Hi in this video we're gonna be talking about sex chromosomes. So uh first let's talk about sex determination, which you may think, oh that's so easy. Xxx Y. I know female, male, but actually different organisms determine sex differently. Now there are two types of sex is that we're going to talk about. The first is the home a comedic sex. And this is a sex that only uses one type of chromosome. Now this is referring to types of chromosomes, not numbers. The home of immediate sex could have 30 x chromosomes and still be home a comedic sex because it has only excess. So for instance, human females are an example of this. Then the second sex is called the hetero comedic sex and this is a sex that has two types of chromosomes for us. This is X. And Y. Now remember just like the home a comedic sex, there could be 30 X chromosomes and 20 Y chromosomes, but it's still a hetero comedic sex because there's only two types no matter how many numbers there are. So um human females are the homo comedic and human males are hetero comedic. Now, sex determination systems are different in different organisms determination systems. So um one type is called the X. X. X. O. And this is when the sexes are determined by having either one or two of the same chromosomes. So either they have two exes or they have one X. And that determines male and female. We have the one that were most similar or familiar with. This is X. X. X. Y. Um And this is one or two different types of chromosomes. And this is obviously the one we're most familiar with because this is humans X. X. Is female and Xy is male. And then you have this unusual one called Zzz W. Which you've probably never heard of before. And the reason that it uses diseases because the male and females kind of switched spots. So males are actually the homo comedic sex, meaning that the male is easy. And the female is the hetero comedic sex. Which means that it's the W. And this is different than humans where the female is the homo comedic and the male is the hetero comedic. So this is given the special one official label and I'll show you an image of what that looks like in a second. Um And then organisms can be classified as mono is jesus. And this is where an organism contains both male and female sexual organs. Um These are sometimes called hermaphrodites. And then Dionysius. And this is when an organism contains either male or female but not both. So that is some classifications of sexual determination systems in different organisms. So here's an example of the X. X. X. O. Z. W. Sort of systems here. So the ones who are most familiar with is human X. X. X. Y. And you can see here that if you take each gamete here's female, here's mail you do opponent squares you get two females to males. Then another example is the X. X. X. O. These are common in insects. And if you do the same thing with the females which are X. X. And the males which are X. Blank. It's just oh or whatever you want to say, you get two females and two males. And then finally you have a Zw and Z Z. Now this is different because Z. W. Is the female. So if you have your female gametes, they're different. And then the male gametes are the same. So you get two males here and here and two females here and here. So in case you just need a cheat sheet or you're a little confused. Remember the females are either going to be X. X. Either in this system or in this system or Z. W. In this one? And the males will be X, Y x zero or Z. Z. Depending on which system that you are using. So those are the three determination systems. So with that, let's now turn the page
Human Sex Chromosomes
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Okay, so now let's focus a little bit more on human sex chromosomes. So humans follow the XX Xy with males determined by the xy chromosomes. Now, these chromosomes are determined by the gametes combinations that you get. And remember gametes are formed through mitosis. Now my aosis, this means in normal mitosis with all the other sort of autism or other chromosomes. The non sex chromosomes, they line up during mitosis and they separate into the daughter cells, but the X. X. And the X. Y. Aren't copies of each other, right? But they still line up and they still separate independently independently into the hap Lloyd daughter cell. So each daughter cell gets one chromosome, either the X or the Y. Now, sometimes this doesn't happen properly, separation of the chromosomes doesn't happen. And um so what that means is that one daughter cell will get, you know, both the X and Y and one will get none. Now this is given a special term called non disjunction. And this is when the chromosomes don't separate properly during mitosis. And when non disjunction occurs, there are many disorders, medical disorders and genetic disorders that occur. So one you're going to hear about is called Klinefelter and it's written as 47 because there's now 47 chromosomes rather than 46 because there's two Xs and A Y. This is an extra sex chromosome. These typically present as male because they have male genitalia, but they're usually infertile. They fail to reduce produce sperm and they have, like, other fanatic pick um uh symptoms as well. Couldn't kind of long fingers and arms. Um and just like development of breasts sometimes. And um so these can obviously like you can see them, but it's because of this extra X chromosome. Now I've written 47 X. X. Y. But actually claim filters can sometimes present as even more X. Is like X. X. X. X. X. Y. And in that case is they typically prevent present with much more severe phenotype. So um both physically but also things like infertility and stuff like that. Now the second big one that you'll hear about is Turner syndrome and this is 45 X. So there's one last chromosome because there's only one X. And these typically have female genitalia, but there's also other cognitive and sexual impairments. They're usually typically infertile. Um and they have much reduced cognitive ability. And so it's generally not a great center of the half. Um And then sometimes in these unusual cases You can actually get mosaics of individuals. And these are somatic cells that display two different numbers of sex chromosomes. So example, some of your cells could be 45 x, which would be resemble of Turner syndrome. And some of them are just normal Or 46 xy for a male. And so you can get these situations where a single individual actually has multiple cells that, you know, either, one type or the other in kind of a bunch of different combinations. Um like I said, these are fairly rare but they do occur and you may hear about them. So let me back up and this is remember a carry a type. This is what a stereotype looks like. It's just this map of the chromosomes of an individual. And you can see that the autos OEMs which are all of these here that are labeled one through 22, there's two pairs. They look exactly like they should. But you'll notice the six chromosomes which are the X. And the Y. There's an extra one. And so what is it when you are X. X. Y. Remember which disorder? That is right. It is going to be climb belters syndrome. So this person has climbed filters. Now remember there can be multiple exes here. So if there were there was another X chromosome still be client shelters, just uh just more severe. It could have just one. So we cross this out one X. No. Why, what would that be? Turner syndrome? So you may be presented with a carrier type and asked to identify, you know, what disorder is this? So make sure you understand how to read these and know the numbers of sex chromosomes found in these disorders. Now when we come back, human sex is so email or female handle the chromosomes differently. So the Y chromosome determines male ness obviously because all males have the Y. Now the the reason for that is because the Y chromosome has this special region called the sex determining region. And um this is a region that produces that that's a gene and it produces a protein called the testis determining factor TDF protein. And this protein is what allows males to develop the way they do. It develops males determines balance. Females. On the other hand get two copies of the X. Chromosome, but you only need one right? When males have one they have an X. One X chromosome. Yeah they have a Y. But it's little and it doesn't really do anything other than just determined males. It doesn't really have a lot of genes on it. But the X chromosome is really important and it has a ton of genes on it. And if you have two copies of it you get double the dose. So in females who get two copies what actually has to happen is they have to shut one off. And so they shut one off by forming these things called Bar bodies. And Bar bodies are inactivated X. Chromosomes and they're an activated because the D. N. A. And the protein that makes up that chromosome, remember if you've heard this, you may have heard this term before, but this is chrome a tin and that equals D. N. A. Plus protein. So that actually condemns it. So it gets really tight together, it squishes all together and because it gets so squished together that actually will inactivate it. It means that the genes on the air cannot be transcribed. They cannot be made into proteins. And so it just remains these like squished up our bodies now. So every female has two X chromosomes. But one of them is inactivated in every single cell that a female has. Now. When does this inactivation occur? So it actually occurs not like as soon as the females formed technically at fertilization in the formation of the or the combination of the egg and sperm, which is when you first get those two X chromosomes. But instead it happens later on in development. And this is called the lion hypothesis. And so it occurs very early in development. But it's still not the very beginning. And so because it's not the very beginning there's actually multiple different somatic cells that are present at the time this happens. So you have, you know, a few dozen cells will say. And so when you activate the one X chromosome in these like dozen cells, what happens is that not all the cells and activate the same copy? So you have X. One and X. Two. And we'll just say half the sales and activate this 1.5 the cells and activate this one. And so normally it doesn't matter right if they all have the same alleles on them, you're not going to notice the difference. But sometimes in some organisms, the alleles on the X chromosomes are different. So if you inactivate one that gives you a different phenotype than if you were to inactivate the second one. And so an example of this or what it's called, it's called a mosaic phenotype because the cells aren't all presenting the same phenotype, some of them half of them present one and have them present the other. So an example of this is actually a calico cat calico cats have different coloration and this one you can see there's an orange and a black phenotype here. And this is determined by cells. Now the alleles or the gene for coloration is actually found on the X chromosome So early in development. The X one let's say has the black alil and the X two has the orange alil. So half the cells that inactivate this one will show up as orange because black is now inactivated, behalf of the cells that inactivate this one will show up as black because they had activated the orange chromosome, the orange gene. And so you end up getting. And so as the organism grows, these two cells go on to make different regions either the back or the legs or the head or right above the eye or right above the second toenail. On the back, right leg, I mean all these different cells go on to create all these different regions of the pat. And so what you get is you get this genotype where these cells here are presenting the orange and these ones here are you can see the black and it's because of which X chromosome they inactivated very early in this cat's development. The other thing that's really interested about this is that calico cats because they're formed based on inactivating one of the X chromosomes are all female because the male only has one X. And one Y. And therefore does it in activate one of them. So calico cats are all female and this has to do with an activation of the X chromosome. So I think that's really cool. So, oh sorry, I didn't mean to zoom in for you, but there you go. You got a better picture of the cat. But this is really neat, I think it's interesting. Um And so with that let's now move on.
Which of the follow sex chromosomes can be describes as homogametic?
In the ZZ/ZW sex determination system, the male is…?
In humans, which region on the Y chromosome determines maleness?
Sperm producing factor (SPF)
Testis determining region (TDR)
Sex Determining Region (SRY)
Barr Body (BB)
During meiosis the XY sex chromosomes segregate independently. Which of the following represents the gametes chromosomes after meiosis?
Two haploid cells that each contain X or Y
Two diploid cells that both contain an XY
A single diploid cell that contains XY
Four haploid cells – two contain X and two contain Y