Gene mapping

one of the very interesting things about genetic recombination is that you can actually use genetic recombination to figure out the distance between genes on a chromosome and if you're you to do this to all the genes on a chromosome you could actually map out the croeso and figure out exactly where the genes are and we're going to explore that concept so we're looking at a pair of homologous chromosomes now let's just say that the orange chromosome is the paternal chromosome so it comes from the father and let's say that the yellow one is the maternal chromosome and just to remind yourself so these are sister chromatids that means that they're identical chromatids they have identical genes on them so those colored circles that I do represent just some genes that I randomly picked and you can see that on the two sister chromatids I drew them in the same color because they're the same and for that matter these are also sister chromatids these two yellow chromatids are also sister chromatids and I'm just going to review a bit to give us some context so normally in the cell the paternal chromosome would just look something like that with the centromere middle and the maternal chromosome would also look something like that but during meiosis or mitosis when they replicate this chromosome will turn into that so it's going to duplicate itself and each one of those strands is a sister chromatid and so will the maternal chromosome turn into something that looks like that but this is still considered one chromosome and this is considered one chromosome one that can be a little bit confusing because how could this be one chromosome and this also be one chromosome if this x has double the material as the original strand and the answer to that question is because we count chromosomes by the amount of centromeres that are present so this has one centromere it's considered one chromosome this X has one centimeter so it's considered one chromosome however they're not quite the same this on top it's a you know just one chromatid and the bond chromosome wasn't above two chromatids so try not to get confused even though we're calling this one chromosome was really made up of two identical sister chromatids but anyway that was a bit of a digression let's focus again on our homologous chromosomes so I picked a couple of genes that I just you know put on and we're going to focus on three genes in particular and to make this a bit more real or relevant let's just say that the green genes represent I don't only be complexion maybe like dark complexion versus a lighter complexion and on the maternal chromosome I also drew it in green to remind us that these are homologous chromosomes or homologous alleles in other words these are both alleles that code for complexion but different shades of green because they're probably different versions so these green genes also code for complexion and let's say that this darker purple or magenta gene codes for let's say here color so that means that the lighter purple on the maternal chromosome also code for your color and then let's say that the blue genes code for eye color so if dark blue in the paternal chromosome and light blue on the maternal chromosome and let's then focus on the sister chromatids where genetic recombination occurs so these two strands are going to swap genetic information between them maybe like a chunk on bottom will swap maybe some something in middle slot maybe something on top will swap actually genetic recombination also occurs between sister chromatids however sister chromatids are identical so it would be a no consequence anyway so let's look at the two chromatids where genetic recombination is happening the two that I circled let's take a closer look at those so here are the two chromatids that are going to exchange genetic information and undergo recombination and I want to ask you a question so I'm going to give you two choices and I want you to try to figure out which one is more likely so for the first choice let's look at the purple and green genes and the question is what's uh what's more likely is it more likely that the purple and green jeans undergo recombination and when I say that I mean that the jeans that are originally on one chromosome separate so I need to say that for example this purple gene will get separated from that gene and this lighter purple gene will get separated from that gene so is it more likely that the purple and green jeans we combine or the way I like to view it get separated or is it more likely that the blue and purple genes recombine or get separated and the way to think about this is well look at the distance between them so the distance between the purple and green jeans is something like that so in order for recombination to occur with respect to the purple and green jeans it would have to happen somewhere over here or on the other side it doesn't matter it doesn't make a difference so you have this whole distance whereas if you wanted recombination to occur with respect to the purple and blue jeans you've a very small area to work with it would have to happen somewhere here so somewhere along you know this line of the chromosome so that means that it's much more likely for recombination to happen with respect to the purple and green jeans that is more likely because you have this entire distance to work with if you if these two chromatids swap genetic information anywhere along this stretch of the chromosome so the the green and purple jeans will separate or recombine and the blue and purple jeans are less likely to recombine because recombination would have to occur only in this little sliver of chromosome and that's just smaller than you know the other part of the chromosome that we were looking at so we just learned a very important concept and that is that the further part two genes are the more likely it is that they will recombine I'm actually not sure if that's the proper way to use the word but I just use it that way so I'm going to put it in quotes and again when I say recombine I mean that two chromosomes that were originally on the same sorry two genes that were originally on the same chromosome get separated and then the next thing we learned is that the closer to genes are to each other I'll abbreviate each other just e o the less likely it is that they will be combined and now I'm going to introduce you to some terms the centimorgan is the unit of measurement that we use to measure distance on a chromosome and another way to say centimorgan is a genetic map unit or m dot u dot which stands for a map unit and I'll give you the official definition of a centimorgan because I think it's important it kind of ties in distance to what it has to do with recombination and so a centimorgan is the distance between genes just going to abbreviate between like that for which one product of meiosis in 100 is recombinant and to put that in simpler terms it means that if two genes are one centimorgan apart it means that one out of 100 times or 1% of the time that meiosis happens those two genes will be recombinant or separate or recombine and we'll actually do an example of this to illustrate what this means so here we have our two chromatids again and let's just say that the distance between the purple and green genes is 25 map units remember a map unit is the same thing as a centimorgan and let's say that the distance between the blue and purple genes is 6 map units so this is clearly not drawn to scale very well but it's just an estimate so let's first focus on the purple and green genes so if there are 25 map units apart so remember if two genes are one map unit apart that means that 1% of the time they'll recombine so these are 25 map units of parts so that means that 25% of the time that my this happens recombination will occur with respect to the purple and green genes so let's see what that looks like so we're going to see that in this spot right over here the chromis the chromatids just swap information so let's draw what that would look like so let's draw our paternal chromosome or at least part of it and then we'll draw our actually I'm going to draw that a little bit higher because I need more room so here's our paternal chromosome and then our maternal chromosome or chromatid and then since they kind of swapped in this spot so I'm going to draw yellow right over here that's the part that came from the maternal chromosome and then I'm going to draw orange over here that's the part from the paternal chromosome and now let's fill in our genes so the blue genes will just stay where they were so we just leave them put and the same goes for the purple genes but then we have that piece of maternal chromosome so we get that hunter green gene over here and then we get the lime-green gene right over here so this is what I mean by recombination being a separation of genes so this purple gene and that lime-green gene were on the same chromosome before but they got separated they're not on the same chromosome anymore and the same applies to these two genes now let's look at the blue and purple genes so there are six map units apart so that means that 6% of the time that meiosis happens these the purple and blue genes will get separated so we're going to say that upwards of this spot the chromatids swap information so let's draw that so here we'll draw a part of our paternal that's the paternal chromosome and eternal one and they swapped somewhere like over here so let's fill that in so that's the part of the maternal chromosome that lands up sorry that's yeah that's the part of your maternal chromosome that ends up on the paternal one and the orange over here and I'll let's fill in our genes so let's first fill in the ones that just stay put so we have that lime-green gene that we have that hunter green gene and our Purple's also stay put but the blue jeans swap chromosomes so we have our dark blue gene over here and our light blue gene over here and again take note of how they swap places or how they separate so these two genes were together on the same chromosome before but not anymore and the same for these two genes so let's just tie this back into the bigger concept going on if we were to do a statistical analysis of how often certain recombinations happen that can help us map out the genes on a chromosome