WEBVTT

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

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wondering a very natural
question, how is gender

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determined in an organism?

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

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

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

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environmental.

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Not all reptiles, but
certain cases of it.

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It could be maybe the
temperature in which the

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embryo develops will dictate
whether it turns into a male

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or female or other environmental
factors.

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

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

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

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

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

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

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characteristics that
differentiate

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a man from a woman.

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

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to work together.

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And to some degree,
your second answer

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would be more correct.

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

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It's actually whole chromosomes
determine it.

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So let me draw a nucleus.

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That's going to be my nucleus.

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

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So 22 of the pairs of
chromosomes are just regular

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non-sex-determining
chromosomes.

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

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10, 12, 14.

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I can just keep going.

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And eventually you
have 22 pairs.

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

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And those are just our standard
pairs of chromosomes

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that code for different
things.

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

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which we learned before
you get one from

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each of your parents.

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

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same versions of the genes,
but they code

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for the same genes.

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

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other gene of the
homologous pair.

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Although you might have
different versions of eye

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color on either one and that
determines what you display.

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But these are just kind of the
standard genes that have

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nothing to do with our gender.

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And then you have these two
other special chromosomes.

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I'll do this one.

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

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

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homologous.

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

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is short and the brown
one's long?

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And that's true.

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They aren't homologous.

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

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

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call that the x chromosome.

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Let me scroll down
a little bit.

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

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chromosome.

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And to figure out whether
something is a male or a

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female, it's a pretty
simple system.

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

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So let me write that down.

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

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

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the nucleus for a man.

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

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

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

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dad, you will be a male.

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

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

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And so we could actually even
draw a Punnett square.

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This is almost a trivially easy
Punnett square, but it

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kind of shows what all of the
different possibilities are.

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

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sex-determining chromosome.

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She's got two x's.

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

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

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capital-- and has
a Y chromosome.

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And we can do a Punnett
square.

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What are all the different
combinations of offspring?

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

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with this X chromosome
from your dad.

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This would produce a female.

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Your mom could give this other
X chromosome with that X

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chromosome.

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That would be a female
as well.

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

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chromosome.

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

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

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

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And it works out nicely
that half are female

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and half are male.

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But a very interesting and
somewhat ironic fact might pop

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out at you when you see this.

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Who determines whether their
offspring are male or female?

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Is it the mom or the dad?

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

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

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

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gender of the offspring.

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

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

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towards the egg.

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

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them have a Y chromosome
in them.

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And obviously they
have others.

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And obviously if this guy
up here wins the race.

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Or maybe I should
say this girl.

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

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into a female.

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

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develop into a male.

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

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and probably the most famous
example of this

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is Henry the VIII.

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

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

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

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with producing a male
heir to kind of take

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over the family name.

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

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And they become very
disappointed and they tend to

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blame their wives when the wives
keep producing females,

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but it's all their fault.

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Henry the VIII, I mean
the most famous case

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was with Ann Boleyn.

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

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

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And then he found a reason
to get her essentially

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decapitated, even though
it was all his fault.

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He was maybe producing a lot
more sperm that looked like

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that than was looking
like this.

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

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

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

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Henry the VIII's fault.

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

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

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for the lack of a male heir.

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Now one question that might
immediately pop up in your

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

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just our sex-determining traits
or are there other

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stuff on them?

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So let me draw some
chromosomes.

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

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chromosome.

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

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although it is kind of
famously gene poor.

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

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

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chromosomes.

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It only codes for on the
order of 78 genes.

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

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

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determining what
the gender is.

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

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it called the SRY gene.

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You don't have to know that.

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

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male sexual organ.

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

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coding for things that will
eventually lead to the

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development of the testicles.

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

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you'll end up with a female.

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And I'm making gross
oversimplifications here.

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

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a role in determining sex.

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But you do have other traits
on these genes.

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And the famous cases all deal
with specific disorders.

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So, for example, color
blindness.

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The genes, or the mutations
I should say.

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So the mutations that cause
color blindness.

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

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maybe a little bit
inappropriate.

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Color blindness and
also hemophilia.

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This is an inability of
your blood to clot.

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Actually, there's several
types of hemophilia.

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But hemophilia is an
inability for your

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blood to clot properly.

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And both of these are mutations
on the X chromosome.

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And they're recessive
mutations.

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So what does that mean?

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

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

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chromosomes have to have the
hemophilia mutation in order

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for you to show the phenotype
of having hemophilia.

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

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her genotype.

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She has one regular X chromosome
and then she has

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

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superscript there for
hemophilia-- she has the

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hemophilia mutation.

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She's just going to
be a carrier.

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Her phenotype right here is
going to be no hemophilia.

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She'll have no problem
clotting her blood.

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

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she gets two versions of
this, because this

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is a recessive mutation.

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Now this individual will
have hemophilia.

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Now men, they only have
one X chromosome.

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So for a man to exhibit
hemophilia, to have this

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phenotype, he just needs
it only on the one X

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chromosome he has.

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And then the other one's
a Y chromosome.

00:11:05.040 --> 00:11:07.980


00:11:07.980 --> 00:11:11.150
So this man will have
hemophilia.

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

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

00:11:18.120 --> 00:11:22.210
infrequent mutation that arises
on an X chromosome--

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

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A male or a female?

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

00:11:29.620 --> 00:11:33.490
Well if this is a relatively
infrequent allele, a female,

00:11:33.490 --> 00:11:35.640
in order to display it, has
to get two versions of it.

00:11:35.640 --> 00:11:40.400
So let's say that the frequency
of it-- and I looked

00:11:40.400 --> 00:11:43.360
it up before this video--
roughly they say between 1 in

00:11:43.360 --> 00:11:46.460
5,000 to 10,000 men exhibit
hemophilia.

00:11:46.460 --> 00:11:51.020
So let's say that the allele
frequency of this is 1 in

00:11:51.020 --> 00:12:00.130
7,000, the frequency of Xh, the
hemophilia version of the

00:12:00.130 --> 00:12:01.100
X chromosome.

00:12:01.100 --> 00:12:04.490
And that's why 1 in 7,000 men
display it, because it's

00:12:04.490 --> 00:12:07.190
completely determined whether--
there's a 1 in 7,000

00:12:07.190 --> 00:12:09.810
chance that this X chromosome
they get is

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the hemophilia version.

00:12:11.250 --> 00:12:14.120
Who cares what the Y chromosome
they get is, cause

00:12:14.120 --> 00:12:17.130
that essentially doesn't code at
all for the blood clotting

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factors and all of the things
that drive hemophilia.

00:12:19.700 --> 00:12:20.870
Now, for a woman to get

00:12:20.870 --> 00:12:23.140
hemophilia, what has to happen?

00:12:23.140 --> 00:12:26.120
She has to have two X
chromosomes with the mutation.

00:12:26.120 --> 00:12:31.020
Well the probability of each of
them having the mutation is

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1 in 7,000.

00:12:33.630 --> 00:12:38.110
So the probability of her having
hemophilia is 1 in

00:12:38.110 --> 00:12:46.150
7,000 times 1 in 7,000, or
that's 1 in what, 49 million.

00:12:46.150 --> 00:12:49.790
So as you can imagine, the
incidence of hemophilia in

00:12:49.790 --> 00:12:52.880
women is much lower than
the incidence of

00:12:52.880 --> 00:12:54.220
hemophilia in men.

00:12:54.220 --> 00:12:56.600
And in general for any
sex-linked trait, if it's

00:12:56.600 --> 00:13:00.990
recessive, if it's a recessive
sex-linked trait, which means

00:13:00.990 --> 00:13:03.530
men, if they have it, they're
going to show it, because they

00:13:03.530 --> 00:13:06.740
don't have another X chromosome
to dominate it.

00:13:06.740 --> 00:13:08.640
Or for women to show
it, she has to have

00:13:08.640 --> 00:13:09.870
both versions of it.

00:13:09.870 --> 00:13:18.150
The incidence in men is going to
be, so let's say that m is

00:13:18.150 --> 00:13:20.060
the incidence in men.

00:13:20.060 --> 00:13:23.725


00:13:23.725 --> 00:13:24.975
I'm spelling badly.

00:13:24.975 --> 00:13:29.560


00:13:29.560 --> 00:13:33.330
Then the incidence in
women will be what?

00:13:33.330 --> 00:13:36.040
You could view this as the
allele frequency of that

00:13:36.040 --> 00:13:38.000
mutation on the X chromosome.

00:13:38.000 --> 00:13:40.110
So women have to get
two versions of it.

00:13:40.110 --> 00:13:42.680
So the woman's frequency
is m squared.

00:13:42.680 --> 00:13:44.680
And you might say, hey, that
looks like a bigger number.

00:13:44.680 --> 00:13:45.280
I'm squaring it.

00:13:45.280 --> 00:13:48.610
But you have to remember that
these numbers, the frequency

00:13:48.610 --> 00:13:51.250
is less than 1, so in the
case of hemophilia,

00:13:51.250 --> 00:13:53.200
that was 1 in 7,000.

00:13:53.200 --> 00:13:58.680
So if you square 1 in 7,000,
you get 1 in 49 million.

00:13:58.680 --> 00:14:01.240
Anyway, hopefully you found that
interesting and now you

00:14:01.240 --> 00:14:05.210
know how we all become
men and women.

00:14:05.210 --> 00:14:11.470
And even better you know whom to
blame when some of these, I

00:14:11.470 --> 00:14:16.340
guess, male-focused parents
are having trouble

00:14:16.340 --> 00:14:17.590
getting their son.

00:14:17.590 --> 00:14:18.336