[Music] hello everyone this is andy and welcome back to med school eu we're going to continue on with the last section of the cell in the biology from the imats specifications and this is going to be a very short video on the concept called chromosome maps i personally don't think anything's going to come up on the imat exam about this topic because it is quite advanced genetics however i'm going to give you a quick introduction from what i learned in the undergraduate class classes about genetics and we learned a little bit about chromosome mapping and how it came to being a concept and how we actually place different alleles on parts of a chromosome so the definition of chromosome mapping is basically using the frequency the recombinant offspring frequency and using that in order to map out where the chromosome genes are located so if we have a carrot type of you know humans and they would go from largest to smallest and they will continue on until we have the sex chromosome at the very end basically we know from the the gene mapping we know on which part so if we enlarge this we know exactly on which part or we can approximate on which part the genes are located in and that's what's called chromosome mapping and we're going to go through the process of how scientists were able to make these discoveries and make assumptions on where the genes are located in each of the chromosomes and the significance of this concept so the first person that began to play with this concept was called alfred startvent and alfred startfriend basically took off doing experiments with dragonflies drosophila dragonflies and using those dragonflies he was able to see which genes in the offspring were recombinant and which frequencies occurred in the offspring and therefore he realized that using those frequencies you're able to map out a linkage map on the chromosome to see exactly where the genes are located and which ones are going to be go through recombination and which ones will not so if we take a look at an example here first this first one right here and if we're measuring three different genes so let's say this is a section of a gene or a chromosome and each of these lines represents the location of a gene so we will have for example gene a right here then we would have gene b on the other side and gene c right in the middle so when we have these three he estimated in his dressophilia experiments is that between a and c there was an eight percent chance of a recombination and i'll explain what that means between c and b there was a two percent chance of recombination and between a and b there was a nine point six percent chance of recombination now i'm going to explain what these numbers mean and why the numbers the further the distance between a gene the higher the number and why the number is lower between closer genes well basically his concept states that if if you are going to map out genes the first thing you're going to do is go through recombinant offspring frequencies so you're going to have a parent and another parent male and female they're going to reproduce and they're going to form different offspring right and these offspring will have various frequencies of genes and you're going to pick a gene and see so we're going to pick a b and c the three different genes and we're going to see their recombination frequencies and we have determined that the recombination between these would be a and c would recombine eight percent of the time a and b would recombine nine point six percent of the time and c and b would recombine two percent of the time now what does recombination mean and we we talked about the process of recombination in meiosis it happens in meiosis one in the prophase one stage and what occurs is that two genes so if we have one like this and the blue one like that they basically do a crossover right here so it happens a crossover and now the yellow one would have part or segment of the blue and the blue one would have a segment of the yellow and these would go through the process of this would be called re-combination that occurs in prophase one of uh meiosis one and recombination the significance of it is that what are the chances that gene a and gene c would recombine and the chances are eight percent of the time now what are the chances of a and b recombining it's a little bit higher why is it higher because the distance between them is bigger and recombination is basically the the splitting of the gene so what is the chance if a chromosome has a c and b what is the chance that recombination will result in a being separated from b it's 9.6 because if you separate here a would remain on the original chromosome and b and c would go on and b on the other chromosome because of recombination however if we had a split right here which happens two percent of the time c and a would remain on the chromosome but b would be on the new chromosome because of recombination and this only happens two percent of the time however this here happens 9.6 percent of the time so looking at another example right here with the white chromosome let's just uh imagine that we have 100 we have 100 germ cells that go through meiosis and they're going to produce 400 gametes if you don't know how i got 400 look back in the previous video about the topic of meiosis and how it produces gametes now we're going to end up with 400 gametes and if a recombination event occurred in the space separating two given genes in 10 of those cells so 10 genes so let's say we have given genes right we have gene a let's mark them by the color so we got gene a we got gene b right here and this one i believe it's this color uh well let's mark it with white because it's it's a darker color and that's going to be gene c if if recombination event occurred in the space separating two given genes so let's say it would be a and b and and 10 of those cells so in 10 of the germ cells we're going to have the separation between a and and b so 10 germ cells go through recombination and they separate a and b so what we end up with is uh 20 recombinant chromatids 20 chromatids would be producing during phase one and 20 gametes would eventually receive recombinant chromosomes and what you have is 20 out of 400 that would equal to be 5 so how did i get 20 chromatids well remember that when you produce the gametes each germ cell would produce four gametes so you'd have one two three four and if you remember only one if you have so at the beginning if you have uh a chromosome only one of the chromatids would go through a process of recombination not both only one goes through the process of recombination meaning that once you have separated everything and you've gone through meiosis completely meiosis one and two you're gonna end up with these regular ones and you're gonna have one and 2 here that would have recombination that's why only 2 it's 20 not 40 but it would be 20 chromatids being produced now 20 out of 400 gametes is going to have 5 of separation between gene a and gene b and we could conclude that these gametes are five so the units that they use whoa that's a crazy five look at that now let's try this again yes now so five and they call them the units of separation are called map units so m u so there would be 5 m u apart right here and that's the significance of it now obviously if you are going from a to c then the number would be much higher because the this uh the significance of having split right here is a lot higher than having the split right here and typically these two would only they would be together if they're going through a recombination they would still remain intact 95 percent of the time and they would only go through this separation five percent of the time and that's why we labeled b a lot closer to c because c would have something higher maybe something like uh 48 and remember you cannot have anything higher than 50 so your recombination frequency cannot be higher it cannot exceed 50 it has to be below that now why is that well because only one sister chromatid goes through recombination not both only once so you're you can have a maximum of 50 percent of recombination offspring frequency so if if it is at 50 that simply means that they would be separated all the time every single time that that specific sister chromatid goes through a reca recombination it would split the two genes apart and now of course if if we're looking at b and c here we would have to take 48 because between a and c it's 48 percent now between b and c it would have to be 48 minus 5 it would split 43 of the time so that that's how we determine the location of each gene on the chromosome and it's it's done through the recombination of offspring frequencies and that's that's typically the significance of this video is this chromosome mapping and how it's done there's a lot a lot more detail to it in in genetics however just a slight introduction to the topic since it is on the imat specifications they these are the little things that you should know okay so we have finally concluded with the topic of the cell in the next video and in the next lecture we're going to begin lecture three called bioenergetics and in that first video we're going to talk about the energy currency of cells called atp [Music] you