Biology: Gene Mapping - Recombination Frequencies

Mapping Clarity

01/28/2012

~ Robert99

I found this topic very confusing until I watched this video.

Gene Mapping & recombination

11/22/2010

~ elcee421

This video would have been more helpful if you would have went through an actual pedigree that showed how to identify recombination in different generations.

not good!

02/23/2010

~ hatav

i paid for a series and i got more confused than before watching these! the guy just makes me nervous i do not know weather it is the tone of speak ,the voice or his stupid toys or paper throwing or what .
i am a medical student and i should say this was not sutable for me at all!

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Remember what this whole idea is all about. Can we tell where genes are in chromosomes? And, what I'm going to show you in this lesson is yes you can, in a sense. In a sense relative to each other. Now, let's back over what Morgan had found when he did a dihybrid cross expecting an independent assortment from a test cross and expecting four different genotypes and four different phenotypes, and instead getting some rather bizarre results. It was very clear that his two genes were linked. They were not independently assorting. But, if they were linked, he should have gotten 50/50. And, instead, he got something else. Let's look at the numbers. He got 965 and 944 of the two phenotypes that he expected which were wild and black vestigal. Okay? He got 206, and this is where the puzzle was, and a 185 which acted as if - the genes were behaving as if this was an independent assortment. In other words, a gray; or in other words a wild body but vestigial...I'll say gray cause that's what I said in the last lesson - gray, it's the wild body, and the other one was 185, black, normal. So, if we divide it up like this and ignore this we are great - interlinked. So we got to say they're linked. I mean, look at the overwhelming numbers. In fact, here's the numbers. What you have here is what looks to be about 17 percent. You can calculate it out. That's 206 plus 85 over 2300 okay. Seventeen percent of somehow these things sorting themselves not as if they are linked on the same chromosome, but as if they are on different chromosomes. And so Morgan students suggested that wow, maybe there is some - and this is before we knew about prophase one of meiosis and homologous pairing and crossing over. So this is a big intuitive leap.
Could there be some kind of situation where, on occasion, genes change places or pieces of genes change places? And, the answer is absolutely yes. Let's go back over what you know about meiosis. I'm going to put these back together because we need to review something or see it for the first time. Remember the chromosomes when they go through meiosis one and in prophase one a process called synapses occurs and what happens is the chromosomes come together. I want you to see an interesting thing, because there's going to be a basic principle here that Studevant Morgan's student also intuitive [sic]; and it's this. Remember where the chromosomes bump into each other are called chiasmata. And, you remember the chiasmata are places where crossing over is going to occur. So, here's the thing, you guys. If these two are bumping into each other here, what about these genes here? Because they are fairly tightly linked to the genes at the top of the chromosome, these genes, because of the nature of the chiasmata have very little likelihood of also forming a chiasma. So, the principle that was suggested and the principle we now know is true, is that the closer two genes are together the less likely they are to crossover relative to each other.
Well, let's see what we're talking about here. That suggests that whereas the fact that we have genes that are linked, and we have the body gene here, and what was the wings, the wing gene here, since these genes are relatively far apart on my chromosome, and this is way far apart and I'm going to show you the reality in a minute; but, I'm making a point here. Then, there's a very strong likelihood that these two genes may crossover and, you know what, let me do this another way. I'm going to flip the page and make it clean for you. I'm going to make them close and you will see what I mean. Here, this will work better. Might as well be accurate. Check it out. Here's the body genes up here and we'll put the wing genes here. Now, we'll make these pretty close. That means that if the chiasma forms right here, the body genes may crossover relative to each other. I'll just cross over one since we're going for accuracy here. And these may not cross over that frequently, why? These down here; because the odds are they won't bump that frequently. Every once in a while they may bump; but generally speaking they are not going to bump that frequently. Make sense?
Well, let's think about what Morgan found. He found that 17 percent of the time the body gene crossed over and left the chromosome where the wing gene was, that it was linked to. Not that there's no wing gene on this chromosome but the linkage pattern was broken and you got new combinations. We call them recombinants. So there were recombining genes. There were combinations that we weren't expecting in link genes because 17 percent of the time this red one jumped over this yellow one, this yellow jumped over to this red. Seventeen percent of the time what? Relative to these. Let's see what that means. Well we've made up a number and we're going to convert this thing into something called the map unit. And, here's what we are going to say. We can now say, and you'll see the how relative position of this in a sec, that if this is the Drasophala chromosome we can say that these two genes, body and wing, are 17 map units apart, why? Because they cross over 17 percent of the time. It's a very convenient - what's a map unit? It's an arbitrary number of two genes relative to each other. But, wait there's more. Where do we put these? Well, it's all a relevant thing. But, watch this. The distance is the key, but watch this. There's another mutation we want to map, okay and so what we're going to do is take this mutation and it's going to be an eye color mute and we're going to map it relative to body, okay. So, we're going to take a gray body kind of thing and we're going to cross it with this eye color called cinnabar. So, once again, we're going to see how often they are linked - we know they are linked. We're going to see how often they cross over - this is body and this eye - relative to each other. And, here's what we are going to find out. The number we find out is going to be smaller than 17. In fact, it's going to be nine. Nine percent of the time gray body, black body crosses over relative to cinnabar.
So, let's see what I got here. Here's my chromosome. Cinnabar, here's gray body. I'll put gray body right here. Body - nine percent of the time. So that's nine map units. But, I have to ask you a question. Is it nine map units this way? Or is it nine map units this way? Because, if it's nine map units this way remember what I had over here? That I arbitrarily put right there. What did I put there? That's a gene locust, a gene place for wings. So, wait a minute. Can you think of a way I can figure out if the nine units for body goes on this side of the body gene color or this side? Suppose I did Cinnabar with wings? Well think, look, let me draw this one more time. If Cinnabar is nine from body, and body is 17.5 from wings, and then Cinnabar is way out here, then it should be about 26 crossover frequency. Right? But, if it's in this direction than it's going to be somewhere in the middle. And guess what we find? We find that crosses for this give us the following numbers. We find out that Cinnabar crosses over with wings 9.5 percent of the time and crosses over with body roughly 9 percent of the time. It's going to be right in between them. There's my original, my body wing 17 percent. We find out because of the way this thing crosses over with body and wings that it's going to be right about there. Great, purple on purple. It's going to be right about there. Nine percent from the body because of the 9 percent crossover frequency and 9.5 percent from the vestigal. And, you're saying wait a minute 9 + 9 is 18 . What are you doing here that's not 17. These numbers are close and this all-relative position. But, look at what we can do. Keep doing these cross over frequencies you can map out an entire geno and guess what? We're real close with fruit flies. You look it up in one of your books. Wait until you see all the genes we've mapped only by doing crossover frequencies.
Mendelian Genetics and Mutation
Linked Genes and Genetic Mapping
Gene Mapping Using Recombination Frequencies Page [2 of 2]