Chromosomal Rearrangements: Translocations - Online Tutor, Practice Problems & Exam Prep

Translocations involve the movement of chromosomal segments between chromosomes, with two main types: reciprocal and Robertsonian. Reciprocal translocations can lead to viable or non-viable gametes depending on how chromosomes segregate during meiosis. In contrast, Robertsonian translocations, often linked to familial Down syndrome, involve the fusion of acrocentric chromosomes, resulting in a loss of short arms but maintaining genetic integrity. This can lead to trisomy 21 in offspring, highlighting the importance of understanding chromosomal mutations in genetics.

Reciprocal Translocation

So translocations pretty much describe when a chromosomal segment is moved to a different chromosome. This can be, they can actually just switch parts, so two completely separate chromosomes can switch parts, or just one can move to the other. But it's essentially that chromosome being moved to a different location or translocation. Now there are two types. The first type we're going to talk about is reciprocal translocations. And these are when two chromosomes trade acentric fragments. First, do you remember what acentric means? Right? It means without a centromere. So this is a fragment that does not contain a centromere in it. And there are three ways that these translocations are sorted into gametes because the reason we study these is that they're inherited or they can be. And so, when we think about translocations happening, it's important to understand how that happens, but that's easy enough to conceptualize. Right? It's a portion of a chromosome without a centromere is switched with another portion of a chromosome without a centromere. But that doesn't necessarily do anything unless it's inherited, and so when we talk about these chromosomal aberrations and mutations, we have to talk about them in terms of gametes, of how they're inherited. So whenever this translocation occurs, the way that they're sorted into gametes can be classified in three ways. So when I talk about these, I'm going to talk about the normal chromosomes and the translocated chromosomes. So the first one, if it doesn't make sense, I'm going to go over a really detailed image of this, so just hold on.

We have adjacent one segregation, and this is when the gametes formed have one normal and one translocated. And notice the numbers here because you start out with two chromosomes. Here we go. And if these two change spots, then you have a normal one, a translocated one, a normal two, and a translocated two. Right? Because these two are the ones that underwent translocation. And the numbers actually matter because in one situation you're referring to homologous chromosomes, and in another, you're referring to nonhomologous chromosomes. And if it doesn't make sense, there's an image to explain it, but just sort of know that these are how the niamites form in adjacent one segregation, and these are inviable. Adjacent two segregation is the same thing, but notice the numbers here are different. You have one normal and one translocation, but these are homologous, whereas these are not homologous, and but they're both inviable because you don't have one full set of chromosomes. And then you have alternative segregation, and these you have one set of gametes that are both translocated and one set that are normal, and so the gametes are all viable. Let's walk through an example of this.

So like I said, we start out with chromosomes. We have normal one, normal two, and a type of translocation has occurred here. Right here and here. So now we have a translocated one where this is a, b, g, and h. We have a translocated two with e, f, c, and d. And these are marked by their the letters. So the letters are representing different genes in this case. We have one chromosome that has a, b, c, or d. We have another that has e, f, g, and h, and these are two separate chromosomes. These are not homologous. Right? Because they have different genes on them. Anything with an a, b, c, or d is homologous, but these are actually two separate chromosomes, chromosome one and chromosome two. And so there's a homologous pair. Right? So if we were drawing these, like how they how we normally see them, you would see them like this. And I guess this is supposed to be red. Hold on. E f g h e f g h. So these are the homologous pairs, each one of these. But the black and the red are two different chromosomes. But in this case, a translocation has occurred here. Right? These two have switched over, and these two have switched over. So now you get a normal one, n 2, two normal chromosomes. Right? This one here and this one here. And you have two translocated ones, the ones that translocated here, and the ones that translocated here. And this is important. So now, when you get the gamete, or you get the cell, essentially, what happens is this is what you have. You have one normal, this is an n one, and you have two normals n 2, and you have your translocated t 1 and t 2.

Robertsonian Translocations

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.

Which of the following represents the chromosomal segregation into gametes after a reciprocal translocation caused by adjacent-1 segregation? N=Normal chromosome T = Tanslocated chromosome

• A. (N2 T1) and (N1 T2)
• B. (N1 T1) and (N2 T2)
• C. (T1 T2) and (N1 N2)

Which of the following ways reciprocal translocated chromosomes are sorted produces viable gametes?

• A. Adjacent-1 segregation
• B. Adjacent-2 segregation
• C. Alternative segregation

An individual heterozygous for a reciprocal translocation has the following chromosomes. Which chromosomes do the gametes receive after alternative segregation? 

A B • C D E F G                        J K • L M N O P 

A B • C D N O P                       J K • L M E F G 

• A. A B • C D E F G and J K • L M E F G
• B. A B • C D N O P and J K • L M E F G
• C. A B • C D N O P and J K • L M N O P

An individual heterozygous for a reciprocal translocation has the following chromosomes. Which chromosomes do the gametes receive after adjacent-1 segregation? 

A B • C D E F G                J K • L M N O P 

A B • C D N O P               J K • L M E F G 

• A. A B • C D E F G and J K • L M E F G
• B. A B • C D N O P and J K • L M E F G
• C. A B • C D N O P and A B • C D E F G

An individual heterozygous for a reciprocal translocation has the following chromosomes. Which chromosomes do the gametes receive after adjacent-2 segregation? 

A B • C D E F G             J K • L M N O P 

A B • C D N O P            J K • L M E F G 

• A. A B • C D E F G and J K • L M E F G
• B. A B • C D N O P and J K • L M E F G
• C. A B • C D N O P and A B • C D E F G

How many chromosomes does a person who is a carrier for familial down syndrome caused by a robertsonian translocation have?

• A. 45
• B. 46
• C. 47
• D. 48

Here’s what students ask on this topic:

What are the main types of chromosomal translocations?

There are two main types of chromosomal translocations: reciprocal and Robertsonian. Reciprocal translocations involve the exchange of segments between two non-homologous chromosomes, often resulting in viable or non-viable gametes depending on how chromosomes segregate during meiosis. Robertsonian translocations occur when the short arms of two acrocentric chromosomes break off and the long arms fuse to form a single chromosome. This type is often associated with familial Down syndrome, where the fusion can lead to trisomy 21 in offspring.

How do reciprocal translocations affect gamete viability?

Reciprocal translocations can affect gamete viability based on how chromosomes segregate during meiosis. There are three main segregation patterns: adjacent-1, adjacent-2, and alternate. Adjacent-1 and adjacent-2 segregations result in gametes with unbalanced genetic material, leading to non-viable offspring. In contrast, alternate segregation produces gametes with balanced genetic material, resulting in viable offspring. The key factor is whether the gametes contain a complete set of genes, despite the translocation.

What is a Robertsonian translocation and how does it relate to Down syndrome?

A Robertsonian translocation involves the fusion of the long arms of two acrocentric chromosomes, with the short arms being lost. This type of translocation is linked to familial Down syndrome, a rare form of Down syndrome. In this condition, a parent carries a balanced Robertsonian translocation, which can lead to an unbalanced translocation in the offspring. This results in trisomy 21, where the child has three copies of chromosome 21, causing Down syndrome.

What is the difference between balanced and unbalanced Robertsonian translocations?

In a balanced Robertsonian translocation, the individual has a normal phenotype because they have the correct amount of genetic material, despite the structural rearrangement of chromosomes. This person typically has 45 chromosomes but appears normal. In an unbalanced Robertsonian translocation, there is an abnormal amount of genetic material, leading to phenotypic abnormalities. For example, in familial Down syndrome, the child inherits an unbalanced translocation, resulting in trisomy 21 and the associated symptoms of Down syndrome.

How do chromosomal translocations contribute to genetic disorders?

Chromosomal translocations can contribute to genetic disorders by disrupting the normal arrangement and function of genes. In reciprocal translocations, the exchange of segments can lead to unbalanced gametes, causing developmental issues or miscarriages. Robertsonian translocations can result in conditions like familial Down syndrome, where the fusion of chromosomes leads to an extra copy of chromosome 21. These disruptions can affect gene expression and lead to various genetic disorders, highlighting the importance of understanding chromosomal mutations in genetics.