Chromosomal Rearrangements: Inversions: Study with Video Lessons, Practice Problems & Examples

An inversion is a chromosomal mutation where a segment of DNA is reversed, maintaining the same genetic information but altering its order. There are two types: paracentric inversions, which do not include the centromere, and pericentric inversions, which do. Inversions can lead to an inversion heterozygote, where one chromosome is normal and the other is inverted. These mutations often arise from an inversion loop during meiosis. Scientists utilize balancer chromosomes, which contain multiple inversions, to prevent crossing over and study genes without altering their stability.

Inversions

Hi. In this video, we are going to be talking about inversions. Okay. So an inversion is when the chromosome keeps all the same genetic information, but the order of it is switched around. So an inversion is when the orientation of a segment is reversed or flipped. Now there's going to be 2 types. We have the paracentric inversion and the pericentric inversion. Now these are really similar, so make sure you don't mix them up. Para and peri, so para- and peri-. Now para- is going to not include the centromere. So, an inversion that happens outside of the centromere. Peri- includes the centromere. So when the segment that's being flipped also includes the centromere. So let's look at the example. So if we had a chromosome here, we'll call it A B C D, this is going to be our normal version, and it has a centromere right here. A paracentric inversion is when the part that's being flipped, which in this case is C and D, there's no centromere. Right? Like, we can cut here and flip this chromosome around and make it normal, and we can cut there and flip it around and make it paracentric. Right? The DNC, we flip it around, there's no centromere involved. Pericentric is when there is a centromere involved. So you can see that the C and the B, they are inverted. Supposed to be A B C D, but instead it's A C B D, but this inversion includes the centromere. So when it flips, we have the centromere included. So, that's the pairing. Now, when this happens, what we usually get is one normal chromosome and then one inverted chromosome, either paracentric or peri-, it doesn't matter. Often we get one normal, one inverted. And what we call that is an inversion heterozygote. Okay. So we have one normal and one inverted. Now this heterozygote, we typically think of actually referring to genes or alleles, and we think heterozygote we see we think this. Right? One dominant and one recessive allele. But, that is not what this means. Right? So, if we look at these above, if we take this and this, or let me just write it again here. So, if we have one chromosome that looks like this, one normal, and one that looks like this, Or, hold on. I wrote that wrong. Let me write my letters correctly. C B D. There we go. So, if we have one normal and one pericentric, we can see that actually in this case, all of the alleles are dominant. Right? We have this homozygous dominant for both chromosomes. Right? So, it's not that the alleles are heterozygous, it's just that the chromosomes are heterozygous. Now, how do inversions happen? Well, a lot of times what happens to them is an inversion loop forms. And an inversion loop literally kind of just looks like a ribbon. And you can see right here during this part, this section and this section can easily flip over. Okay. And, sometimes you will see it drawn like this. Right. And during this section, you can imagine how easily it is for a gene that is here to get put really close to a gene that's here. Okay. So these are examples of inversion loops, and that is what forms and what allows these inversions to take place. Okay. So the twisting of one end pairs with an uninverted one or pairs with an uninverted end, and that ends up causing inversions. Now, if scientists want to study inversions, then what they often do, or they want to study genes in general, not just inversions, just any kind of gene. So a geneticist wants to study that. But they don't want any kind of crossing over. They really don't want to mess with the gene. They really want to keep everything as stable as possible. One of the things that they'll do is use a balancer chromosome. Now, a balancer chromosome is a chromosome that scientists have created. It contains a ton of inversions in it. Just inversion after inversion after inversion. And the reason that it does is because it wants to prevent crossing over. So it suppresses crossing over, so that scientists can study a gene or a chromosome without crossing over occurring. So, balancer chromosomes are inverted chromosomes that scientists have made to help in their studies. So, here is an example of an inversion heterozygote. We can see we have one normal chromosome. We have A, B, C, and D in their correct order with the centromere here. And we have one inversion A, C, B, and D. Now note, this is heterozygotes referring to one normal and one inverted chromosome and has nothing to do with the alleles, which in this case are all homozygous dominants. Okay, So make sure you don't mix that up because students get really confused about that because they hear heterozygous and they're already thinking alleles. In this case, inversion heterozygotes focus on chromosomes. Okay, so with that, let's move on.

A person has a WT chromosome with the following segments. A B C • D E F G H. Which of the following shows how the chromosome would look after an paracentric inversion?

• A. A B C • D E F G H
• B. A B E D • C F G H
• C. A B C • D G H
• D. A B C • D G F E H

A person has a WT chromosome with the following segments. A B C • D E F G H. Which of the following shows how the chromosome would look after a pericentric inversion?

• A. A B C • D E F G H
• B. A B E D • C F G H
• C. A B C • D G H
• D. A B C • D G E F H

Here’s what students ask on this topic:

What is a chromosomal inversion and how does it differ from other types of chromosomal mutations?

A chromosomal inversion is a type of mutation where a segment of DNA within a chromosome is reversed end to end. Unlike deletions or duplications, which remove or add genetic material, inversions maintain the same genetic information but alter its order. There are two types of inversions: paracentric, which do not include the centromere, and pericentric, which do. This rearrangement can affect gene function and regulation, potentially leading to genetic disorders or evolutionary changes. Inversions are unique because they can create inversion heterozygotes, where one chromosome is normal and the other is inverted.

What are the differences between paracentric and pericentric inversions?

Paracentric inversions and pericentric inversions are two types of chromosomal inversions. Paracentric inversions do not include the centromere in the inverted segment, meaning the inversion occurs entirely within one arm of the chromosome. In contrast, pericentric inversions include the centromere, meaning the inversion spans both arms of the chromosome. This distinction is crucial because the involvement of the centromere can affect the chromosome's behavior during cell division, potentially leading to different genetic outcomes and complications.

How do inversion loops form during meiosis and what is their significance?

Inversion loops form during meiosis when homologous chromosomes pair up. If one chromosome has an inversion, it must loop out the inverted segment to align properly with its normal homolog. This looping can facilitate the inversion process and lead to the formation of inversion heterozygotes. The significance of inversion loops lies in their potential to suppress crossing over within the inverted region, which can stabilize certain genetic traits and be useful in genetic studies. However, improper alignment can also lead to genetic abnormalities.

What is an inversion heterozygote and how does it differ from a typical heterozygote?

An inversion heterozygote is an organism that has one normal chromosome and one chromosome with an inversion. This differs from a typical heterozygote, which refers to having two different alleles at a specific gene locus. In the case of inversion heterozygotes, the term 'heterozygote' refers to the structural difference between the chromosomes rather than differences in alleles. This structural heterozygosity can affect genetic stability and crossing over during meiosis, potentially leading to unique genetic outcomes.

What are balancer chromosomes and how are they used in genetic research?

Balancer chromosomes are specially engineered chromosomes that contain multiple inversions. These inversions suppress crossing over during meiosis, which helps maintain genetic stability. Scientists use balancer chromosomes to study specific genes or genetic traits without the complications of recombination. By preventing crossing over, balancer chromosomes ensure that the genetic material remains unchanged, allowing researchers to observe the effects of specific genes or mutations more accurately. This tool is particularly valuable in model organisms like Drosophila melanogaster.