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  • Hi. It's Mr. Andersen and in this video I'm going to talk about genetic recombination

  • and gene mapping. And it centers on the work of Thomas Hunt Morgan who used fruit flies

  • to show that genes just don't travel by themselves. They actually travel on chromosomes. And as

  • those chromosomes undergo what's called crossing over, genes from one chromosome are actually

  • going to swap position with genes from another chromosome. And so before we get to that we

  • should talk about fruit fly genetics for just a second. And so on the left we have a wild

  • fruit fly. That's what they normally look like. And on the right is a mutant. There

  • are two mutations in the one the right. Now only coloration, but you can see that it also

  • has these really small what are called vestigial wings. And so if we look at the genotypes,

  • the one on the right is little b little b. So it has that black coloration. The one on

  • the left we simply add a plus to it. And that implies that it's of the wild type. We could

  • also look at vestigial wings. Maybe the genotype of the one on the left has one of the wild

  • type normal wings but it has one of the vestigial genes. It still has normal wings on the left.

  • And that's because the wild type is going to be dominant in this case. And so let me

  • show you the quintessential cross that Morgan did that was so puzzling. And so what he has

  • is a normal wild type on the left, but it's hybrid for both of these genes. And so you

  • can think of this as like a F1 generation. And then he's simply doing a test cross with

  • it. So he's crossing it with a mutant fly that is mutant and homozygous recessive for

  • both of those traits. And so Morgan understood the work of Mendel and so he set up his Punnett

  • square like this. And so on the top he's going to show all the possible gametes that we could

  • get from this one parent. So you could have both of the wild type genes. Or you could

  • have both those recessive mutant genes. Or you could a combination of the two. So we

  • could have one wild one recessive. Or vice versa. Now this parent over here only can

  • give its recessive genes and so we could represent that on the other side like this. And so he

  • knew that there are only four possibilities that we could get out of this. And those first

  • two are going to look like that. And we call those what are called the parental phenotypes.

  • Why is that? Because this one looks like that parent and this one looks like that parent.

  • In other words there's no recombination. But on these other two alternatives right here

  • and right here, what we're getting is a recombination of those parents and so we call these simply

  • the recombinant phenotypes. But that shouldn't have been confusing to him. If we look at

  • the Punnett square, we have four different squares and so we would expect that 50 percent

  • are going to be parental and 50 percent are going to recombinants. But when he did this

  • cross what he found is that there is actually 17 percent recombinants and 83 percent that

  • were of the parental type. And so was Mendel wrong? Was all of this wrong? No. It's just

  • that the model wasn't good enough. And so he thought about this idea of 17% and what

  • it meant for a really, really long time. And then finally one of his students Alfred Sturtevant,

  • and I couldn't find a good open source picture of him, but he's always smoking a pipe, so

  • we'll say this represents Alfred Sturtevant. One night he just blows off his homework and

  • he figures it out. The whole thing. He figures it out. To understand it you really have to

  • understand what's going on during meiosis. And so if we look at these two parents, so

  • this is the double mutant on this side and this is the hybrid on the other side, let's

  • look at each of those and figure out what gametes could they produce? And so if we look

  • at the one on the right, we know that it can only produce these two gametes. But since

  • we're seeing a frequency of recombination that's less than 50 percent, that implies

  • that these two genes are found on the same chromosome. We know this now. Thomas Hunt

  • Morgan and Alfred Sturtevant had to kind of work through this. But if we look at what

  • does that mean? These two genes are found on the same exact chromosome. So if we go

  • through all the steps of meiosis, remember what happens first during interphase is that

  • we copy all of the DNA. And then it divides in half and then it divides in half again.

  • And since those genes are on the same chromosome, I see just one possible gamete that could

  • be produced. In other words you're going to get one of each of those recessive genes.

  • Now let's look at that hybrid parent. And we know and Thomas Hunt Morgan knew since

  • he saw some of those recombinants we had to have all four of these possible gametes. And

  • so let's put the dominant or the wild type genes on this one chromosome. And the recessive

  • on another. So how do I know that I have both of the wild type on one chromosome and both

  • of the recessive on another? Remember this is the F1 generation. And so it's receiving

  • this chromosome from a parent that was pure for both of these genes and vice versa for

  • the pure mutant parent as well. And so let's go through the steps of meiosis again. And

  • so what happens during interphase is that we copy them. Then there's one division and

  • then there's another division. And so how many gametes do you see? Well this one is

  • exactly the same as that one. And it's not based on orientation of the chromosomes because

  • again they're both found on the same chromosome. And so this was puzzling. But then eventually

  • they settled on this idea of crossing over. What if there were crossing over between these

  • chromosomes? What if somehow this chromosome wrapped around this chromosome during meiosis?

  • And they could see that under the microscope. They could see this occurring. If these crossed

  • over what you could get is bits of this chromosome actually being crossed over to that one. And

  • so what we could now produce is a chromosome that has the wild type for coloration but

  • it has the recessive gene for this vestigial wing and vice versa over here. And so Sturtevant,

  • it's brilliant coming to this kind of idea, that if that crossing over of that occurs

  • between the different genes, then we would have recombination, genetic recombination.

  • But if it doesn't occur in that part of the chromosome there is going to be no recombination.

  • And so where does that 17 percent come from? Well this is roughly 17 percent of that area

  • of the chromosome. That's where it's coming from. If those genes were closer together

  • that frequency of recombination would be closer. If they were really far apart, it's more likely

  • that it is going to split in the middle. And so we can use this one cross to figure out

  • the frequency of recombination. And then they were able to use that to build a gene map.

  • And so if you look at a chromosome, if we look at that frequency of recombination, let's

  • say it's 17 percent, that implies that it's an arbitrary distance of 17 map units apart

  • on the chromosome. Let's say the frequency of recombination is less than that. That means

  • the genes are closer together. What if the frequency of recombination is greater than

  • that? It means that it's farther apart. What if it's exactly 50percent? Remember that's

  • what we were thinking about. If it was independent assortment that would mean that those two

  • genes are found on different chromosomes. And so we can use that to really map a chromosome.

  • And so let's look at some of the data that they gathered. They found that the distance

  • between the vestigial and that black coloration gene, the frequency of recombination is 17

  • percent. They then compared that to another gene called the cinnabar which has to do with

  • eye coloration of the fruit fly and they got these frequencies of recombinations as well.

  • And so when you're figuring out a gene map what I would encourage you to do is always

  • start with the highest frequency of recombination. So I'm going to start with this one. And just

  • choose to put them on that chromosome. We'll say 17 units apart. So we're going to put

  • the vestigial and the black apart by 17. Now let's go to another one. So let's figure out

  • where the cinnabars are. Well if we start with the vestigial gene we know it's going

  • to be 8 map units apart from that. So I could say maybe it's going to be over here or I

  • could say it's going to be over here. So we have these two different alternatives. And

  • so which of those actually fits with that last frequency of recombination? Well if I

  • put it way over here, then we're going to have a frequency of recombination between

  • that and the black. We know it to be 9 percent. But it's going to be a way larger number than

  • 9 percent. And so I can narrow it down to this is where our gene map fits. Now let me

  • give you a problem of your own. So now I've given you these four genes and their frequency

  • of recombination. I would encourage you to pause the video here and then you try to map

  • out where each of those genes are found on the chromosome. I'll pause. And then let me

  • show you what the right answer is. And so what I would do is again start with the largest

  • frequency of recombination. I'm going to put B & C really far apart. So I'll put B on one

  • side, C on the other. What's the total distance of the chromosome? Remember it's going to

  • be 50 map units. And now I could work backwards. And so now let me figure out, so I've got

  • B and C. Where is D going to be? Well I can't put D way out here because I don't have enough

  • map units to do that. So I'm going to have to put it over here. And once I've got D,

  • I've got to figure out where A is. So I could work backwards to that. Well I know that A

  • can't be way out here on this side, so I know that A has to be somewhere over here. So that

  • would be the relative map distance or the relative gene map based on frequency of recombination.

  • And so Sturtevant and Morgan did that over years and they were able to map out where

  • the genes are found on the chromosomes. Now we don't do it this way anymore. What do we

  • do today? We simply sequence the DNA. Once we sequence the DNA we can figure out where

  • the genes are. But the cool thing is that as we compare that you could go right here

  • to the fly base I was looking up, the vestigial gene, we know exactly where it is. But that

  • maps up perfectly with the work of Morgan and Sturtevant. And so that is genetic recombination.

  • It allows us to create gene maps. And I hope that was helpful.

Hi. It's Mr. Andersen and in this video I'm going to talk about genetic recombination

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B1 INT US recombination chromosome frequency vestigial recessive gene

Genetic Recombination and Gene Mapping

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