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  • Hi. It's Mr. Andersen and in this podcast I'm going to talk about X-Inactivation.

  • Sometimes we call this lyonization and that's kind of giving credit to the person who discovered

  • most of this which is Mary Lyon. But I want to start the whole thing by talking about

  • calico and tortoise shell cats. Calico cats like this one and tortoise shell cats like

  • this are almost always female. Which puzzled scientists for a long time until we finally

  • figured out how that works. And it puzzled Mary Lyon as well. Here she is. She worked

  • at Cambridge. But basically what she found is that when she was working with lab rats,

  • some of them would be a white. Some would be a grey. And some would be kind of a tortoise

  • shell between to two. But what she found is that most of those tortoise shell mice were

  • females. And so most of them had two X chromosomes. And she was also working on chromosomes and

  • why some disorders are more prevalent in females then they are in males. And so we know that's

  • sex-linkage. But basically what she found is that when you look into the nucleus of

  • a cell, so this is in the nucleus, in every cell of every female, one of the X chromosomes

  • will be active. And so it works perfectly. But the other one is going to be inactive.

  • It's going to fall apart. And so if you're a female, in all of your cells, only one of

  • the X chromosomes is active and the other one is inactive. It's okay though. Because

  • you're fine. On guy's side, same thing. We only have one X chromosome and so sex chromosomes

  • like this are kind of designed to just work on their own. And when it breaks apart like

  • that we call that a bar body. Or that X chromosome is now inactivated. And so let me kind of

  • talk you through how this works. Well basically when you were born, well before you're born,

  • when you're a zygote, that zygote, if you're a female will have two X chromosomes. And

  • those X chromosomes will be copied when the cell goes through mitosis. And so we go from

  • having one cell to two. And then to four. And then to eight. And I could keep doing

  • this, but basically when you get a big ball of cells like this, these are called stem

  • cells. They're cells that can become any other cell. But right around this time when you

  • start to get this big blastula there's basically a flip of the coin that goes on in each of

  • the cells. And so one of the X chromosomes will simply wad up and become inactivated.

  • So this one might wad up. And it's fifty-fifty chance which are the ones that are going to

  • wad up. So it might be that one. It might be that one. So it's totally random which

  • one's going to wad up in each of these. Now the one that's left, it's going to be able

  • to express all of the genes that are found on that. And what's interesting is that all

  • of the cells that come from this cell, likewise, are going to have one X chromosome that's

  • inactivated. And then one X chromosome that's activated. And so basically this stem cell,

  • once that flip of the coin is occurred, it's for the rest of the life of the cell and all

  • the cells that come from that. And so that tells us why a calico cat is female. And let

  • me talk you through that. So if you're a male cat you could basically be an orange or a

  • black cat. Or you could be a white cat if you're lacking the pigment on the hair. And

  • so let's look at this. So it's found on the X chromosome. If you have the black gene on

  • your X chromosome, since you only have on X chromosome, then you're going to be a black

  • cat. If you have the orange gene on your X chromosome then you're going to be an orange

  • cat. And it's pretty straight forward. Because in every cell there's going to be an X chromosome

  • with that gene making that pigment color. Now let's look at the female side. So if you're

  • a female cat and you have two X chromosomes, if both of them are black gene, then you're

  • going to have a black cat. Now what you should know is that in each of the cells of this

  • cat that produces this black pigment on it's skin, it's going to be a different X chromosome

  • that's working in each of those. So that flip of the coin has occurred. Likewise if we've

  • got you know an orange gene on each of those X chromosomes, same thing. It's going to be

  • orange. But what's weird is if you're heterozygous for it. So if you're this, if your a female

  • who has one X chromosome with a black gene and one with a orange gene, well, how could

  • this occur? Well simple. Let's say we match this, a black cat with an orange cat, and

  • then all of the offspring are going to be like this. And so what is it going to look

  • like? Well, this is not a great animation or a great picture, but basically what you

  • will have is splotches of black and splotches of orange. And so let's look at this black

  • right here. Basically what happened is at the stem cell stage, the orange gene basically,

  • this orange gene became a bar body. So it became inactivated and all the cells that

  • came from that produced black pigment. I think that's really cool. Likewise in one's where

  • the black one wadded up, all of the cells that come from that are going to produce orange

  • pigment. And so that's X inactivation. It's rare that we see this occurring and that's

  • just because sex cells are designed to work on their own. The X chromosome. Now two quick

  • questions you should be thinking right away. Is number one, what about the white? Why do

  • we get this white coloration. Well if you remember when I talked about genetics there

  • was a process called epistasis. And so basically there's going to be a set of genes that determine

  • the color of the pigment that's added to the hair. But there's going to be another gene

  • or set of genes that are actually going to tell us, is that color added to the hair or

  • not. And that's called epistasis. And so basically in these cells we're not actually adding the

  • pigment. But up here we've got X inactivation. Another question you should be thinking to

  • yourself is at the beginning I said almost all calico cats that you'll ever see are going

  • to be female. Well how could we ever have a male calico cat? It's pretty simple, but

  • maybe non-intuitive. But basically what you could get is a XXY male. And so how does that

  • occur? Well during meiosis you could get non disjunction of the two X chromosomes. And

  • so basically you could get one that is of the orange. One that is of the black. And

  • so you could have a, we call this Klinefelter's syndrome in humans. But you could have a calico

  • male. But again, it's going to be incredibly rare. So that's X inactivation. It's rare

  • that you could see this in humans. However, I was reading and there's a disorder in humans

  • where they lack sweat glands. You could imagine that's really, really dangerous because you

  • can't regulate your temperature. And what I found was that if you have or are heterozygous

  • for this in some females, they'll actually have patches of their skin where they have

  • sweat glands and patches where they don't. And that's just due to X inactivation. And

  • I hope that's helpful.

Hi. It's Mr. Andersen and in this podcast I'm going to talk about X-Inactivation.

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B1 US chromosome gene orange calico pigment black

X Inactivation

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    James posted on 2015/06/15
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