PROFESSOR: Hello and
welcome to the
help session on pedigrees.
Today, we will be working
out a problem together.
If you have not yet had a chance
to work it on your own,
please do so now, and
return to this video
when you are done.
Now that you've had a chance to
look at this problem, let's
work it out together.
The first part of this question
asks, what is the
mode of inheritance that is
observed in this pedigree?
So, we know that there are two
main types of modes of
inheritance.
It can either be dominant
or recessive.
And from there it can either
be autosomal or X-linked.
If a disease follows a dominant
inheritance pattern,
generally, it must be present
in every generation.
So here we notice that the
disease is present in the
first generation, but it's not
present in anyone in the
second generation.
However, then it reappears
in the third generation.
This suggests that the
disease is recessive.
So now, do we think
this disease is
autosomal or is it X-linked?
So we look at the pedigree
again, and upon closer
inspection, we notice
that the affected
individuals are only males.
This is a key characteristic
of an
X-linked recessive disease.
However, we're also given more
information that tells us that
this individual here does not
carry an allele associated
with the affected phenotype.
This rules out the possibility
that this disease could be an
autosomal recessive disease.
Thus, the most likely mode of
inheritance is X-linked recessive.
Great, so moving on to the
second part of this problem,
we are asked to determine the
genotypes of individuals
number one and number three.
So, if we look at this pedigree,
we know individual
one is female and not affected
by the disease.
Female individuals contain two
X chromosomes, and since she
is not affected she must contain
at least one large R.
So we'll call the allele
associated with the
disease small r.
We do not yet know what her
second X chromosome could be.
It could be either another
large R or it
could be a small r.
And she also would still not
be affected by the disease.
This male right here is affected
by the disease.
So he must have an
X with a small r.
Remember that males have one
X chromosome and one Y
chromosome.
So now, making our way down to
individual number three.
Individual number three does
not have the disease and is
female, so again, must have
two X chromosomes.
One of the X chromosomes must
come from her father and the
other from her mother.
The only X chromosome that her
father can pass along to her
is X small r.
In order for her to not be
affected with the disease, her
other X chromosome must
have a large R.
She was able to get this
genotype with her mother
having either of these
two genotypes.
To make sure that both of these
genotypes are possible
for this mother, we need to
examine her other children.
Her son here is unaffected.
So his genotype must have been
X large R, Y. It's possible
for him to get this genotype
when the mother has either of
these two genotypes.
Similarly, this daughter could
be unaffected just as this
daughter was unaffected.
So the mother can have either
of these two genotypes.
And we're unable to rule
out one of them.
So let me just write up
the answer over here.
All right, so far, so good.
Next, we're asked to calculate
the probability that
individual A is affected.
Individual A is over here at
the bottom of the pedigree.
In order for individual A to be
affected, she must have the
following genotype.
The only way for her to get two
little r's is for her to
get one from her father and
one from her mother.
We know this is possible because
her father is affected
with the following genotype
of Y, X small r.
Her mother is not affected.
So for her to get this small
r, her mother must be a
carrier for the disease,
and have the genotype X
large R, X small r.
So in order to determine the
probability that A is affected
with both X small r's, we must
also know the probability that
her mother was a carrier
for the disease.
Let's call her mother
individual C.
So, we know the genotypes
of her parents.
Her mother was a carrier.
And her father did not have the
disease, so his genotype
was Y with X large R. So to
calculate the probability that
individual C is a carrier for
the disease, let's go back
over here and draw out
a Punnett square.
On one side, we're going to
write the genotype of her
father, which was Y because
he's a male.
And then, he was not affected
by the disease, so it was X
with a large R. Her mother
was a carrier.
So she had one copy that was X
large R, and one copy that was
x little r.
So next, we want to know
the probability
that she was a carrier.
The only ways you can get
females is by looking at this
top row here.
So, for now, we're going to
ignore the bottom row.
Of the two possible ways to
generate a female, only one of
them is a carrier.
So there's a 50% chance that
a female will be a carrier.
So this is a 50% chance that
individual C is a carrier,
which we will denote with this
half filled in circle.
All right, so now that we know
the probability that C was a
carrier, we need to know the
probability that her progeny,
A, was affected with
the disease.
Again, looking back over here,
her progeny needs to inherit
the X small r from the
mother and the X
small r from the father.
So, once again, we can draw
a Punnett square to better
understand this.
Individual A's father
was indeed
affected by the disease.
So his genotype was X small r
and Y. Her mother, we just
calculated the probability of
her being a carrier, so she is
X large R, X small r.
Now, we're looking at individual
A, who is a female,
so again, we can ignore
the bottom row because
this will be males.
Here are the progeny
which are female.
Of the two possibilities, only
the one on the right, X small
r, X small r is affected
by the disease.
So there is a 50% chance that
A will be affected by the
disease given that her mother
is a carrier of the disease.
Now to finish up this part of
the problem, we need to
multiply these two probabilities
together because
we're looking at the probability
that C is a
carrier, and the probability
that A is
affected with the disease.
This gives us our answer
of one fourth.
All right, so, moving on to the
last part of the problem.
We need to calculate the
probability that B is affected
with the disease.
So this isn't that hard now that
we've drawn out all of
these Punnett squares already.
So, let's take a look back at
our pedigree over here.
If B is going to be affected
with the disease, he must have
the genotype Y, X
with a small r.
He gets the Y from his father
and he must get this X with a
small r from his mother.
So again, we need the mother,
C, to be a carrier.
So let's go back to our Punnett
squares over here.
We know that there is
a 50% chance that
individual C is a carrier.
Now we need to look at
individual B being affected by
the disease.
So we can go ahead and fill
in the bottom part of this
Punnett square.
So of these two males, that are
possible, only one of them
is affected with the disease.
So again, there is a 50%
probability that that child
will be affected with
the disease.
Again, we are going
to multiply these
probabilities together.
So the probability that the
mother is a carrier, and that
the son is affected with the
disease, gives us one quarter.
That concludes our problem
on pedigrees.
Thank you for joining us.
