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4. Genetic Mapping and Linkage

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Multiple Cross Overs and Interference

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Hi in this video, I'm gonna be talking about multiple crossovers and interference. So as we just as you probably just learned, tracking multiple crossovers is more difficult because there could be a double crossovers and they're hard to find unless you know the order of the jeans and you've done a lot of work and um but we still need to know about them. So multiple crossovers occur when more than one crossover causes two or more changes in the gamma genotype. So a single crossover event, Is this a multiple crossover event? Is this where you have something like this happening twice? And these are much harder to detect, but they also happen less frequently and so you can calculate um information on how likely it is. A double crossover will happen using the product law, which we've talked about before. So um you can double count, I'll show you the product for example, in the first, but the example that we've gone through before in the previous video, this was in the tri hybrid cross video. Now, if you don't understand what I'm about to say and you're confused, I really highly suggest you go back and look at this because it explains it very well at the end, but you have to double count the double crossovers when calculating the recombination frequency and this helps you accurately map the genes. Like I said, you don't know what I'm talking about about. Double counting the double crossovers, then go back and watch this video now. But if you know what I'm talking about and you're ready to learn something else. What we want to talk about next is interference, interference is when crossovers in one region affect the chance of a crossover in a second region. So it would be like if you have jeans, A B and C. And A B. C. And two chromosomes say that if this crossover event happens, then it's much less likely that a second crossover event here will happen. And so um independent crossovers happen a lot of the time and this means there's no interference. And you can calculate this using a double recombination frequency and this is the product law thing I was just talking about. So the question would look like this if the frequency of crossing over is 20% for genes A and B. So we'll just say this is exactly like this And 30% for Jeans B&C. So the chance of this happening is 20% and the chance of this happening, it's 30%. Then what is the frequency of a double crossover? So the frequency of both happening? Well, you just use the product loss. So you take 20% and you turn it into a decimal and do the same thing for 30 and you multiply them together and multiplied by 100. So your answer here is 60%. So you do 20 times 200.3, um times 100 that's 6%. So the frequency of a double crossover happening here and here is 6%. And this is a good formula to know because you definitely may see a question like this. Now this is the predictive, right? This is the ideal situation, no interference occurring. And so if you want, if you're kind of doing it, if you're setting up an experiment and you want to know, you know what percentage of double crossover should I expect, you can use this formula. But sometimes if you have interference this means that the calculated value. So the 6% we just calculated isn't actually what you observed in your experiment. And so this means that interference is affecting the data. So what you do is you you a different formula. This formula uses the coefficient of coincidence and that's here. This is the C. O. C. What I just highlighted in this paragraph. So the formula for interferences I equals one minus the C E O C. Now the C O. C. Is going to be the observed double recombinant over the expected. So in this formula that this is going to be what you see and this is gonna be what you calculated, which is 6% and you do one minus. So if we're doing this, calculating the interference, say we observed four. But we expected six say this is this exact same experiment that we talked about earlier where the chance was 20% here and 30% here. So you did This times 100 and that equals 6%. So that's going to be your expected. But you did the experiment and what you got was you got four offspring instead of six. And so then you use this interference formula. And so that's gonna be 1 -4/6 because 0.33 you times that by 100 to get a percentage 33% back up here. 33%. So you say, okay, the interference is 33%. But what does that actually mean in a real sentence? Well, that means there were 33% fewer double crossovers occurred than expected, meaning that crossing over at one location either the A. Or the B. Decrease the chance of crossing over either than adjacent location. Either the B. Or the sea. Now I don't know which one, I don't know which one was more likely or which you know, I can't tell you that. But I can tell you that crossing over one of these locations, decreased it by 33%. Or decreased it at adjacent location, meaning that we got 33% less double crossovers than we expected. So that's genetic interference and you really need to know how to calculate it. So genetics is a lot of formulas is a lot more math than you're probably expecting. But these formulas are important and you do need to know how to calculate these. So with that let's now move on

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Problem

A female with the following genotype can produce a number of different gametes. Choose the gamete produced if no crossovers have occurred. Genotype = a b + / + + c

A

a + c

B

+ b c

C

a b +

D

+ b +

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Problem

A female with the following genotype can produce a number of different gametes. Choose the gamete produced if a single crossover has occurred. Genotype = a b + / + + c

A

+ + c

B

+ b c

C

a b +

D

a b c

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