Okay. So now let's talk about the second type of translocation, and this is a Robertsonian translocation. This is a little bit different and it's actually the source of familial Down syndrome, which is an interesting case of Down syndrome. It's very rare, but it does exist. Here's the definition of it; it's a little confusing, but hopefully, the image will make it clearer. This occurs when there are breaks at 2 short arms of 2 non-homologous, acrocentric chromosomes. Remember, the 2 short arms here are breaking off, and that leaves 2 long arms, forming a single chromosome. Instead of having these, what you end up with is 2 long arms and 2 short arms from a translocation here. Now, there are 2 forms of this. There's the balanced form and typically in familial examples of this, this is going to be the parent. The balanced form has this translocation, so it looks like this but results in no phenotypic problems. This parent does not have Down syndrome; they are just carrying that translocation, which allows it to happen. This is because you have both copies. You have the long arms of both copies of chromosomes, and even if the structure isn't correct, all the genes are there. So there are really no problems. In the unbalanced form, which is usually the child in this familial Down syndrome case that I've been talking about, there's a chromosomal imbalance. You have this and now you have too many copies of the same chromosome.

Let's go through this example. Let's first talk about the normal case. Normally, you have chromosome 21 and chromosome 14, so this is going to be the example of inherited or familial Down syndrome. Normally, when a normal person has chromosome 21 and chromosome 14 and they create a gamete, they get one of each. So you have one copy of chromosome 21 and one copy of chromosome 14, and this is the gamete. That's what happened. Now I wrote it in gray and blue. It could have been green and black. Doesn't really matter. But essentially, you only get one copy of chromosome 21 and one copy of chromosome 14. That's how meiosis works. It replicates, divides, and divides again. So you're haploid, so this is haploid because it contains one copy of chromosome 21 and one copy of chromosome 14. This is normal. But what happens in a Robertsonian translocation that's balanced? It appears normal, but what has happened is you have one copy of chromosome 21 and one copy of chromosome 14, but there's been a Robertsonian translocation between the other two forms of chromosome 21 and chromosome 14. So the green and the black are now on the same chromosome, so you have 2 long arms of chromosomes 14 and 21 and 2 short arms of chromosomes 14 and 21. This person appears normal because they have the long and short arm of each chromosome 21 and chromosome 14. They have the exact same amount of genetic material as this normal person over here, but the structures are all messed up and different. This results in 45 chromosomes. The reason we say this is 45 instead of 46 is that the two short arms are so small and they're acrocentric. So they don't have a centromere attached to them. That means during division, they're actually just lost. So you end up with one copy of chromosome 21, one full copy of chromosome 14, and just the long arms of chromosomes 14 and 21 as the second copy. But because these small arms here are so small, they don't really contain that much genetic information, and the genetic information they do contain is actually repeated elsewhere in the genome. So it's kind of just like this region that's not necessary for life. Obviously, it's important, but it's not necessary. And so if these people appear normal, but they only have 45 chromosomes, but they contain enough of the genetic material still to appear normal even though they've lost the short arms of chromosomes 14 and 21.

So when this person creates gametes, their gametes look a lot different than when the normal person creates gametes. Here are the different gametes they can form. They can form just exactly like the other normal non-Robertsonian translocation person did. They can contain a gamete that contains chromosomes 21 and 14, and that's here. And this is very normal. It looks exactly the same, and they have the ability to produce that because you get one copy of chromosome 21 and one of chromosome 14. There's another option where you get one copy of chromosome 14 and chromosome 21, but the Robertsonian translocation actually contains a copy of chromosome 14 and chromosome 21. So you just get that Robertsonian translocation. Because you have chromosome 21, chromosome 14, but this one also is chromosome 14, chromosome 21, so it has the same amount of genetic material. Then you have this option where you get chromosome 21 normally, but then the chromosome 14 chosen is actually this one. And so now you have 2 copies of this chromosome 21 and one copy of chromosome 14. And this could happen with chromosome 14 too. It could happen either way. But I've shown you this way because this is what ends up causing Down syndrome. Or what could happen is you could just get chromosome 14 or just chromosome 21. And this is more rare because this means that something else has happened for you not to get chromosome 21, but it is possible. So these are the 4 gamete possibilities. So if you were to mate or essentially fertilize, you have this one gamete coming together with this one, so you get 2 copies of chromosome 21 and 2 copies of chromosome 14, and this offspring is going to appear normal because they have the exact amount of genetic material that they should. Then you have this mating where you get chromosome 21, chromosome 14, and you get this Robertsonian translocation of chromosome 14 and chromosome 21. What you get is chromosome 21, chromosome 14, chromosome 14, and chromosome 21. And this, because it has the same amount of information, is normal. It also appears normal, but it's a carrier for that Down syndrome because it has this, and this makes it a carrier. Now, you can have this combination where you have 21, 14 from this parent, and you also get 21, 14, 21 from this parent. And that means that you have 3 copies of chromosome 21 and 2 of chromosome 14, and that is Down syndrome. Down syndrome is trisomy 21. So now you have 3 copies of chromosome 21 because of the Robertsonian translocation, and this means that you inherited Down syndrome from a parent that didn't have Down syndrome. And then finally, you have this rare case, where you have 21, 14 and then just 14, and what you end up with is 21, 14, 14 or 21, 21, 14, and either way, you're missing part of the information, and this is non-viable. And so these are the 4 offspring, and this is how this happens. I want to go back for a second and explain how this would happen. So remember that when during meiosis, you have 21, 14, 21, and 14. Now these are going to replicate. And then they're each going to go to 1 gamete. So in this case, where you have 21 and 14, 21 goes to 1 gamete, then this means that you have an extra 14 that's left by itself. And that's how you get this gamete here. So it does happen. So I just wanted to make sure that was clear because I'm not sure I explained that very well. But anyways, this is how a Robertsonian translocation Robertsonian translocation means that you have 2 long arms of 2 different chromosomes now attached to each other, which is here. And that is how it can lead to the inheritance of these trisomes, especially like Down syndrome. Now, familial Down syndrome is about 5% of all Down syndrome cases, so it's very rare, but it does exist. Right? 5% is a significant number for these types of cases. And so Robertsonian translocations do happen in humans and do cause diseases fairly commonly for how rare you would expect them to be. So that's Robertsonian translocation. With that, let's now move on.