hello this is Ed Chapman and this video cast is going to take a pretty close look at recalculating recombination frequencies based on how often crossing over takes place okay now before we can jump into this please remember these facts that a stem cell for example a cell inside of an ovary or a cell inside of a test is getting ready to go through meiosis it's going to start out as a diploid cell and when it completes meiosis it's going to make hloy gametes okay diploid cells two copies of each gene one on each homologous chromosomes okay we call each of these copies alals and they can be alike or they can be different right before meiosis Begins the chromosomes are replicated which means each chromosome is then made up of two identical sister chromatids and finally during prophase one of meiosis tetrads form and chromatids that are really close together that are not homologous to each other can cross over which means they can trade their tips all right so let's look at a situation here where we have a gene where we have sorry we have a chromosome with two length genes so this is a chromosome right here okay and this is its homologous chromosome here and the gene the genotype for this organism is Big a little a big b little B so it's a DI hybrid and because it has two chromosomes we can say it's 2N number is two so here's one of the chromosomes and here's the other chromosome all right now we're going to take this cell through interphase and each of the chromosomes are going to duplicate themselves so what we end up with over here now is one chromosome with two identical chromatids and the other chromosome with the same situation two identical chromatids this little dot right here represents the centrom meter all right now we're going to go through meiosis one and what's going to happen is the homologous pairs going to separate so in this picture we have I've drawn a circle around that represent one cell and another Circle here to represent the other cell and notice that now each cell only has one chromosome in it so they're now haid now meiosis 2 is going to separate the sister chromatids so these two sister chromatids are going to separate and over here these two sister chromatids are going to separate so what we end up with because there is no crossing over all right um in this example for there's no crossing over we end up with one set of gametes right here that are just like one of the parents so we call these paren because they're big a big b and we have another possibility over here from the other cell which is little a little B so that's the other parental chromatid chromatid notice okay this is real important that there are no recombinants which means there are no gametes possible that are big a little b or little a big b which would be the the recombinat they're not possible now if the genes are linked and there's no crossing over this is what a test cross would look like so here is the genotype of the one the one we just looked at on the previous slide and we're going to cross it with a completely recessive individual whenever you do this it's called a test cross all right so this individual can make two different gametes okay big a big b or little a little B all right now remember because the genes are linked okay they're linked so we can't put a big a and a little B together because remember the big a and the Big B are on the same chromosome so they can't get away from each other all right so this individual here can make this gam or this gam and of course we only have one possible gamet from this individual here so The Offspring out of a 100 we're going to predict we're going to get half of them are made by combining this gamet and this gamet and the other half is going to be made by combining this gamet and this gamet so we can have little a little a little B little b or big a little a big b little B which look just like the parents see see why we call these um parental okay the genotypes of these individuals are parental genotypes because they look just like the parents there are no recombinants okay so if we calculated it number of recombinants is zero divided by 100 Offspring so we have no crossing over happening all right now what if the genes are linked and we have all of them crossing over exact same situation okay we have two chromosomes one's carrying big a big b and one's carrying little a little B we go through um replication so now we have two chromatids for each of them we go through um prophase one of meiosis and this is where we get crossing over and I've tried to represent that here by crossing over these two chromatid ends here so what you end up with is an original okay it's like we just started with but the other chromatid is no longer identical because of the crossover now this one's carrying a little B and the Big B jumped over here so now we have parental parental okay and inside here we have the two recombinant chromatids okay then that Arrow there is wrong the recombinant one should be right here so ignore that all right so do you see the difference between recombinant chromatids and the parental chromatids okay so if the genes are linked and it's 100% crossing over this is what you're going to end up with you're going to get okay just like we got before the two parental um this is excuse me let me get rid of that we have the two parental possibilities for gametes plus we have two recombinate possibilities and for this one just like before there's only one possibility now there's four ways you can combine these and they're of equal probability so we're going to get an equal probability of a parental a parental and a recombinant and a recombinant so we have 50 combinant out of 100 offsprings so we've got 50% crossing over all right um so at least 50% so if all of them cross over what you can't really anything above 50% really doesn't make sense because once you get past half of them crossing over um I'll show you in then a future slide what that looks like all right so if 50% or more of these original cells that are going through meiosis are crossing over then the recombinat turn out to be just as likely as the parental phen genotypes so statistically Gene a and Gene B Gene B behave just like they were not linked so that's what I've drawn up here okay we have one gene carrying a another little a and on a completely separate Gene we have the B's so you can see here is as you go through the replication and they're unlined they behave like independent particles that's much more like what mendal explored with his PE Plants all right now the farther apart to Loi remember Loi is the location of genes on a chromosome the farther apart to low sigh are the more likely a crossover event will occur between them so you notice here Gene a has a low sigh right here and Gene B has its Locus right here there's a lot of real estate between them so it's there's a lot of places for a crossover break to take place so that means Gene a and Gene B are very likely to cross over all right whereas genes C and D here okay are close together so there's only a few there's much less real estate here for the break to take place so much less likely for a crossover event to take place between C and D now Morgan's team used the frequency of recombinations to map the locations of genes within the fruit fly genome but just remember the this these Gene maps that Morgan's lab produced only show relative loc locations not actual locations on the chromosome so for example he could figure out that um the gene controlling short and tenny was closer to the gene controlling wing shape than it was to the genee controlling leg shape but he couldn't actually figure out where they were located on the chromosome he could only figure out the relative distances so for example he would do that by by doing crosses between plants that had excuse me between fruit flies that had um the mutant antenni and the mutant wings and he would find that this has 133% recombination so that's the recombination frequency so that's how he would decide that these are 13 map units apart whereas the combination frequency between the short n Tenny and short legs is 31 map units apart okay so he would get 31% crossing over between those two all right now um if he did a comparison between short arist the short antenna and purple eyes you notice that's over 50 map units apart so these are if this is going to cross over so much that you can't really tell for sure that they're on the same chromosomes on the same chromosome by comparing the purple I Gene to the aristy one but you can tell on they're on the same chromosome by comparing it to a closer one all right so I'm hoping that makes sense but between these you can figure out the different map units so for example between the gray body the gene controlling the body color and the gene controlling purple versus red eyes we have a distance here that is equal to 54.5 minus 48.5 so that would be the the predicted percent recombination between these two so I'm hoping this helps you make sense of what's meant by recombination frequencies and how Morgan's team figured out how to um map the location of genes without actually being able to to image them or to um decode them like we're going to learn about in our next unit of study on the molecular basis of inheritance thanks for listening