Pedigree for determining probability of exhibiting sex linked recessive trait | Khan Academy

- [Instructor] We are
told the pedigree chart

represents the inheritance
of color blindness

through three generations.

And we see this here.

The standard convention is a square

is male, circle is female.

If it's colored in, that means
that they exhibit the trait,

in this case it's color blindness.

So Bill exhibits color blindness.

His phenotype is color blind,

while Bonnie does not
exhibit color blindness.

Color blindness is an
X-linked recessive trait.

If Barbara is expecting another child,

so this is Barbara right here,

what is the probability
that it will be colorblind?

So pause this video and see if you can

figure that out on your own.

All right, now let's work
through this together.

So they're asking us about
their next child here.

What is the probability that
it is going to be colorblind?

And to help us with that,

we can try to figure out the
genotypes of Tom and Barbara.

So Tom is pretty straightforward.

He is male, we know that
'cause there's a square there.

So X, he has an X chromosome
and he has a Y chromosome.

And color blindness is an
X-linked recessive trait.

And so let me just make
clear what's going on.

So I'll do lowercase C for
colorblind, colorblind.

And I could do a capital
C for the dominant trait,

which is not colorblind,

but since they look so similar,

I'll just use a plus for not colorblind,

not color, not colorblind.

And so Tom, his phenotype,
he is colorblind,

and he only has one X chromosome,

what the colorblind trait is linked to.

And so that must have the
recessive allele right over there.

So this is Tom's genotype.

But what about Barbara?

Well, we know Barbara's going to have

two X chromosomes because
Barbara is female.

And we know that both of
them can't be lowercase C

because then Barbara would
exhibit color blindness,

but how can we figure
out her actual genotype?

Well, we could look at her parents.

So Bill over here is going to have

the same genotype as Tom,

at least with respect to color blindness.

He is male, so he has an X
chromosome and a Y chromosome.

And because he exhibits color blindness,

that X chromosome must have the recessive

colorblind allele associated with it.

Now, Bonnie, we do not know.

She will be XX, will
have two X chromosomes.

Like Barbara, we know that both of these

can't have the recessive allele

because then Bonnie would be filled in,

she would exhibit color blindness.

But we don't know whether she is a carrier

or whether she isn't.

But let's just think about where

Barbara got her chromosomes from.

One of her X chromosomes
comes from her father.

And the other one comes from her mother.

So if she got this X
chromosome from her father,

her father only has one
X chromosome to give,

the one that has the colorblind allele.

So if this is from her father,

it must have the colorblind allele here.

And we know that the one from her mother

does not the colorblind allele

because if it was like this,

then Barbara would be
colorblind, and she isn't.

So we know that this must be a plus here.

It is the dominant non-colorblind allele.

And so now we know both of their genotypes

and we can use those to then figure out

the possible outcomes for their offspring.

So for example, Tom can contribute

a X chromosome that has
a colorblind allele,

or a Y chromosome.

And Barbara, right over here,

can contribute an X chromosome

that has the colorblind allele,

or an X chromosome that has
the non-colorblind allele.

Barbara is a carrier.

And so let me just draw a
little Punnett square here.

And so we have four possible
outcomes for their children

and they're all equally likely.

So you can get the X
chromosome from Barbara

that has the colorblind allele

and the X chromosome from Tom

that has the colorblind allele.

You could have the X chromosome

from Barbara with the colorblind allele,

and the Y chromosome from Tom.

You could have the
non-colorblind X chromosome

that does not have the
colorblind allele on it,

and get the colorblind
X chromosome from Tom.

Or you could have the
non-colorblind X chromosome

and the Y chromosome from the father.

So there's four equal scenarios.

And so in how many of these scenarios

is the offspring colorblind?

Well, here we have a colorblind female.

She has two of the recessive alleles,

so that female will be colorblind.

This is a female carrier,

but they will not show the
phenotype of being colorblind.

This over here is a colorblind male,

has only one X chromosome

and it has the colorblind allele on it.

And this is a non-colorblind male.

So out of four equal outcomes,

two of them have the
offspring being colorblind.

So two out of four, that
would be a 50% probability

that the offspring will be colorblind.