Chromosomal Mutations: Aneuploidy: Study with Video Lessons, Practice Problems & Examples

Chromosomal mutations involve changes in chromosome structure or copy number, primarily categorized as aneuploidy and euploidy. Aneuploidy, such as trisomy (2n + 1) and monosomy (2n - 1), results from nondisjunction during cell division, leading to conditions like Down syndrome and Turner syndrome. Gene balance is crucial; aneuploidy disrupts this balance, causing severe phenotypes due to altered gene dosage. In contrast, polyploidy maintains balance across all chromosomes, resulting in less severe abnormalities. Understanding these concepts is essential for grasping genetic disorders and their implications.

Aneuploidy

So remember, chromosomal mutations refer to alteration in chromosome structure or the number of chromosomal copies. What we're discussing now is chromosomal copies. There are two types of chromosomal mutations, and we'll reference the copy number, and this is aberrant euploidy. And in this video, which we're not going to talk about, but this is working with the entire set of chromosomes. So if an organism has 40 sets of chromosomes, then there's going to be abnormal numbers of all 40. But what we're going to focus on is aneuploidy, and this changes the part of a single or a few chromosomes. So if the organism has 40 chromosome sets, then in an aneuploidy, only one or two of those chromosomes are going to have abnormal numbers. Aneuploidy refers to organisms found in some chromosomes, but not all of them, so not the whole set. And there are many different types. You can have trisomic, so I wrote 2n+1, so this is for diploid organisms. Right? So if you have 2n, that indicates diploid, but then you have one extra chromosome, plus one. An example of this is Down syndrome, right, where you have an extra chromosome of number 21, or Klinefelter's in humans, where you have that extra X chromosome, sex chromosome. Another term is monosomic, again, if it's diploid, 2n is diploid, but now it's minus one, so it has one less chromosome than a normal diploid organism. This is Turner Syndrome, which only has one X instead of two. If you're not familiar with these syndromes yet, don't worry about it. These are just examples if you are familiar with them. The really important thing to know is the terms and these numbers and what those numbers mean. Then you have nolesomic, which is 2n minus two for a diploid organism and can also refer to this as disomic, which is a haploid organism. Notice this is only n and not 2n's. So that suggests haploid minus one. So it's a haploid minus one chromosome. So it's actually lost an entire one of those chromosomes. Here's an example of what these look like: we have trisomic. You can see there are two diploids, which this organism is normally diploid, but they have this plus one, so they have this extra chromosome here. Here you have monosomic, so again, your diploid organisms, but it's minus one, so you have you're missing this chromosome here. Nolesomic is 2n-2, which might be harder to detect because here you have your two diploids. But because we're dealing with these organisms here, we know there's supposed to be a blue chromosome here, but there's not because both are missing, minus two. Disomic, now we're working with haploid. So, here are our normal half haploids, and we have plus one. So here is our plus one. So those are the different types of vocab words you're going to have to memorize. Now, here's nondisjunction. So nondisjunction is the failure of chromosomes to separate properly during division, and this is the cause of aneuploidy. It can occur in meiosis, which is most common during the formation of the gametes, but it can also occur in mitosis as well. If this mitosis is happening very early in development, like within the first few hours or days of development, this mitosis is occurring, and it just divides the chromosomes wrong. So, nondisjunction is the cause of these up here. And that's chromosomes not separating properly during or during division. Now, any aneuploid, so the ones we're talking about now, ones with only one chromosome added or missing, These are typically more abnormal than the type we talked about before, which are polyploids, and these are ones with the entire chromosome set. So all 40 chromosomes, for instance, have an extra copy. Whereas, aneuploids would be there, it'd still be 40, but it would just be 41. So why is that? Why is only having one chromosome extra so much worse than having 40 extra copies? Well, this actually has to do with a concept called gene balance. And this is the ratio of genes on one chromosome to genes on the other chromosome. So, if you normally have two chromosomes, and you have like three genes on each, And you have multiple chromosome sets. Right? Here we go. Now, it is less balanced to just get one extra copy here. Right? So now, I have nine copies of all of these genes, whereas I only have six of these. But if I got an extra chromosome here, then I have nine copies of these. And so, the balance of the number of genes on each chromosome is actually really important. So you want to have that balance. So the more so if you only have one extra copy, then all of your chromosomes have the same number of alleles, except for that one chromosome which has been added, and that gets you extra, and that really throws off your gene balance. Whereas, if you just have an extra copy of every chromosome, then the number of genes present in that cell is not the same, but it's equal across the chromosomes. And so you want that equal across the chromosome, even if that means having an extra copy of every chromosome or a less or missing a copy of every chromosome, because you want that gene balance. Whereas, if you have aneuploidy, where you only have one set, say, chromosome 9 that has an extra copy. Well, chromosome 9 now has extra alleles that none of the other chromosomes have, and that causes a more severe phenotype. And this has to do with gene dosage, and that's the relation between number of genes and the amount of gene product. So if, say we go back to, let me erase these circles here so we can see this better. So we go back to our original example here. We start out with two, we start out with the diploid organisms, these red chromosomes and the black chromosomes, and they each have the same number of alleles. Right? Same number of genes. Well, this means, say, that six gene copies are going to be produced, and six of this one. So we'll say chromosome number one and chromosome number two. So the red and the black chromosome will each produce six copies. But if you get an extra chromosome. Right? So if you get here, then that's going to mean how many are produced? It's going to mean nine are produced. And that means that you're going to have, more product than you should compared to every other gene. Right? Because six copies are still going to be produced down here. So you're going to get more of every gene on this chromosome, and that's going to give you a higher gene dosage.

Which of the following chromosomal mutations increases the amount of genetic material from only some chromosomes?

• A. Aberrant Euploidy
• B. Aneuploidy
• C. Monoploidy
• D. Tetraploidy

True or False:Aneuploids are more abnormal that polyploids

• A. True
• B. False

A species has 2n = 20. How many chromosomes will be found per mutant cell in an monosomic organism.

• A. 10
• B. 19
• C. 20
• D. 21

Here’s what students ask on this topic:

What is aneuploidy and how does it differ from euploidy?

Aneuploidy refers to the presence of an abnormal number of chromosomes in a cell, such as having one extra (trisomy) or one missing (monosomy) chromosome. This results from nondisjunction during cell division. Euploidy, on the other hand, involves changes in the entire set of chromosomes, such as having an extra set (triploidy) or missing a set. While aneuploidy affects only specific chromosomes, leading to severe phenotypes due to gene dosage imbalance, euploidy maintains a balance across all chromosomes, resulting in less severe abnormalities.

What causes aneuploidy and what are some examples of conditions resulting from it?

Aneuploidy is caused by nondisjunction, the failure of chromosomes to separate properly during cell division, either in meiosis or mitosis. This leads to cells with an abnormal number of chromosomes. Examples of conditions resulting from aneuploidy include Down syndrome (trisomy 21), where there is an extra chromosome 21, and Turner syndrome (monosomy X), where there is only one X chromosome instead of two. These conditions result in various developmental and physical abnormalities due to the imbalance in gene dosage.

How does gene dosage affect the severity of aneuploidy?

Gene dosage refers to the number of copies of a gene present in a cell and the amount of gene product produced. In aneuploidy, the imbalance in gene dosage occurs because one chromosome has extra or missing genes compared to the others. This disrupts the gene balance, leading to severe phenotypes. For example, in trisomy, the extra chromosome produces more gene products than normal, causing developmental issues. In contrast, polyploidy, where all chromosomes have extra copies, maintains gene balance across the genome, resulting in less severe abnormalities.

What is nondisjunction and how does it lead to aneuploidy?

Nondisjunction is the failure of chromosomes to separate properly during cell division, either in meiosis or mitosis. This results in gametes or cells with an abnormal number of chromosomes. When such gametes are involved in fertilization, they produce zygotes with aneuploidy. For example, if a gamete with an extra chromosome 21 fuses with a normal gamete, the resulting zygote will have trisomy 21, leading to Down syndrome. Nondisjunction can occur at various stages of cell division, but it is most common during meiosis.

Why is aneuploidy more severe than polyploidy?

Aneuploidy is more severe than polyploidy because it disrupts gene balance. In aneuploidy, only specific chromosomes have extra or missing copies, leading to an imbalance in gene dosage. This causes severe phenotypes due to the disproportionate amount of gene products. In contrast, polyploidy involves changes in the entire set of chromosomes, maintaining a balance across all chromosomes. Although polyploidy results in a larger number of gene copies, the relative balance between different chromosomes is preserved, leading to less severe abnormalities.