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- 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.