In this video, we're going to introduce X inactivation. Recall from our previous lesson videos that females actually have two X chromosomes, in comparison to males, who only have one X chromosome and a Y chromosome. Because females have two X chromosomes, they inherit double the number of X-linked genes in comparison to males, who 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. The reason for this is because female cells will randomly turn off or inactivate one of their X chromosomes during early development. When one X chromosome in females is turned off or inactivated, it will not be expressed. That's why males and females still have relatively the same amount of expression of their X-linked genes. X inactivation, the turning off of X chromosomes, only occurs in female cells; it does not occur in male cells. The Barr body is the scientific term referring to the highly condensed inactive X chromosome found in female cells. This random X inactivation in female cells can result in a female expressing different alleles of an X-linked gene in different cells because the two X chromosomes in female cells may have different alleles. Depending on which X chromosome is inactivated, there could be different expression in different cells.
A classic example of X inactivation is these calico cats, which have color patches due to X inactivation. Notice in the illustration below a calico cat, which has white, orange, and black color patches in its fur. Specifically, the reason it expresses the black color patch is that early in the embryonic development of the cat, one of its two X chromosomes is going to be inactivated. These two red structures represent the two X chromosomes in this female calico cat. One of them is going to be randomly inactivated. If one of these X chromosomes contains an allele or a gene that controls fur color, and this allele represents black fur, if it is expressed then the cat will have black fur. If the other X chromosome has an allele for orange fur, in the areas of the cat that have black fur, the X chromosome with the black allele is going to be expressed, whereas the X chromosome with the orange allele will be inactive and will not be expressed; it will become a Barr body. Again, the black version is going to be active here, leading to black fur on the calico cat in that region.
However, if we were to look at one of the orange patches on the cat and examine the cells within it, what we would notice is that it's the chromosome—the X chromosome with the black fur allele—that is actually inactive. The black version here will be inactive and become the Barr body, whereas the X chromosome with the orange fur allele is actually going to be active. In the regions where the cat has orange fur, the orange allele is active on that X chromosome. In the regions of the cat that have black fur, it's the X chromosome with the black allele that will be active. This demonstrates how X inactivation, the random turning off or inactivation of one of the X chromosomes in female cells, can lead to different expressions of different alleles. This concludes our introduction to X inactivation, and we will 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.
