Sex-Linked Traits

Chromosomal basis for gender. Sex-linked traits.

By this point in the biology
playlist, you're probably

wondering a very natural
question, how is gender

determined in an organism?

And it's not an obvious answer,
because throughout the

animal kingdom, it's actually
determined in different ways.

In some creatures, especially
some types of reptiles, it's

environmental.

Not all reptiles, but
certain cases of it.

It could be maybe the
temperature in which the

embryo develops will dictate
whether it turns into a male

or female or other environmental
factors.

And in other types of animals,
especially mammals, of which

we are one example, it's
a genetic basis.

And so your next question is,
hey, Sal, so-- let me write

this down, in mammals it's
genetic-- so, OK, maybe

they're different alleles, a
male or a female allele.

But then you're like, hey, but
there's so many different

characteristics that
differentiate

a man from a woman.

Maybe it would have to be a
whole set of genes that have

to work together.

And to some degree,
your second answer

would be more correct.

It's even more than just
a set of genes.

It's actually whole chromosomes
determine it.

So let me draw a nucleus.

That's going to be my nucleus.

And this is going to be
the nucleus for a man.

So 22 of the pairs of
chromosomes are just regular

non-sex-determining
chromosomes.

So I could just do, that's one
of the homologous, 2, 4, 6, 8,

10, 12, 14.

I can just keep going.

And eventually you
have 22 pairs.

So these 22 pairs right there,
they're called autosomal.

And those are just our standard
pairs of chromosomes

that code for different
things.

Each of these right here is a
homologous pair, homologous,

which we learned before
you get one from

each of your parents.

They don't necessarily code for
the same thing, for the

same versions of the genes,
but they code

for the same genes.

If eye color is on this gene,
it's also on that gene, on the

other gene of the
homologous pair.

Although you might have
different versions of eye

color on either one and that
determines what you display.

But these are just kind of the
standard genes that have

nothing to do with our gender.

And then you have these two
other special chromosomes.

I'll do this one.

It'll be a long brown one, and
then I'll do a short blue one.

And the first thing you'll
notice is that they don't look

homologous.

How could they code for the same
thing when the blue one

is short and the brown
one's long?

And that's true.

They aren't homologous.

And these we'll call our
sex-determining chromosomes.

And the long one right here,
it's been the convention to

call that the x chromosome.

Let me scroll down
a little bit.

And the blue one right there,
we refer to that as the y

chromosome.

And to figure out whether
something is a male or a

female, it's a pretty
simple system.

If you've got a y chromosome,
you are a male.

So let me write that down.

So this nucleus that I drew
just here-- obviously you

could have the whole broader
cell all around here-- this is

the nucleus for a man.

So if you have an x chromosome--
and we'll talk

about in a second why you can
only get that from your mom--

an x chromosome from your mom
and a y chromosome from your

dad, you will be a male.

If you get an x chromosome
from your mom and an x

chromosome from your dad, you're
going to be a female.

And so we could actually even
draw a Punnett square.

This is almost a trivially easy
Punnett square, but it

kind of shows what all of the
different possibilities are.

So let's say this is your
mom's genotype for her

sex-determining chromosome.

She's got two x's.

That's what makes her your
mom and not your dad.

And then your dad has an x and
a y-- I should do it in

capital-- and has
a Y chromosome.

And we can do a Punnett
square.

What are all the different
combinations of offspring?

Well, your mom could give this
X chromosome, in conjunction

with this X chromosome
from your dad.

This would produce a female.

Your mom could give this other
X chromosome with that X

chromosome.

That would be a female
as well.

Well, your mom's always going
to be donating an X

chromosome.

And then your dad is going to
donate either the X or the Y.

So in this case, it'll
be the Y chromosome.

So these would be female,
and those would be male.

And it works out nicely
that half are female

and half are male.

But a very interesting and
somewhat ironic fact might pop

out at you when you see this.

Who determines whether their
offspring are male or female?

Is it the mom or the dad?

Well, the mom always donates an
X chromosome, so in no way

does what the haploid genetic
makeup of the mom's eggs, of

the gamete from the female, in
no way does that determine the

gender of the offspring.

It's all determined by whether--
let me just draw a

bunch of-- dad's got a lot of
sperm, and they're all racing

towards the egg.

And some of them have an X
chromosome in them and some of

them have a Y chromosome
in them.

And obviously they
have others.

And obviously if this guy
up here wins the race.

Or maybe I should
say this girl.

If she wins the race, then the
fertilized egg will develop

into a female.

If this sperm wins the race,
then the fertilized egg will

develop into a male.

And the reason why I said it's
ironic is throughout history,

and probably the most famous
example of this

is Henry the VIII.

I mean it's not just the
case with kings.

It's probably true, because most
of our civilization is

male dominated, that you've had
these men who are obsessed

with producing a male
heir to kind of take

over the family name.

And, in the case of Henry the
VIII, take over a country.

And they become very
disappointed and they tend to

blame their wives when the wives
keep producing females,

but it's all their fault.

Henry the VIII, I mean
the most famous case

was with Ann Boleyn.

I'm not an expert here, but the
general notion is that he

became upset with her that she
wasn't producing a male heir.

And then he found a reason
to get her essentially

decapitated, even though
it was all his fault.

He was maybe producing a lot
more sperm that looked like

that than was looking
like this.

He eventually does produce a
male heir so he was-- and if

we assume that it was his
child-- then obviously he was

producing some of these, but for
the most part, it was all

Henry the VIII's fault.

So that's why I say there's a
little bit of irony here.

Is that the people doing the
blame are the people to blame

for the lack of a male heir.

Now one question that might
immediately pop up in your

head is, Sal, is everything on
these chromosomes related to

just our sex-determining traits
or are there other

stuff on them?

So let me draw some
chromosomes.

So let's say that's an X
chromosome and this is a Y

chromosome.

Now the X chromosome, it does
code for a lot more things,

although it is kind of
famously gene poor.

It codes for on the order
of 1,500 genes.

And the Y chromosome, it's the
most gene poor of all the

chromosomes.

It only codes for on the
order of 78 genes.

I just looked this up, but who
knows if it's exactly 78.

But what it tells you is it does
very little other than

determining what
the gender is.

And the way it determines that,
it does have one gene on

it called the SRY gene.

You don't have to know that.

SRY, that plays a role in the
development of testes or the

male sexual organ.

So if you have this around, this
gene right here can start

coding for things that will
eventually lead to the

development of the testicles.

And if you don't have that
around, that won't happen, so

you'll end up with a female.

And I'm making gross
oversimplifications here.

But everything I've dealt with
so far, OK, this clearly plays

a role in determining sex.

But you do have other traits
on these genes.

And the famous cases all deal
with specific disorders.

So, for example, color
blindness.

The genes, or the mutations
I should say.

So the mutations that cause
color blindness.

Red-green color blindness, which
I did in green, which is

maybe a little bit
inappropriate.

Color blindness and
also hemophilia.

This is an inability of
your blood to clot.

Actually, there's several
types of hemophilia.

But hemophilia is an
inability for your

blood to clot properly.

And both of these are mutations
on the X chromosome.

And they're recessive
mutations.

So what does that mean?

It means both of your X
chromosomes have to have--

let's take the case for
hemophilia-- both of your X

chromosomes have to have the
hemophilia mutation in order

for you to show the phenotype
of having hemophilia.

So, for example, if there's a
woman, and let's say this is

her genotype.

She has one regular X chromosome
and then she has

one X chromosome that has
the-- I'll put a little

superscript there for
hemophilia-- she has the

hemophilia mutation.

She's just going to
be a carrier.

Her phenotype right here is
going to be no hemophilia.

She'll have no problem
clotting her blood.

The only way that a woman could
be a hemophiliac is if

she gets two versions of
this, because this

is a recessive mutation.

Now this individual will
have hemophilia.

Now men, they only have
one X chromosome.

So for a man to exhibit
hemophilia, to have this

phenotype, he just needs
it only on the one X

chromosome he has.

And then the other one's
a Y chromosome.

So this man will have
hemophilia.

So a natural question should be
arising is, hey, you know

this guy-- let's just say that
this is a relatively

infrequent mutation that arises
on an X chromosome--

the question is who's more
likely to have hemophilia?

A male or a female?

All else equal, who's more
likely to have it?

Well if this is a relatively
infrequent allele, a female,

in order to display it, has
to get two versions of it.

So let's say that the frequency
of it-- and I looked

it up before this video--
roughly they say between 1 in

5,000 to 10,000 men exhibit
hemophilia.

So let's say that the allele
frequency of this is 1 in

7,000, the frequency of Xh, the
hemophilia version of the

X chromosome.

And that's why 1 in 7,000 men
display it, because it's

completely determined whether--
there's a 1 in 7,000

chance that this X chromosome
they get is

the hemophilia version.

Who cares what the Y chromosome
they get is, cause

that essentially doesn't code at
all for the blood clotting

factors and all of the things
that drive hemophilia.

Now, for a woman to get

hemophilia, what has to happen?

She has to have two X
chromosomes with the mutation.

Well the probability of each of
them having the mutation is

1 in 7,000.

So the probability of her having
hemophilia is 1 in

7,000 times 1 in 7,000, or
that's 1 in what, 49 million.

So as you can imagine, the
incidence of hemophilia in

women is much lower than
the incidence of

hemophilia in men.

And in general for any
sex-linked trait, if it's

recessive, if it's a recessive
sex-linked trait, which means

men, if they have it, they're
going to show it, because they

don't have another X chromosome
to dominate it.

Or for women to show
it, she has to have

both versions of it.

The incidence in men is going to
be, so let's say that m is

the incidence in men.

I'm spelling badly.

Then the incidence in
women will be what?

You could view this as the
allele frequency of that

mutation on the X chromosome.

So women have to get
two versions of it.

So the woman's frequency
is m squared.

And you might say, hey, that
looks like a bigger number.

I'm squaring it.

But you have to remember that
these numbers, the frequency

is less than 1, so in the
case of hemophilia,

that was 1 in 7,000.

So if you square 1 in 7,000,
you get 1 in 49 million.

Anyway, hopefully you found that
interesting and now you

know how we all become
men and women.

And even better you know whom to
blame when some of these, I

guess, male-focused parents
are having trouble

getting their son.