In a series of two-point mapping crosses involving five genes loc...

Question

In a series of two-point mapping crosses involving five genes located on chromosome II in Drosophila, the following recombinant (single-crossover) frequencies were observed:

pr–adp 29%
pr–vg 13
pr–c 21
pr–b 6
adp–b 35
adp–c 8
adp–vg. 16
vg–b. 19
vg–c 8
c–b. 27

Given that the adp gene is near the end of chromosome II (locus 83), construct a map of these genes.

Accepted Answer

Hi everyone. Let's look at our next problem. It says a student conducted a sequence of two point crosses for four genes, specifically genes A. B. C. And D. These jeans are the following recombination frequencies and we have a list of different recombination frequencies for different combinations of two genes at a time, A. And B. A. N. C. And so forth. Then it says considering the given data, which of the following statements is true. And were given four statements regarding the locations of these genes on chromosomes. Are they on the same or different chromosomes from each other? So we need to think through and recall what recombination frequencies tell us about whether or not genes are on the same chromosomes from each other. And to do that. It could be helpful to think through that meaning of recombination frequencies and what it tells us about gene linkage. So I like to imagine a Hetero sickos individual for two imaginary jeans, B. And A. So this individual is big B. Little B. Big a little A. And the result of um two gametes uh Big B. Big A. And little B. Little A. Well this hetero size individuals, B. And A. Are on two separate chromosomes. They independently assort this homo Vegas individual can then produce four types of gametes. Big be big A big a little a little bi big a little B. Little A. Well the big be big A. And little B. Little A. As we see up here are parental type gametes. They're the same combinations of genes B. N. A. As the parents of our hetero status individual. But Big B. Little A. And B. Little B. Big A. Our recombinant gametes, they are two different combinations. They show a crossing over of these genes. And we see that when we have the genes independently, a sorting that these recombination meets Are going to be 50% or 50% of the time these four types of gametes will be equally likely each equally likely to be produced as each other. So in a case where we have independent assortment, 50% of the gametes will be recumbent. Now let's imagine a scenario where the two genes are on the same chromosome. There will be less likelihood of you have to have crossing over for recombinant gammas to be produced. Um It's supposed to independent assortment. And of course the closer the genes are together the less likely there will be a crossover event. The fewer recumbent gametes you'll see. So 50% recombination frequency indicates independent assortment and location on different chromosomes. Less than 50% indicates they're on the same chromosome with the lower the recombination frequency. The closer the closer they all together. So remembering that let's evaluate our answer choices. Choice A says jean B. Is located on a different chromosome from genes A. C. And D. So let's look at jean b here Are A&B. Cross shows 50% recombination recombination frequency which indicates a different chromosome when we look at B. And C. That shows a. 20% recombination frequency, which indicates B. And C. Are on the same chromosome. So we can go ahead and eliminate answer choice A. Since B appears to be on the same chromosome as C. Choice B says all genes are located on the same chromosome. However, we can go ahead, eliminate that because we see we have at least three crosses that show a 50% recombination frequency indicating a location on a different chromosome. Choice C. Says jean, C. And D. Are on separate chromosomes from genes A and B. Well, let's look at this. So gene sea and Jean A. We'll start with show a 50% recombination frequency indicating a separate chromosome. So that's all right. Green C. And B. Show a 20% recombination frequency. So c. and B appear to be on the same chromosome. So that means answer C. Is not correct. And finally we have choice. Do you hear choice D. Says gene A. Is located on a different chromosome than the other genes. When we look at our recombination frequencies with gene A. We show A. And B. Is 50% A. And C. Is 50% and A. And D. Is 50%. So all these different crosses with gene A. Show a 50% recombination frequency indicating that yes, you expect to be on a different chromosome than the others. So choice D. Is our answer choice. So given these recombination frequencies, which of the following statements is true. Choice D. Gene A is located on a different chromosome than the other genes. See you in the next video.

Key Concepts

Genetic Mapping

Recombination Frequency

Drosophila Genetics

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