Genetic Variation During Meiosis: Videos & Practice Problems

Meiosis generates genetic diversity through two key processes: crossing over and independent assortment. Crossing over occurs during prophase 1, where homologous chromosomes exchange DNA segments, forming non-identical sister chromatids. Independent assortment happens in metaphase 1, allowing homologous pairs to align randomly, creating numerous genetic combinations. The formula for calculating combinations is $2^n$, where n is the haploid number of chromosomes. Errors in these processes, such as nondisjunction, can lead to aneuploidy, resulting in genetic disorders like Down syndrome.

Genetic Variation During Meiosis

In this video, we're going to talk about genetic variation during meiosis. And so recall from our previous lesson videos that meiosis results in 4 haploid cells that are all genetically different from one another, and so meiosis creates genetic variation. And so it turns out that meiosis actually creates genetic diversity or genetic variation via two main events. And so the first main event that creates genetic diversity during meiosis is crossing over. And the second main event that creates genetic diversity during meiosis is independent assortment. Now in this video, we're going to focus mainly on the process of crossing over. But later in our course, in a different video, we'll talk about the process of independent assortment. Now crossing over is the process in which pairs of homologous chromosomes actually exchange their genetic material, essentially swapping segments of DNA. And so you can see that during crossing over, homologous chromosomes are going to cross over and swap or exchange their DNA. Now this process of crossing over is ultimately going to form non identical sister chromatids. And crossing over occurs specifically during prophase 1 of meiosis 1. And so it's important to note that crossing over, this process that creates genetic diversity, only occurs during meiosis 1, specifically prophase 1 of meiosis 1. But crossing over does not occur during mitosis, which recall mitosis creates genetically identical, not genetically diverse cells.

Now when your professors are explaining, crossing over and when your textbooks describe crossing over, they're likely going to mention these other terms called synapsis and chiasma. Synapsis is the process where homologous pairs of chromosomes actually align themselves and align their DNA sequences at similar alleles to prompt crossing over. And so synapsis is really just this alignment of the DNA sequences between homologous chromosomes. And chiasma is really just the site of crossing over, the attachment site between 2 homologous chromosomes, allowing these homologous chromosomes to cross over their genetic material, exchange genetic material. But the chiasma is really just the site of crossing over. And so, within this word chiasma, you can see the root chi, which is actually a Greek letter that resembles an x. And so what you'll notice is that the chiasma really does resemble the formation of an x, and it represents the site of crossing over.

And so if we take a look at our image down below, notice that we're showing you one replicated chromosome over here that has 2 identical sister chromatids, and we're showing you another replicated chromosome over here that also has 2 identical sister chromatids. And notice that these, replicated chromosomes are very, very similar in size and shape. They carry the same genes, but not necessarily the same versions of the genes or the same alleles. And so that makes these 2 chromosomes here homologous chromosomes. And, again, you can see here that for this particular gene a, they have the same version of the gene, the capital A version of this gene. But notice that for gene b, the blue chromosome has the capital B version of the gene, whereas the red chromosome has the lowercase b version of the gene. And so homologous chromosomes are going to be similar in size, shape, carry the same genes, but not necessarily the same versions of genes.

Now what's important to note is that, again, this sister chromatid is exactly identical to this other sister chromatid, and, this is going to be the case before crossing over takes place. And the same goes for this sister chromatid here on this one and this one over here. They are identical to each other before crossing over takes place. And so, again, the process of synapsis is where the homologous pairs of chromosomes are going to align their DNA sequences at similar alleles. And so you can see that synapsis is shown here in the background here, with the yellow background, where it's just showing the alignment here of the chromosomes to prompt crossing over to take place. And then notice over here in the middle image that we have this overlapping, region of the homologous chromosomes called the chiasma. And this represents, again, the site of crossing over, where crossing over is going to take place. And so notice that at the very very end of crossing over that we have a chromosome that is mostly blue but has a little bit of the pink chromosome because there has been an exchange of the genetic material. And then notice over here we have a chromosome that is mostly red, but has a little bit of blue, again, because there has been an exchange of the genetic material in the process of crossing over. And so this process of crossing over takes identical sister chromatids and ends up converting them into non identical sister chromatids. And so notice that this chromatid here is no longer identical to the chromatid that's over here because there has been this exchange of genetic material between these pairs of homologous chromosomes. And so crossing over here, we're showing crossing over at one particular gene, but crossing over can actually occur between 100 or thousands of genes, between homologous chromosomes, and crossing over occurs randomly with every single event of meiosis. And so this is going to create an enormous amount of genetic diversity, swapping little bits and pieces of chromosomes, between homologous pairs of chromosomes. And so again, crossing over is one of the main events that creates genetic diversity during meiosis. And later in our course, we're gonna talk about the second event that creates genetic diversity during meiosis, independent assortment. And so that concludes this video, and I'll see you all in our next one.

Which of the following processes occurs when homologous chromosomes cross over in meiosis I?

• A. Two sister chromatids get tangled, resulting in one re-sequencing its DNA.
• B. Two sister chromatids exchange identical pieces of DNA.
• C. Maternal alleles are "corrected" to be like paternal alleles and vice versa.
• D. Corresponding segments of non-sister chromatids from homologous chromosomes are exchanged.

Answer: (D)

Crossing over involves each of the following EXCEPT:

• A. The transfer of DNA between two non-sister chromatids.
• B. The transfer of DNA between two sister chromatids.
• C. The formation of a synaptonemal complex.
• D. The alignment of homologous chromosomes.
• E. All of the above are involved in crossing over.

Answer: (B)

Independent Assortment

So now that we've covered crossing over in our last lesson video, the first main event that creates genetic diversity during meiosis. In this video, we're going to talk about the second main event that creates genetic diversity during meiosis, and that is independent assortment. Independent assortment occurs specifically during metaphase 1 of meiosis 1. During metaphase, the chromosomes align themselves in the middle of the cell. Specifically during metaphase 1 of meiosis 1, the chromosomes align themselves in homologous pairs in 2 rows on the metaphase plate. Independent assortment refers to the ability of these pairs of homologous chromosomes to independently and randomly align themselves on the metaphase plate during metaphase 1 of meiosis 1. This results in an enormous amount of possible genetic combinations during meiosis. Independent assortment, along with crossing over, is part of the reason why meiosis results in 4 haploid cells that are all genetically different from one another. Independent assortment helps to create more possible genetic combinations, making it very difficult to get 2 cells that are genetically identical during the process of meiosis.

It's actually possible to calculate the number of combinations due to independent assortment by using the equation 2n, where n represents the haploid number of chromosomes in a cell. We'll be able to get some practice applying this equation here to calculate the number of combinations due to independent assortment when we get to our image down below.

If we take a look at our image down below, notice that we're showing 2 possibilities for independent assortment as it occurs during meiosis. Notice that possibility number 1 is over here generating combination 1 and combination 2, and possibility number 2 is over here generating combination 3 and combination 4. This first row right here represents metaphase 1 of meiosis 1. During metaphase 1 of meiosis 1, homologous chromosomes are going to pair up and align themselves in 2 rows on the metaphase plate. You can see that one possibility for these chromosomes to align themselves is that all of the maternal chromosomes inherited from the mother line up on one side, and all of the paternal chromosomes inherited from the father line up on the other side. This is one possibility if these homologous chromosomes independently and randomly align during metaphase 1. Another possibility for how these chromosomes can independently and randomly align is shown here, where they're not lined up on one side. Each of these possibilities is going to result in a different genetic combination.

In the representation of metaphase 2 of meiosis 2, the chromosomes are aligning in a single file line, like they do in mitosis. This results in different genetic combinations. Each possibility in this phase represents different genetic combinations and they all resulted from independent assortment, how these chromosomes can independently and randomly align themselves during metaphase 1 of meiosis 1.

Ultimately, independent assortment, the random alignment of homologous pairs of chromosomes, is going to create a lot of possible genetic combinations. In human cells, there are actually 46 chromosomes which would create significantly more possibilities. You can again calculate the number of possible combinations by using the formula 2n. In this example, if the haploid number of chromosomes is 2, then 22 which is 4, represents the number of possible genetic combinations. This concludes our brief introduction to independent assortment, how it occurs during metaphase 1, and how it consists of homologous chromosomes independently and randomly aligning to create an enormous amount of possible genetic combinations during meiosis. We'll be able to get some practice applying these concepts as we move forward in our course. So, I'll see you all in our next video.

Genetic Variation During Meiosis Example 1

Alright. So here we have an example problem that says for a species with a haploid number of 23 chromosomes, which would be humans, humans have a haploid number of 23 chromosomes. How many combinations of maternal and paternal chromosomes are possible for the gametes based on the independent assortment of chromosomes during meiosis? And we've got these 4 potential answer options down below. So really what this problem is trying to ask us is how many possible genetic combinations are there if the haploid number of chromosomes is 23? And so what we need to recall from our last lesson video is that there is an equation to calculate the number of possible genetic combinations due to independent assortment. And so that equation is right here. The number of possible genetic combinations due to independent assortment is equal to 2 raised to the power of n, where n is equal to the haploid number of chromosomes. And so for this equation, all we need to do is take 2 and raise it to the power of n23, again, the haploid number of chromosomes, and we're told that the haploid number of chromosomes is 23. So 223. And when you type this into your calculator, 223, you'll get an answer of 8,388,608 possible genetic combinations. And this is a lot of possible genetic combinations when it comes to just independent assortment on its own, and this is every single time meiosis occurs. And so the likelihood that these 4 gamete cells that result are going to have the same genetic combination is really unlikely if there are 8,388,608 possible genetic combinations. And so of course, when we look at the answer options, option d says about 8,000,000. And of course, that is going to be the closest one to the correct answer here. So option d here is going to be the correct answer to this example. And that concludes this example. So I'll see you all in our next video.

How many genetically unique gametes can be created in an organism with 4 chromosomes?

• A. 256.
• B. 23.
• C. 16.
• D. 1 million.
• E. 4.

Answer: (C)

During which of the following processes does independent assortment of chromosomes occur?

• A. In meiosis I only.
• B. In meiosis II only.
• C. In mitosis and meiosis I.
• D. In mitosis and meiosis II.
• E. In meiosis I and meiosis II.

Answer: (A)

Independent assortment of chromosomes is a result of which of the following processes?

• A. The random way each pair of homologous chromosomes lines up at the metaphase plate.
• B. The random combinations of eggs and sperm during fertilization.
• C. The random distribution of the sister chromatids into the two daughter cells.
• D. The diverse combination of alleles that may be found within any given chromosome.

Answer: (A)

Nondisjunction

In this video, we're going to introduce nondisjunction. And so nondisjunction is an error that can occur during either meiosis 1 or meiosis 2, when chromosomes fail to separate from each other, which can result in aneuploid cells. Now aneuploid cells are cells that are going to contain either too many or too few chromosomes. And, again, aneuploid cells can be the result of a nondisjunction. Now nondisjunctions resulting in aneuploid cells can lead to genetic disorders. For example, trisomy 21 or Down syndrome, or it could even lead to cell death. And so let's take a look at our image down below which is showing you nondisjunction during anaphase 1 of meiosis 1 or, anaphase 2 of meiosis 2. And so notice, here at the top we're showing you a cell undergoing meiosis 1, and here at the bottom we're showing you a cell undergoing meiosis 2. And again, nondisjunction can occur either during meiosis 1 or meiosis 2 when these chromosomes are going to fail to separate properly. And so here what you can see is during metaphase 1 of meiosis 1, the homologous chromosomes are going to, randomly and, independently align themselves in 2 rows on the metaphase plate. And so what you'll see here is in this image, we're showing you the nondisjunction of a small blue chromosome. And so this small blue chromosome, if anaphase were to occur properly, it should shift over to this side of the cell. However, if a nondisjunction occurs of the small blue chromosome, notice that the small blue chromosome is actually going to go to the same side as its homologous chromosome pair. And so this side over here is going to be missing the homologous chromosome, whereas this side over here is going to have an additional homologous chromosome. And so what you'll notice is that this cell here, which would represent the cell here at the top, is going to have too many chromosomes. And so if you have too many chromosomes, that is going to be a type of aneuploid cell. And so notice that this cell over here only has one replicated chromosome, so it has too few chromosomes. And so you can see something similar occurring with meiosis 2 down below. And again in meiosis 2, the chromosomes all line up in 1 single file row. And this time, you can see that there's nondisjunction of the large red sister chromatids. And so, usually during, meiosis 2, this sister chromatid would go, to the bottom and the other sister chromatid would go to the top. But if there is nondisjunction, then notice that this bottom sister chromatid here is not going to separate. It's going to fail to separate. And so, notice that both sister chromatids would end up over in the same cell, and this cell would be lacking a sister chromatid. And so again, you would end up getting a cell that has too many chromosomes and a cell that has too few chromosomes. And so, again, if a cell has either too many or too few chromosomes, they're referred to as aneuploid cells. And again, this nondisjunction can lead to genetic disorders such as, again, trisomy 21 or Down syndrome. And trisomy 21, as you can see here, tri is a root that means 3, and, trisomy 21 is referring to having 3 copies of chromosome 21. And so if we take a look at this karyotype that we have over here on the right, notice that there are 2 pairs of every single chromosome except for this pair right here. And this pair, notice that there are actually 3 copies of this chromosome, chromosome 21, and that leads to, the genetic disorder trisomy 21 which is Down syndrome. And so we're showing you a baby here, cute baby with Down syndrome. And so, what you'll notice here is that, we have finished our lesson here introducing nondisjunction and how nondisjunction is an error that can occur during meiosis, where chromosomes or sister chromatids failed to separate resulting in aneuploid cells with either too many or too few chromosomes. And so we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Non-disjunction means that chromosomes:

• A. Do not cross over during synapsis.
• B. Do not replicate during interphase.
• C. Have serious mutations.
• D. Do not separate correctly from one another during anaphase.

Answer: (D)

During meiosis, if nondisjunction occurs during anaphase II (shown below in cell #3) what are the possible gametes that can be produced from cell #3?

• A. A only.
• B. B only.
• C. C only.
• D. A & C.
• E. B & C
• F. A & B
• G. All three.

Answer: (D)

________ cells have an extra or missing chromosomes after meiosis due to nondisjunction during Meiosis I.

• A. Somatic cells.
• B. Gametes.
• C. Diploid Cells.
• D. Aneuploid Cells.
• E. Haploid Cells.
• F. Sperm Cells.

Answer: (D)

Here’s what students ask on this topic:

What is crossing over and when does it occur during meiosis?

Crossing over is a process during meiosis where homologous chromosomes exchange segments of their genetic material. This exchange results in non-identical sister chromatids, contributing to genetic diversity. Crossing over specifically occurs during prophase 1 of meiosis 1. During this phase, homologous chromosomes pair up and align their DNA sequences at similar alleles, a process known as synapsis. The actual site of crossing over is called the chiasma, where the chromosomes physically exchange segments. This process is crucial for creating genetic variation in the resulting gametes.

How does independent assortment contribute to genetic variation?

Independent assortment is a process during meiosis that contributes to genetic variation by allowing homologous chromosomes to align randomly at the metaphase plate during metaphase 1. This random alignment results in numerous possible genetic combinations in the resulting gametes. The number of possible combinations can be calculated using the equation $2^n$, where n is the haploid number of chromosomes. For example, in humans with 23 pairs of chromosomes, the number of possible combinations is $2^{23}$, which equals 8,388,608 possible combinations, significantly contributing to genetic diversity.

What is nondisjunction and what are its consequences?

Nondisjunction is an error during meiosis where chromosomes fail to separate properly, occurring either in meiosis 1 or meiosis 2. This failure results in aneuploid cells, which have either too many or too few chromosomes. Consequences of nondisjunction can include genetic disorders such as Down syndrome (trisomy 21), where there are three copies of chromosome 21. Aneuploidy can also lead to cell death or other developmental abnormalities. Nondisjunction during meiosis 1 affects homologous chromosomes, while during meiosis 2, it affects sister chromatids.

How can the number of genetic combinations due to independent assortment be calculated?

The number of genetic combinations due to independent assortment can be calculated using the equation $2^n$, where n represents the haploid number of chromosomes. For example, in humans, the haploid number is 23. Therefore, the number of possible genetic combinations is $2^{23}$, which equals 8,388,608. This large number of combinations contributes significantly to genetic diversity in the resulting gametes, making it highly unlikely for two gametes to be genetically identical.

What is the role of synapsis in crossing over?

Synapsis is the process during prophase 1 of meiosis 1 where homologous chromosomes pair up and align their DNA sequences at similar alleles. This alignment is crucial for crossing over, as it ensures that the homologous chromosomes are properly positioned to exchange genetic material. The alignment facilitates the formation of the chiasma, the site where crossing over occurs. Synapsis ensures that the genetic exchange is precise, contributing to the creation of non-identical sister chromatids and increasing genetic diversity in the resulting gametes.