- [Instructor] Hemophilia is
an X-linked recessive trait
that affects blood clotting.
If someone has hemophilia, their
blood has trouble clotting.
If a carrier woman and a
hemophiliac man have a daughter,
what is the percent chance
that she, the daughter,
will have hemophilia?
So, if you're so inspired,
pause this video,
and try to work through this on your own.
All right, now let's work
through this together.
And to do this, let's
think about the genotypes
of both the mother and the father.
So, let's start with the mother.
And since it is an
X-linked recessive trait,
we're going to be dealing
with the sex chromosomes.
So, let's just remind
ourselves that the mother,
because she's female, is going to be XX,
have two X chromosomes.
The father,
the father is going to
have an X chromosome
and a Y chromosome.
But now let's think about the hemophilia.
They say it's a carrier woman.
So, a carrier woman means that
one of her two X chromosomes
would have the hemophilia allele,
and the other one wouldn't.
Carrier implies that she
doesn't show the trait.
She doesn't have hemophilia,
but she's carrying one of these
X-linked recessive hemophilia alleles.
So, we could say that
one of her chromosomes,
one of her X chromosomes
does not have hemophilia.
I use capital H for the dominant allele,
which you would say is no hemophilia.
We tend to use the
capital for the dominant.
And then the lowercase h,
well, this would be the X chromosome
that has the hemophilia allele.
And I did it lowercase h
because it is a recessive trait.
And so this is
hemophilia.
And now, what about the father?
Well, it's X-linked,
and the father only has one X chromosome.
And they say it's a hemophiliac man.
And so that one X chromosome
is going to have the hemophilia allele.
And so the reason why
he exhibits hemophilia
is that there is no other X chromosome
with the dominant allele to be dominant
over the hemophilia allele.
But now let's cross these
two with a Punnett square.
So, the father to the
offspring could contribute
the X chromosome that
has the hemophilia allele
or a Y chromosome.
And the mother can
contribute the X chromosome
that doesn't have the hemophilia allele
or the X chromosome that does
have the hemophilia allele.
And now I'll draw my
little Punnett square here.
So, there you go.
And so what are the different scenarios?
Well, this top-left scenario
would be I get the X chromosome
from the mother that does not
have the hemophilia allele,
and I get the X chromosome from the father
that does have the hemophilia allele.
This scenario is the X
chromosome from the mother
with the hemophilia allele
and the X chromosome
from the father with
the hemophilia allele.
This scenario is the X
chromosome from the mother
without the hemophilia
allele, so no hemophilia,
or with the non-hemophilia allele,
and the Y chromosome from the father.
And this is the X
chromosome from the mother
that has the hemophilia allele
and the Y chromosome from the father.
Now, what is the percent chance that she,
that a daughter would have hemophilia?
So, which of these
scenarios show a daughter?
Well, the daughters are
going to be the XX scenarios.
So, these are the two scenarios
in which they have a daughter.
These two, because there's a Y chromosome,
these would be them having a son.
So, out of the two scenarios
where they have a daughter,
one of those scenarios
you have two of the recessive alleles.
And so this is a situation
where you have a daughter,
daughter with hemophilia,
hemophilia,
right over here.
And so, if we know they have a daughter,
what is the percent chance
that she will be a hemophiliac?
Well, one out of two,
so there is a 50% chance.
Now, what's useful about
this Punnett square
is you could answer
all sorts of questions.
You could say what is the percent chance
that they have a hemophiliac son?
Well, one out of these four
scenarios is a hemophiliac son.
Then you would say there's a 25% chance
that they have a hemophiliac son.
Similarly, there is a 25% chance
that they have a non-hemophiliac son.
If someone said what is the percent chance
that they have a hemophiliac offspring?
Well, that would be these two
scenarios right over here.
So, two out of the four scenarios
are hemophiliac offspring,
so that would be 50%.
If you said, given that they had a son,
what is the percent chance that
they don't have hemophilia?
Well, you would say, given
that they have a son,
so that would be those two scenarios,
one out of those two scenarios,
the son does not have hemophilia.
So, that would again be a 50%.
So anyway, hopefully this was useful.
