24,530 --> 00:17:26,550
So which is it?

320
00:17:26,550 --> 00:17:29,470
Is Mendel right, my hero?

321
00:17:29,470 --> 00:17:31,740
Or is this chromosome
theory right?

322
00:17:31,740 --> 00:17:34,850
Because they can't be both
perfectly right.

323
00:17:34,850 --> 00:17:38,060
So which is it?

324
00:17:38,060 --> 00:17:39,370
Oops, we've run out of time.

325
00:17:39,370 --> 00:17:40,960
[LAUGHTER]

326
00:17:40,960 --> 00:17:42,210
PROFESSOR: Next time.