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So we're going to continue our
discussion about genetics and

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patterns of inheritance.
We're going go relatively quickly

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compared to last time.
We've got a lot to get through

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today so I'm going to go
relatively quickly.

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And I printed a handout for you
because I'm going to rely on the

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PowerPoint to a greater degree than
I have before.

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So I want you to have them in case
you want to take notes on them.

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OK?  And we will do that
occasionally for particular lectures.

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Now, importantly,
we teach you about inheritance,

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not so much because we think you
need to know about peas but rather

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the rules of inheritance which were
derived from such studies but

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applied to many other studies.
Including in agricultural settings,

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in research settings in the
laboratory involving fruit flies and

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mice, but more relevant to you to
humans.  The rules that Mendel laid

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down apply to you and allow us to
predict inheritance patterns based

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on visible traits but also disease
traits.  And you'll see in next

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lecture that the information that
we've given you helps us understand

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inheritance of various forms of
genetic disease and predict,

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for example, frequencies within such
pedigrees.

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OK.  So, firstly,

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I wanted to review some terms which
you need to know.

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And I'll use a lot today.
The genotype which is the alleles

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present for a given gene in an
individual.  OK?

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The genes.  The alleles.
The term homozygous which means

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having two of the same alleles.
Heterozygous, having two different

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alleles for a given gene.
The phenotype which refers to the

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trait.  That's what the researcher
or the individual observes.

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The manifestation of the activity
of those genes,

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of those alleles,
the observed trait.

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The first filial, or F1,
generation which is the product of a

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cross of individuals who are
homozygous for different alleles of

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a given trait.
I think this should say gene,

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not given, given.  Such that the F1
individual is heterozygous for that

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gene.  That is,
has two different alleles.

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If you cross two F1s together you
generate the F2 generation.

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The products of an F1 intercross.
And based on Mendel's First Laws,

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which I told you about last time and
I'll reiterate in a second,

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the ratio of the genotypes that you
see in the F2 generation is 1:2:1.

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Homozygous for one allele,
heterozygous and homozygous for the

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other allele; 1:2:1.
And the ratio of phenotypes is 3:1.

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That assumes that there is a simple
dominant recessive relationship

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between the two alleles.
We closed with that last time.

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The dominant is the allele that
determines the phenotype in a

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heterozygous individual.
Big S, little S you're smooth.

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Big S is dominant over little S.
Recessive allele is the opposite.

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In a heterozygote it's not seen.
And, in fact, the only time you see

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the phenotype associated with the
recessive allele is when the

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organism is homozygous for that
recessive allele.

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And finally there's a term
codominance, which I'm going to

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mention now.  They don't really need
to know, Claudette?

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We bring it up in Section,
so I'm going to mention it here.

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And I refer you to the relevant
parts of the book,

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but I'm not going to cover it in
lecture but I'll show you where to

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go in the book to review that.
And that's relevant because Mendel's

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Rules apply as simple dominant
recessive relationships,

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but they actually don't apply in
terms of scoring them when you have

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a codominant situation.
So this is Mendel's First Law laid

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out for you.  When any individual
produces gametes,

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germ cells, sperm,
egg, the alleles, for a particular

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gene which control particular traits
separates, so that each gamete

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receives only one of a pair of
alleles present in the

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original individual.
And, as I said just a second ago,

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these experimental tests work well
when there's a simple dominant

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recessive relationship between those
two alleles.  There are exceptions

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to this in terms of codominance and
incomplete dominance.

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And I refer you to Figures 10.
3 and 10.14 in your book which show

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you examples from fruit flies as
well as, sorry,

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from humans as well as from pea
plants that illustrate both

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codominance and incomplete
dominance.

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Now, to emphasize the simplicity,
really, of Mendel's First Law, I

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want to show you an example which
also derives from your book using

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coins.  OK?  And it's just to
emphasize the point about

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probability.  So what this says is
that you have two,

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if you have two alleles of a given
gene, big A, little A,

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big S, little S, you're going to
hand off to your germ cells one or

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the other with equal probability.
And your mate will do the same such

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that the genotype of the offspring
is the product of the probabilities

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of the genotype of the germ cells.
OK?  So I have two coins here, two

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pennies which have a head and a tail.
All right?  Head.

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Tail.  So if I flipped this coin,
what's the likelihood that this is a

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head?  1:2.  If I flip this coin,
what's the likelihood that it's a

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head?
1:2.  If I put these together,

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like in fertilization, what's the
likelihood that it's both heads?

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1:4.  Very good.  One in two times
one in two is one in four.

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That's really the essence of
Mendel's First Law that the alleles

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segregate randomly into the germ
cells, and you can then figure out

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00:06:22,000 --> 00:06:27,000
Punnett Squares and the phenotype
based on the probabilities of the

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different genotypes that derive from
that simple calculation.

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OK.  We'll come back to a slightly
more complicated version momentarily.

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OK. Now, last time we talked about
smooth and wrinkled peas.

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I don't like this yellow chalk.
Is there white chalk here?

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00:06:55,000 --> 00:07:05,000
We talked about smooth and yellow

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peas, smooth and wrinkled peas.
We talked about genotypes, big S,

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big S, big S, little S, little S,
little S.  Remember that?

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We talked about phenotypes.
What's the phenotype of this pea?

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Smooth.
What's the phenotype of this pea?

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00:07:35,000 --> 00:07:44,000
Smooth, because you know that big S
is dominant over little S.

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And what's the phenotype of this
pea?  Wrinkled.

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Good.  What if I gave you,
I handed to you on your dinner plate

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a smooth pea?
What's its genotype?

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What's its genotype?
Who wants to venture out into the

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unknown here?  What's its genotype?
Come on.  Yes?  Thank you.  You

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don't know what its genotype is
because it could either be big S,

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big S or big S, little S.
It could either be big S,

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big S or big S, little S.  How are
you going to tell?

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How can you tell?  You can test it.
You could do what we call a Test

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Cross with a tester strain.
And a tester strain is homozygous

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for the recessive allele.
OK?  And based on this test we can

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figure out whether we're dealing
with a big S, big S or big S,

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little S pea.  So we're going to do
this using Punnett Squares to

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emphasize the use of Punnett Squares.
We have two possibilities.

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Either our pea is big S,
big S or our pea is big S,

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little S.  Either way we're going to
cross it to a pea,

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really a pea plant,
but a pea that's little S,

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little S.  OK?  Now, when I generate
a Punnett Square what gametes do I

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put over here?  What alleles
do I put over here?

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Little S, little S.
This plant only gives the little S

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allele to its gametes because that's
all it has.  What alleles do I put

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over here?  Big S on both because
this plant only gives big S to its

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gametes.  And so therefore all of
the offspring will be heterozygous,

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big S, little S, big S, little S.
And what will the phenotype be of

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all of the offspring?
Smooth.  100% smooth.

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OK.  And how about over here?
Well, this side of the Punnett

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Square is the same.
This individual only gives little S

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to its gametes.
And how about over here?

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Flip a coin, half of the time it's
big S, half of the time

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it's little S.
If this gamete meets up with this

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00:11:01,000 --> 00:11:06,000
one then that individual is going to
be big S, little S.

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The same thing over here.
With this gives little S, little S,

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this is the same thing.  What are
the phenotypes here?

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Half and half.  50% smooth,
50% wrinkled.

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OK.  That's great.  Now --

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What I want to now consider is a

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second trait.  And after we do this
we're going to consider the

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inheritance of the two traits
together, but let's just introduce

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the second trait now.
The gene is the Y gene.

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There are, again, two alleles,
big Y and little Y.  And the

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phenotypes associated with these
genotypes are yellow

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and green.  OK?
Mendel does a lot of crossing,

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etc., creates pure breading strains,
the parental strains.  And I'm going

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to tell you that their genotypes are
homozygous big Y,

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and these produce yellow peas.
He also produces pure breeding

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strains that only ever produced
green peas, and their genotype is

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little Y, little Y.
You cross these together to generate

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the F1.  What's the genotype with
respect to Y in the F1?

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Big Y, little Y.  They're all big Y,
little Y.  And what's the phenotype

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of the F1?  You don't know.
That's very good, because I haven't

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told you which is dominant over
which.  By convention,

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you might have guessed that big Y
was dominant over little Y.

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And if that were true, and it is,
then what would be the phenotype?

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Yellow.  And from that you can
conclude that Y is dominant over

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little Y.  OK?
I always run out of space.

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Do these boards get smaller every
year?  I mean it's unbelievable

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to me.
Now, if we take these F1s and do an

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intercross to generate F2s,
we're just reinforcing things we

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just learned here,
in the F2 generation we're going to

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generate individuals who are Y,
Y, big Y, big Y, big Y, little Y and

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little Y, little Y.
What will be the frequency of

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generating such individuals
in this cross?

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One quarter will be like this,
a half will be like this, a quarter

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will be like this,
and that would give a ration of

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1:2:1.  The phenotypes associated
with that will be yellow,

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yellow and green.
So the ratio of the phenotypes will

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be 3:1.  OK?  We just redid Mendel's
First Law for you.

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Now, what happens when we consider
the two of these traits together,

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smoothness and color in the same
cross?  Well, this led Mendel to

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propose his Second Law which is
illustrated here,

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which is that alleles of different
genes, the S gene and the Y gene,

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assort independently of one another
in producing gametes.

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What happens for one does not effect
what happens to the other.

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And this is true for genes that lie
on separate chromosomes.

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Although it's not necessarily true
for genes that lie on the same

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chromosome.  I make that point here,
and I'll make it again later because

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I don't want you to come to the
conclusion that two genes always

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assort from one another
independently.

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They do, and Mendel's Second Law is
based on that,

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in some instances,
but they don't always.

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So to illustrate that point we turn
to the coins again.

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I always also lose the coins.
OK.  So now we have our pennies

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once again, heads and tails.
And dimes, heads and tails, yeah?

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OK.  So I'm going to put a penny
and a dime in each pocket.

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What's the frequency of generating
a penny in the head configuration

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and a dime in the head configuration
together?

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00:16:33,000 --> 00:16:37,000
Well, let's do it differently.
What's the frequency of generating

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a head, a penny in the head
configuration here?

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00:16:42,000 --> 00:16:47,000
1:2.  What's the frequency of
getting a penny in the head and dime

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00:16:47,000 --> 00:16:52,000
in the head?  One over four.
It's one in two times one in two.

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00:16:52,000 --> 00:16:57,000
And what's the frequency of getting
penny head, dime head in this hand?

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00:16:57,000 --> 00:17:02,000
One over four.
So what's the frequency of getting

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00:17:02,000 --> 00:17:06,000
penny head, penny head,
dime head, dime head in the zygote?

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00:17:06,000 --> 00:17:11,000
1:16.  Again, simple probabilities.
OK?  And for Mendel's Second Law in

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00:17:11,000 --> 00:17:15,000
its simplest form,
it determines what you would expect

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00:17:15,000 --> 00:17:19,000
to see in such crosses.
And, again, we'll come back to a

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00:17:19,000 --> 00:17:24,000
twist on that in a moment.
Now, in biological terms it looks

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00:17:24,000 --> 00:17:28,000
like this.  And this derives from
your book, so you can

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00:17:28,000 --> 00:17:32,000
go and look at it.
It's adapted from Purves 10.

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00:17:32,000 --> 00:17:36,000
.  And there's a nice little
animation that goes along with that.

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00:17:36,000 --> 00:17:40,000
But we're going to go through this
because there's some subtly here

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that I want you to understand.
So here is the parent.  It's a

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00:17:44,000 --> 00:17:48,000
diploid.  It has the two chromosomes
that are relevant that carry the S

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00:17:48,000 --> 00:17:52,000
gene and the Y gene.
And the parent is heterozygous for

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00:17:52,000 --> 00:17:56,000
both.  It's a double heterozygote.
Big S, little S, big Y, little Y.

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00:17:56,000 --> 00:18:00,000
OK?
Now, when this individual undergoes

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00:18:00,000 --> 00:18:05,000
meiosis, it can generate four
possible haploid gametes.

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00:18:05,000 --> 00:18:10,000
And those have the genotypes of all
four combinations.

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00:18:10,000 --> 00:18:15,000
It can be big S, little Y,
big S, big Y, little S, little Y,

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00:18:15,000 --> 00:18:20,000
little S, big Y.  All four
possibilities.

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00:18:20,000 --> 00:18:25,000
And based on the simple laws of
probability that we just went

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00:18:25,000 --> 00:18:30,000
through with the coins,
those are generated in equal amounts

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00:18:30,000 --> 00:18:35,000
amongst the gametes.
OK?  They're generated in equal

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00:18:35,000 --> 00:18:40,000
amounts amongst the gametes.
Why are they?  Well, let me just

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00:18:40,000 --> 00:18:46,000
show you another slide that
illustrates it in slightly more

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00:18:46,000 --> 00:18:51,000
detail.  When these guys undergo the
first, undergo duplication to

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00:18:51,000 --> 00:18:56,000
generate the two chromatids for each
chromosomes, enter prophase of

215
00:18:56,000 --> 00:19:02,000
meiosis one, the homologs
pair, as you recall.

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00:19:02,000 --> 00:19:06,000
The two chromosomes don't have
anything to do with one another.

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00:19:06,000 --> 00:19:10,000
The homologs pair but the two
chromosomes don't have anything to

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00:19:10,000 --> 00:19:14,000
do with one another.
They're independent of each other.

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00:19:14,000 --> 00:19:18,000
Now, if we consider this chromosome,
the one that carries the S gene,

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00:19:18,000 --> 00:19:22,000
it will line up on the metaphase
plate in an orientation which is

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00:19:22,000 --> 00:19:26,000
random.  In this particular example,
the big S allele is pointing down

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00:19:26,000 --> 00:19:30,000
and the little S allele
is pointing up.  OK?

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00:19:30,000 --> 00:19:34,000
Based on that,
let's fix that.

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00:19:34,000 --> 00:19:38,000
Based on that there is one of two
possible alignments for the

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00:19:38,000 --> 00:19:42,000
chromosome that carries the other
gene.  In our example it's the Y

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00:19:42,000 --> 00:19:46,000
gene.  One orientation would have
the big Y carrying chromosome

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00:19:46,000 --> 00:19:50,000
pointing up, but it's just as likely
that that chromosome could end up

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00:19:50,000 --> 00:19:54,000
the other way around such that the
big Y carrying chromosome

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00:19:54,000 --> 00:19:58,000
is pointing down.
And it's based on the fact that the

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00:19:58,000 --> 00:20:02,000
two chromosomes are independent of
one another when they choose which

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00:20:02,000 --> 00:20:06,000
side of the metaphase plate to go to
that gives us this random assortment

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00:20:06,000 --> 00:20:10,000
in meiosis and the development of
Mendel's Second Law.

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00:20:10,000 --> 00:20:15,000
So if you carry this through then
you generate these gametes in a

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00:20:15,000 --> 00:20:19,000
2:2:2:2 or 1:1:1:1 ratio.
OK?  That's the most important

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00:20:19,000 --> 00:20:23,000
thing.  If you understand that then
you can work it out as to which

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00:20:23,000 --> 00:20:27,000
genotypes you get and which
phenotypes you get using

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00:20:27,000 --> 00:20:32,000
Punnett Squares.
OK?  But it's important that you

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00:20:32,000 --> 00:20:37,000
understand why it is that you get
random assortment of the alleles in

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00:20:37,000 --> 00:20:42,000
such a cross.  Is that clear to
everybody?  It's probably not clear

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00:20:42,000 --> 00:20:47,000
to the guy reading the newspaper.
Oh, the woman reading the newspaper.

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00:20:47,000 --> 00:20:52,000
Usually it's a guy reading the
newspaper but not today.

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00:20:52,000 --> 00:20:58,000
Is this clear?  Are there any
questions?  OK.

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00:20:58,000 --> 00:21:01,000
As I said, you can work through this
with Punnett Squares.

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00:21:01,000 --> 00:21:05,000
I decided not to do it on the board
because it takes a lot of time,

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00:21:05,000 --> 00:21:08,000
but the book shows you, in this
specific example,

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00:21:08,000 --> 00:21:12,000
what you should expect.
Here are the two parentals.

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00:21:12,000 --> 00:21:15,000
They're homozygous for each of the
alleles, big S,

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00:21:15,000 --> 00:21:19,000
big S, big Y, big Y,
little S, little S, little Y,

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00:21:19,000 --> 00:21:22,000
little Y.  In the F1 generation you
create the double heterozygote,

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00:21:22,000 --> 00:21:26,000
big S, little S, big Y, little Y,
and then that generates the four

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00:21:26,000 --> 00:21:30,000
gametes in equal proportion,
as we just discussed.

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00:21:30,000 --> 00:21:34,000
If you cross two of these F1s
together then you can create a

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00:21:34,000 --> 00:21:38,000
Punnett Square which now has 16
squares instead of just four which

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00:21:38,000 --> 00:21:42,000
allows us to consider the products
of all four of these gametes crossed

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00:21:42,000 --> 00:21:46,000
to all four of these gametes.
So we get various combinations of

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00:21:46,000 --> 00:21:50,000
genotypes and we get various
combinations of phenotypes.

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00:21:50,000 --> 00:21:54,000
And you can calculate what those
are based on looking at this.

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00:21:54,000 --> 00:21:58,000
There's actually another way of
doing it, which I find to be simpler,

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00:21:58,000 --> 00:22:02,000
which is illustrated here.
Again, because the traits and the

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00:22:02,000 --> 00:22:07,000
alleles are segregating from one
another independently you can think

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00:22:07,000 --> 00:22:12,000
about this as two Punnett Squares
separately and figure out the

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00:22:12,000 --> 00:22:17,000
consequences of those two traits by
simply multiplying between then.

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00:22:17,000 --> 00:22:22,000
So what I mean by that is if you
consider the S allele in isolation

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00:22:22,000 --> 00:22:27,000
then there are the familiar 1:2:1
ratio of the three different

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00:22:27,000 --> 00:22:32,000
genotypes.
And that's true also of the Y

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00:22:32,000 --> 00:22:36,000
genotypes, 1:2:1 of the three
possible genotypes.

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00:22:36,000 --> 00:22:40,000
In total there are nine possible
genotypes, but when you separate it

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00:22:40,000 --> 00:22:44,000
from S and Y it's 3:3.
Likewise with respect to the

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00:22:44,000 --> 00:22:48,000
phenotypes, three-quarters of these
guys will be smooth,

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00:22:48,000 --> 00:22:52,000
one-quarter will be wrinkled.
And over here three-quarters will

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00:22:52,000 --> 00:22:56,000
be yellow, one-quarter will be green.
If I want to figure out what the

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00:22:56,000 --> 00:23:00,000
frequency is of any compound
genotype, I just have

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00:23:00,000 --> 00:23:03,000
to multiply across.
So if I want to know the frequency

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00:23:03,000 --> 00:23:07,000
of big S, big S,
big Y, big Y, and it's just the

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00:23:07,000 --> 00:23:11,000
product of a quarter times a quarter
or a sixteenth,

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00:23:11,000 --> 00:23:14,000
just like the coins example by the
way.  If I want to know the

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00:23:14,000 --> 00:23:18,000
frequency of being big S,
little S, big Y, little Y,

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00:23:18,000 --> 00:23:22,000
then it's a half times a half or
four-sixteenths,

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00:23:22,000 --> 00:23:26,000
sorry.  A half times a half or a
quarter which equals

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00:23:26,000 --> 00:23:30,000
four sixteenths.
And the other genotypes comprise the

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00:23:30,000 --> 00:23:34,000
remaining sixteenths that we see,
another 11.  And likewise with

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00:23:34,000 --> 00:23:38,000
respect to phenotypes.
If you want to know the frequency

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00:23:38,000 --> 00:23:42,000
of smooth yellow offspring you just
multiple across,

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00:23:42,000 --> 00:23:47,000
three-quarters times three-quarters
gives you nine-sixteenth.

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00:23:47,000 --> 00:23:51,000
Of wrinkled green ones it's a
quarter times a quarter or

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00:23:51,000 --> 00:23:55,000
one-sixteenth.
So, again, when you're asked to do

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00:23:55,000 --> 00:23:59,000
Punnett Squares for two traits that
segregate independently,

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00:23:59,000 --> 00:24:04,000
you might think about doing it using
separate Punnett Squares.

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00:24:04,000 --> 00:24:10,000
OK.  So we've talked about Mendel's
Laws.  We're wrapping up Mendel's

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00:24:10,000 --> 00:24:16,000
Laws here.  The first law is
independent segregation of the two

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00:24:16,000 --> 00:24:22,000
alleles.  The second law with
respect to different genes,

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00:24:22,000 --> 00:24:29,000
they segregate independently from
each other.  OK?

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00:24:29,000 --> 00:24:32,000
Now, as I said,
this is true for genes that are on

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00:24:32,000 --> 00:24:36,000
different chromosomes or actually
genes that are far away on the same

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00:24:36,000 --> 00:24:39,000
chromosome, but it's not always true.
And to emphasize that point we're

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00:24:39,000 --> 00:24:43,000
going to return to
our coin friends.

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00:24:43,000 --> 00:24:52,000
So let me just remind you,

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00:24:52,000 --> 00:24:56,000
let me just remind you what we saw.
So this doesn't apply to genes that

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00:24:56,000 --> 00:25:00,000
are linked together on
the same chromosome.

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00:25:00,000 --> 00:25:04,000
Let's remind you what we determined
for genes that are unlinked,

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00:25:04,000 --> 00:25:08,000
the dime and the penny.  We said
that the frequency of getting in my

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00:25:08,000 --> 00:25:12,000
left hand head,
head and in my right hand head,

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00:25:12,000 --> 00:25:17,000
head, and together was 1:4 and 1:4
and together was 1:16.

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00:25:17,000 --> 00:25:21,000
That was the situation when they
were independent of one another.

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00:25:21,000 --> 00:25:25,000
But let's consider the situation
when I've taped them together.

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00:25:25,000 --> 00:25:30,000
Now I have a penny and a quarter
which are taped together.

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00:25:30,000 --> 00:25:35,000
I have fixed the orientation of
these two coins with respect to one

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00:25:35,000 --> 00:25:40,000
another.  So you can see the heads
are on the same side,

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00:25:40,000 --> 00:25:46,000
the heads are on the same side,
yeah?  OK.  What's the likelihood

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00:25:46,000 --> 00:25:51,000
that this is going to have the penny
in the head configuration?

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00:25:51,000 --> 00:25:57,000
1:2.  And if it is what's the
likelihood that the quarter is going

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00:25:57,000 --> 00:26:02,000
to be in the head configuration?
One.  So what's the likelihood that

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00:26:02,000 --> 00:26:06,000
the coin is going to have head,
head when I turned it up?  1:2.  Not

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00:26:06,000 --> 00:26:10,000
like this example.
1:2.  And so what's the likelihood

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00:26:10,000 --> 00:26:14,000
that it's going to be head,
head in this configuration, in this

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00:26:14,000 --> 00:26:18,000
hand?  1:2.  So what's the
likelihood that it's going to be

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00:26:18,000 --> 00:26:22,000
head, head, head,
head?  1:4.  So the frequencies

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00:26:22,000 --> 00:26:26,000
don't work out,
according to Mendel's Rules,

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00:26:26,000 --> 00:26:31,000
if the two genes are linked, the two
genes are linked together.

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00:26:31,000 --> 00:26:35,000
So thank you for your help.
It pays to sit in the front row

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00:26:35,000 --> 00:26:40,000
every once in a while.
So this is what we just talked

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00:26:40,000 --> 00:26:45,000
about.  In my left hand the
frequency of head,

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00:26:45,000 --> 00:26:50,000
head was one-half, in my right hand
frequency of head,

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00:26:50,000 --> 00:26:54,000
head was one-half, so the product
was a half times a half or a quarter.

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00:26:54,000 --> 00:26:59,000
OK?  Now, if we want to think about
that is sort of genetic terms,

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00:26:59,000 --> 00:27:03,000
let's think about chromosomes.
And genes can be linked on

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00:27:03,000 --> 00:27:06,000
chromosomes like those two coins can
be linked together by tape.

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00:27:06,000 --> 00:27:10,000
So here are two genes, the penny
gene and the quarter gene,

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00:27:10,000 --> 00:27:13,000
two alleles, the head allele and the
tail allele, and they're present on

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00:27:13,000 --> 00:27:16,000
two chromosomes in a heterozygous
individual.  And they're in this

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00:27:16,000 --> 00:27:20,000
configuration such that the head
allele of the quarter is next to the

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00:27:20,000 --> 00:27:23,000
head allele of the penny.
The tail allele of the quarter is

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00:27:23,000 --> 00:27:27,000
next to the tail allele
of the penny.

334
00:27:27,000 --> 00:27:31,000
When this goes through meiosis the
alleles stick together.

335
00:27:31,000 --> 00:27:36,000
The head alleles stick together in
the gametes.  The tail alleles stick

336
00:27:36,000 --> 00:27:40,000
together in the gametes.
It's just like Mary Had a Little

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00:27:40,000 --> 00:27:45,000
Lamb.  Remember that one?
I'm getting curious looks.

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00:27:45,000 --> 00:27:49,000
Mary had a little lamb.  Its fleece
was white as snow.

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00:27:49,000 --> 00:27:54,000
Everywhere that Mary went the lamb
was sure to go.

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00:27:54,000 --> 00:27:59,000
A long way to go for that one,
but anyway.

341
00:27:59,000 --> 00:28:03,000
Everywhere that Mary went the lamb
was sure to go.

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00:28:03,000 --> 00:28:07,000
They stay linked.
OK?  And importantly you don't find,

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00:28:07,000 --> 00:28:12,000
or you almost never find this
situation where the quarter in the

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00:28:12,000 --> 00:28:16,000
head, sorry, the penny in the head
configuration went with the quarter

345
00:28:16,000 --> 00:28:21,000
in the tail configuration.
You don't find those, or you almost

346
00:28:21,000 --> 00:28:25,000
never, and this is the point of the
final phase of the lecture.

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00:28:25,000 --> 00:28:43,000
OK.  Very good.

348
00:28:43,000 --> 00:28:49,000
So let's look at this with respect
to peas.  Why not?

349
00:28:49,000 --> 00:28:56,000
We're going to talk about now
linked genes in peas.

350
00:28:56,000 --> 00:29:03,000
We're going to talk about two genes.

351
00:29:03,000 --> 00:29:08,000
We're going to have our familiar Y
gene which has its two alleles.

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00:29:08,000 --> 00:29:13,000
And now we're going to talk about a
new gene, the D gene,

353
00:29:13,000 --> 00:29:18,000
which likewise has two alleles and
two phenotypes which are dense peas

354
00:29:18,000 --> 00:29:24,000
and light peas.
OK?  Dense peas and light peas.

355
00:29:24,000 --> 00:29:29,000
And it turns out that these two
genes are on the same chromosome and

356
00:29:29,000 --> 00:29:35,000
very close to one another.
So if you think about the

357
00:29:35,000 --> 00:29:43,000
chromosomes in a heterozygous
individual, double heterozygous

358
00:29:43,000 --> 00:29:50,000
individual they would look like that.
If we think about the chromosomes

359
00:29:50,000 --> 00:29:58,000
in a tester strain,
which is homozygous for the

360
00:29:58,000 --> 00:30:08,000
recessive alleles for both.

361
00:30:08,000 --> 00:30:15,000
What's going to happen to the
offspring?  What?

362
00:30:15,000 --> 00:30:22,000
What?  I'm sorry.
First of all, this phenotype is

363
00:30:22,000 --> 00:30:29,000
yellow and dense.
Big D is dominant over little D.

364
00:30:29,000 --> 00:30:35,000
This phenotype is green and light.
OK?  What do the gametes look like

365
00:30:35,000 --> 00:30:39,000
in this Punnett Square?
And why am I drawing four boxes,

366
00:30:39,000 --> 00:30:43,000
only four boxes anyway?  Didn't I
tell you a little while ago that if

367
00:30:43,000 --> 00:30:47,000
you have two alleles,
two genes you have to draw a Punnett

368
00:30:47,000 --> 00:30:51,000
Square with 16 boxes?
Why am I drawing only four?

369
00:30:51,000 --> 00:30:55,000
Because they go together.  They're
linked.  So what do the

370
00:30:55,000 --> 00:31:02,000
alleles look like?
Well, this parent is going to give

371
00:31:02,000 --> 00:31:10,000
either big Y, big D or little Y,
little D.  And this parent is always

372
00:31:10,000 --> 00:31:18,000
only ever going to give little Y,
little D.  OK?  So therefore the

373
00:31:18,000 --> 00:31:33,000
genotype follows, sorry.

374
00:31:33,000 --> 00:31:41,000
And the phenotype.
What am I going to get when I

375
00:31:41,000 --> 00:31:49,000
deconvolute this?
What am I going to get?

376
00:31:49,000 --> 00:31:57,000
The phenotype in that cross,
50% will be yellow dense.  And 50%

377
00:31:57,000 --> 00:32:04,000
will be what?  Green and light.
The very same two phenotypes that I

378
00:32:04,000 --> 00:32:10,000
got in the parents.
These are the parental phenotypes.

379
00:32:10,000 --> 00:32:23,000
And in a test cross like this,

380
00:32:23,000 --> 00:32:27,000
of two genes that are very tightly
linked, you would always regenerate

381
00:32:27,000 --> 00:32:32,000
the parental phenotypes.
They would never separate from one

382
00:32:32,000 --> 00:32:35,000
another.  You'd always only ever
generate the parental --