Okay. So now we're going to talk about other chromatin regulatory mechanisms. We talked about individual amino acids getting modified, but DNA packaging can cause entire chromosomal effects. A great example of this that you're probably already familiar with is X inactivation. We know that females, human females, get two copies of the X chromosome. We don't need both copies, so one has to become inactivated, and that inactivated X chromosome is called a Barr body. Now that entire chromosome is inactivated due to heterochromatin. This is DNA packaging on an entire chromosome level that shuts off an entire chromosome and prevents all those genes from being transcribed in order to prevent a huge overdose of gene dosage in females.
There's a second term that you're going to come across, and that's genetic imprinting. What this means is you get one copy of every chromosome from your father and your mother. One copy will be inherited as inactive, so that entire chromosome will be inactive when you inherit it. Depending on which parent it came from, it can either come from the mom or the dad, they'll also have that as an inactive chromosome. When you get one copy that's inactive and one that's active, that means that genes are expressed as if there's only one allele. Because if you have one active and one inactive, only the allele on the active chromosome will be expressed. If that's a recessive allele, that means that the recessive allele is going to be expressed even if the allele on the inactive one was dominant, but it's not being expressed so you can't see it.
An example of genetic imprinting can be seen in these mice right here; aren't they cute? I'm not a big mouse fan, but anyways these are genetically engineered mice. If you take their genome and sequence it, you're going to get the exact same genome between these mice. They look so different because one of these mice has genetic imprinting, so that means one of their chromosomes is entirely inactive. That results in genetically identical mice looking so different because an entire chromosome has now been inactivated. This results in recessive alleles or a single allele, showing forth when there should have been two, to make them look like each other. So, that is genetic imprinting and different ways of entire chromosomal modifications that can affect gene expression. So, with that, let's now move on.
