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PROFESSOR: Hello and
welcome to the

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help session on pedigrees.

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Today, we will be working
out a problem together.

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If you have not yet had a chance
to work it on your own,

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please do so now, and
return to this video

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when you are done.

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Now that you've had a chance to
look at this problem, let's

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work it out together.

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The first part of this question
asks, what is the

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mode of inheritance that is
observed in this pedigree?

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So, we know that there are two
main types of modes of

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inheritance.

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It can either be dominant
or recessive.

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And from there it can either
be autosomal or X-linked.

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If a disease follows a dominant
inheritance pattern,

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generally, it must be present
in every generation.

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So here we notice that the
disease is present in the

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first generation, but it's not
present in anyone in the

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second generation.

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However, then it reappears
in the third generation.

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This suggests that the
disease is recessive.

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So now, do we think
this disease is

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autosomal or is it X-linked?

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So we look at the pedigree
again, and upon closer

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inspection, we notice
that the affected

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individuals are only males.

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This is a key characteristic
of an

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X-linked recessive disease.

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However, we're also given more
information that tells us that

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this individual here does not
carry an allele associated

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with the affected phenotype.

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This rules out the possibility
that this disease could be an

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autosomal recessive disease.

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Thus, the most likely mode of

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inheritance is X-linked recessive.

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Great, so moving on to the
second part of this problem,

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we are asked to determine the
genotypes of individuals

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number one and number three.

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So, if we look at this pedigree,
we know individual

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one is female and not affected
by the disease.

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Female individuals contain two
X chromosomes, and since she

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is not affected she must contain
at least one large R.

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So we'll call the allele
associated with the

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disease small r.

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We do not yet know what her
second X chromosome could be.

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It could be either another
large R or it

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could be a small r.

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And she also would still not
be affected by the disease.

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This male right here is affected
by the disease.

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So he must have an
X with a small r.

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Remember that males have one
X chromosome and one Y

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chromosome.

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So now, making our way down to
individual number three.

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Individual number three does
not have the disease and is

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female, so again, must have
two X chromosomes.

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One of the X chromosomes must
come from her father and the

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other from her mother.

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The only X chromosome that her
father can pass along to her

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is X small r.

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In order for her to not be
affected with the disease, her

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other X chromosome must
have a large R.

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She was able to get this
genotype with her mother

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having either of these
two genotypes.

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To make sure that both of these
genotypes are possible

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for this mother, we need to
examine her other children.

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Her son here is unaffected.

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So his genotype must have been
X large R, Y. It's possible

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for him to get this genotype
when the mother has either of

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these two genotypes.

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Similarly, this daughter could
be unaffected just as this

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daughter was unaffected.

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So the mother can have either
of these two genotypes.

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And we're unable to rule
out one of them.

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So let me just write up
the answer over here.

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All right, so far, so good.

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Next, we're asked to calculate
the probability that

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individual A is affected.

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Individual A is over here at
the bottom of the pedigree.

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In order for individual A to be
affected, she must have the

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following genotype.

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The only way for her to get two
little r's is for her to

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get one from her father and
one from her mother.

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We know this is possible because
her father is affected

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with the following genotype
of Y, X small r.

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Her mother is not affected.

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So for her to get this small
r, her mother must be a

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carrier for the disease,
and have the genotype X

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large R, X small r.

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So in order to determine the
probability that A is affected

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with both X small r's, we must
also know the probability that

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her mother was a carrier
for the disease.

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Let's call her mother
individual C.

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So, we know the genotypes
of her parents.

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Her mother was a carrier.

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And her father did not have the
disease, so his genotype

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was Y with X large R. So to
calculate the probability that

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individual C is a carrier for
the disease, let's go back

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over here and draw out
a Punnett square.

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On one side, we're going to
write the genotype of her

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father, which was Y because
he's a male.

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And then, he was not affected
by the disease, so it was X

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with a large R. Her mother
was a carrier.

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So she had one copy that was X
large R, and one copy that was

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x little r.

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So next, we want to know
the probability

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that she was a carrier.

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The only ways you can get
females is by looking at this

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top row here.

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So, for now, we're going to
ignore the bottom row.

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Of the two possible ways to
generate a female, only one of

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them is a carrier.

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So there's a 50% chance that
a female will be a carrier.

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So this is a 50% chance that
individual C is a carrier,

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which we will denote with this
half filled in circle.

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All right, so now that we know
the probability that C was a

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carrier, we need to know the
probability that her progeny,

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A, was affected with
the disease.

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Again, looking back over here,
her progeny needs to inherit

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the X small r from the
mother and the X

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small r from the father.

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So, once again, we can draw
a Punnett square to better

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understand this.

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Individual A's father
was indeed

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affected by the disease.

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So his genotype was X small r
and Y. Her mother, we just

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calculated the probability of
her being a carrier, so she is

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X large R, X small r.

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Now, we're looking at individual
A, who is a female,

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so again, we can ignore
the bottom row because

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this will be males.

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Here are the progeny
which are female.

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Of the two possibilities, only
the one on the right, X small

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r, X small r is affected
by the disease.

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So there is a 50% chance that
A will be affected by the

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disease given that her mother
is a carrier of the disease.

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Now to finish up this part of
the problem, we need to

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multiply these two probabilities
together because

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we're looking at the probability
that C is a

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carrier, and the probability
that A is

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affected with the disease.

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This gives us our answer
of one fourth.

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All right, so, moving on to the
last part of the problem.

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We need to calculate the
probability that B is affected

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with the disease.

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So this isn't that hard now that
we've drawn out all of

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these Punnett squares already.

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So, let's take a look back at
our pedigree over here.

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If B is going to be affected
with the disease, he must have

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the genotype Y, X
with a small r.

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He gets the Y from his father
and he must get this X with a

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small r from his mother.

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So again, we need the mother,
C, to be a carrier.

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So let's go back to our Punnett
squares over here.

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We know that there is
a 50% chance that

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individual C is a carrier.

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Now we need to look at
individual B being affected by

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the disease.

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So we can go ahead and fill
in the bottom part of this

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Punnett square.

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So of these two males, that are
possible, only one of them

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is affected with the disease.

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So again, there is a 50%
probability that that child

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will be affected with
the disease.

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Again, we are going
to multiply these

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probabilities together.

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So the probability that the
mother is a carrier, and that

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the son is affected with the
disease, gives us one quarter.

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That concludes our problem
on pedigrees.

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Thank you for joining us.