Bozeman science: Genetic recombination and gene mapping

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 and the one on the right not 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 a little be little be 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 the f1 generation and then he's simply doing a test cross with it so he's crossing it with a mutant fly that's 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 of those recessive mutant genes or you could have 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 we 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 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 fifty percent are going to be parental and fifty percent are going to be recombinants but when he did this cross what he found is that there were actually seventeen percent recombinants an eighty-three 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 seventeen percent 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 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 then so 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 fifty 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 interfaces that we copy all of the DNA and then it divides in half and then it divides in half again and since those jeans were 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 in 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 jeans 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 sturdivant it's brilliant coming to this kind of idea that if that crossing over event occurs between the different genes then we would have recombination genetic recombination but if it doesn't occur during that in that part of the chromosome there's going to be no recombination and so where does that seventeen percent come from well this is roughly seventeen percent of that area of the chromosome that's where it's coming from if those jeans were closer together that frequency recombination would be closer if they're really far apart it's more likely that it's 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 seventeen 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 means that it's farther apart what if it's exactly fifty percent 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 seventeen percent they then compared that to another gene called the cinnabar which has to do with I coloration of the fruit fly and they got these frequency of recombination zaz 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 will say 17 units apart so we're going to put the vestigial in the black apart by 17 now let's go to another one so let's figure out where these cinnabar czar well if we start with a vestigial gene we know it's going to be eight 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 nine percent but it's going to be a way larger number than nine 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 jeans 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 and C really far apart so i'll put be on one side see on the other what's the total distance of the chromosome remember it's going to be 50 map units and now i can work backwards and so now let me figure out so I've got B and C where's d gonna 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 got to figure out where a is I could work backwards to that well I know that a can't be way out here on this side so I know aids got 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 stirred of it and so that's genet genetic recombination it allows us to create gene maps and I hope that was helpful