X-Inactivation - Video Tutorials & Practice Problems
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concept
X-Inactivation
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Video transcript
In this video, we're going to introduce X inactivation. And so recall from our previous lesson videos that females actually have 2 x chromosomes in comparison to males that only have 1 x chromosome and a y chromosome. And so because females have 2 x chromosomes, of course, that means that they're going to inherit double the number of x linked genes in comparison to males, which only inherit one set of those x linked genes. However, even though females have double the number of x linked genes in comparison to males, they do not have double the expression of those x linked genes in comparison to males. And the reason for that is because female cells will randomly turn off or inactivate one of their X chromosomes during early development. And so when this one X chromosome in females is turned off or inactivated, it will not be expressed. And that's why males and females still have relatively the same amount of expression of their x linked genes. And this x inactivation, this turning off of x chromosomes only occurs in female cells. It does not occur in male cells. Now the Barr body is really just the scientific term that is referring to that highly condensed inactive X chromosome that is found in female cells. It turns out that this random X inactivation that occurs in female cells can result in a female expressing different alleles of an X linked gene in different cells because those 2 X chromosomes that are in female cells may have different alleles. And so depending on which X chromosome is inactivated there could be different expression in different cells. And so a classic example of X inactivation are these calico cats which have color patches due to X inactivation. And so notice down below here what we're showing you is a calico cat, which you'll notice has these color patches. Specifically, it has white color patches, orange color patches, and black color patches in its fur. And so taking a look at the black color patch region, the reason that it expresses this black color patch is because early on in the embryonic development of this cat, one of its 2 x chromosomes is going to be inactivated. And so these 2 red, structures that you see here represent the 2 x chromosomes in this female calico cat. And so the 2 X chromosomes here, one of them is going to be randomly inactivated. And so if one of these X chromosomes, contains, an allele or a gene that controls the fur color, and basically this allele here is the allele that represents black fur, if it is expressed then the cat will have black fur. And if the other X chromosome has an allele that says orange the of the cat that have black fur, what happens is the X chromosome with the black allele version is going to be expressed, whereas the X chromosome with the orange allele version is going to be inactive, and it will not be expressed. It will become a Barr body. And so the black version again is going to be active here, and that leads to black fur and the calico cat in that region. However, if we were to take a look at one of the orange patches on this cat and look at the cells with them within them, what we would notice is that it's the chromosome, the X chromosome with the black allele, black fur allele, that is actually inactive. And so the black version here will be inactive and become the bar body, whereas the, X chromosome with the orange allele, orange fur allele, is actually going to be active. And so, in the regions where the cat has orange fur, it's the orange allele is active on that x chromosome. And in the regions of the cat that have black fur, it's the black X chromosome or the chromosome X chromosome with the black allele that will be active. And so you can see here how this X inactivation, this random turning off or inactivation of 1 of the X chromosomes in female cells can lead to different expression of different alleles. And so this here concludes our introduction to X inactivation and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
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Problem
Problem
A Barr body is:
A
An inactive Y chromosome.
B
An active X chromosome.
C
An inactive X chromosome.
D
An active Y chromosome.
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Problem
Problem
In cats, fur color is determined by an X-linked gene; There are two alleles for this gene, one allele is for black fur and the other is for orange fur. In cats, a heterozygote female has calico (black and orange patches) colored fur. What kinds of offspring would you expect from the mating of a black female and an orange male?