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Genetic recombination

Genetic recombination is a process that occurs during meiosis, which is the type of cell division that produces gametes. During this process, homologous chromosomes pair up and form a unit called a tetrad. The chromosomes can then cross over and exchange genetic material, which results in new combinations of alleles. This process is important because it increases genetic variability, which can be beneficial for a species. Created by Efrat Bruck.

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  • blobby green style avatar for user Emily Bovasso
    Aren't there only 22 pairs of homologous chromosomes and 1 pair of sex-linked chromosomes?
    (6 votes)
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    • leaf green style avatar for user Shelby Burridge
      Homologous just refers to pairing, matching up of similar chromosomes. So this could refer to both sex chromosomes and autosomal (non-sex) chromosomes. So we have 23 pairs of homologous chromosomes. Each pair is made up of 1 chromosome from the mother and 1 from the father. 22 pairs of homologous autosomal chromosomes and 1 pair of sex chromosomes.
      (5 votes)
  • female robot amelia style avatar for user Gail  Golston
    Those 2 chromosomes involved in crossing over are not homologous chromosomes correct? What are they? Are they at all related to each other? Also, the chromosomes have duplicated by that point correct?
    (1 vote)
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    • male robot johnny style avatar for user Dododeda
      They ARE homologous chromosomes. Crossing over almost always happens between homologous chromosomes. And yes, the chromosomes have already duplicated by that point. In fact, they duplicated during interphase (before meiosis even begins).
      (9 votes)
  • leaf green style avatar for user Sankalp Godugu
    does crossing over only occur at one chiasma or can it occur at multiple chiasmas?
    (2 votes)
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  • primosaur ultimate style avatar for user Shreya
    Independent assortment is a type of recombination (apart from crossing over). Recombination violates the law of independent assortment. I understand the two ideas separately but together they both seem contradicting. Is one of them wrong?
    (1 vote)
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    • marcimus pink style avatar for user Taylor
      Independent assortment refers to the idea that traits tend to assort independently of each other when gametes are formed. In other words, the probability of inheriting a hair color trait is not influenced by the probability of inheriting an eye color trait, for example. If they assort independently, the probability of each event occurring is unaffected by the other.

      Recombination in the form of crossing over occurs between homologous chromosomes, in order to increase genetic diversity by changing the assortment of genes. It does not violate independent assortment since it doesn't affect how traits and genes are actually assorted in gametes.

      Genetic linkage violates the law of independent assortment since linked traits would be inherited together and would not assort randomly/independently.
      (7 votes)
  • piceratops ultimate style avatar for user Jacob Schluns
    At , she states that the rest of the discussion should also be considered to apply to all of the other chromosomes. Does that include the pair of sex-linked chromosomes? Do they also undergo crossing-over?
    (2 votes)
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    • female robot ada style avatar for user Lidiya
      In the second video on the genetic recombination, the author does describe the recombination that could happen between tips of the X and Y chromosomes during male meiosis but it is a very infrequent event. Based on scientific data, the small homologous regions at the tips of the chromosome seemed to appear there due to the recombination in recent past of primate evolution that could be in a range of million years. In addition, the region of homology is so small that it is highly unlikely that they undergo crossing-over in a common way that applies to other chromosomes.
      (3 votes)
  • hopper jumping style avatar for user Jacob Anderson
    Does anyone have a link to an animation that has mitosis or meiosis with all 46 chromosomes?

    I think I could benefit from seeing the entire complexity/organization.

    Or can someone make one :D?
    (2 votes)
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  • blobby green style avatar for user jschneid0022
    How would homologous recombination (crossing over) affect traits that are normally inherited together?
    (1 vote)
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  • leaf green style avatar for user Shlok Shankar
    Crossing over takes place when , particular and similar regions break from a tetrad and are assisted by the synaptonemal complex to join with the other paired tetrad how does that actually happen ,how does it break ,how does SC play it's part and does this breaking and recombination have a random pattern or consistent pattern?
    (1 vote)
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  • female robot grace style avatar for user OpenMinded737
    you only have 22 homologous chromosomes. so the only different is the heterogous chromosomes of sex. in mamanals right?
    (1 vote)
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  • piceratops seedling style avatar for user marie-reine  haddad
    why the two genes that are separated by 10 map uni show a recombination percentage of 10%?
    (1 vote)
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Video transcript

- [Voiceover] Let's take a look at the nucleus of a cell that's just starting to undergo meiosis. So we have these 46 chromosomes and beforehand each chromosome probably looked something like that. That's the centromere in middle. And then, as meiosis is beginning, they will duplicate into, each chromosome will duplicate into something like that. That's again that centromere in middle. So we have these 23 purple chromosomes, we're gonna say that these are the maternal chromosomes. And then we're gonna say that the blue ones are the paternal chromosomes. They come in homologous pairs. And the nucleus that we're looking at must belong to the cell of a male. because right over here you can see that is the Y chromosome. It's a bit smaller than most of the other chromosomes. I'm gonna digress just for a moment, to clarify a very common point of confusion. And that is that if you look over here at these two chromosomes, well, this is considered one chromosome, but so is this, called one chromosome. And this can be confusing because this is only one chromatid and this is two chromatids. So why are we calling them each one chromosome? And the answer to that question is because we count chromosomes by the number of centromeres. So this single chromatid has one centromere, but these two chromatids are attached in the middle by one centromere. So we also call that one chromosome. Just keep that in mind, it's just a technical point to know that even though they're different, they're still both considered one chromosome. And in this chromosome, the two chromatids are duplicates of each other. So it's just a copy of itself. Anyway, back to our nucleus. So we have these 46 chromosomes, 23 homologous pairs. And they're not, the chromosomes are not necessarily arranged in the way that I drew them. I just drew it that way for the sake of organization. But, we have these pairs and we're gonna focus on one pair of homologous chromosomes. But I want you to keep in mind throughout this video, that whatever we're describing that's happening to this pair of homologous chromosomes, is also happening to the other 22 pairs of homologous chromosomes. It's just that it would be too hard to depict in a video how that's happening, but keep in mind it's not just happening to the pair that we're talking about, but what we're going to be describing is happening to all of the pairs of chromosomes in the nucleus. So here we have our pair of homologous chromosomes. And during prophase one of meiosis, the homologous chromosomes pair up with each other and form a unit called a tetrad. And it's called a tetrad because, well, tetra means four and this unit has four chromatids, right. One, two, three, and four. And the process during which the homologous chromosomes pair up with each other is called synapsis. So during synapsis, the homologous chromosomes will get a little bit closer to each other. Something like that. And at a certain spot, they might actually cross over or overlap. So I'm gonna circle that spot. And that's called the chiasma. And in some cases, another thing happens. This protein complex that resembles something like a railroad track forms. We'll see in a minute why. And this is called the synaptonemal complex. You can actually see the word synapse in there because this happens during synapsis. So we've formed the synaptonemal complex and with the help of the synaptonemal complex, these two chromatids, the ones that are crossing over, will actually swap material downward of that point. So we're gonna get something that looks like that. Look at how the purple chromosome now has some blue over there. And look at how the blue chromosome now has some purple over there. And the way that happened was that the DNA in the chromosome, actually some bonds in that DNA broke and the DNAs just kinda swapped places. So what we just described, this process by which the two chromosomes swap information is called crossing over. Or, another way to say this, is genetic recombination. And let's see why this is called genetic recombination. So we're gonna fast forward to the end of meiosis to where the chromosomes get split into two and all the chromatids get separated into different gametes. And I want to pause and remind you that everything we're describing that's happening to this pair of chromosomes is also happening to all the other 22 pairs of homologous chromosomes. But anyway, so now let's put each one of the chromatids in a different gamete. And look at how we get four different gametes. And we can call these two, gametes recombinant. And we're calling them recombinant because they have a combination of alleles that's new. We haven't had this combination of alleles, even in a parent. And just to clarify things, let's see what the gametes would look like if crossing over did not happen. So let's go back to our original chromosomes. And let's split them. And let's put them into four different gametes. And we are going to get that. And you can see that in this case, we only have two different types of gametes. So we can see how genetic recombination increases genetic variability. Which is usually a good thing.