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

In another set of experiments, a sixth gene, d, was tested against b and pr:

d–b 17%
d–pr 23%

Predict the results of two-point mapping between d and c, d and vg, and d and adp.

Accepted Answer

Hi everyone. Welcome back. Let's look at our next problem. It says four genes are all located on the same chromosome. The genes have the following recombination frequencies. And when we have several different recombination frequencies listed between all the different genes there. Then it says considering the mentioned data, which the following images illustrates the correct gene sequence in the chromosome. And I'm gonna scroll down here so we can see all the different maps. And I'm also going to copy and paste the frequencies because we can't see them once we've scrolled down here. Okay, so there's our frequency list. So the kind of easiest way to start is perhaps to look for the greatest distance, assuming that those two genes will be um they're the farthest apart. And so therefore will be the two N. Genes on our map. So when I look for my farthest distance here, I see that that is the Jeans G. And H. Which have a recombination frequency of 27%. So when I look at my answer choices, I'm going to look for a map that puts G. And H the farthest apart on either end of the chromosome. And I see here in choice A. I've got G. And H. Select like G. And H. Here for this department. When I look at my other answer choices for G. N. H. I see in choice B. G and H are in the middle here closest together. So that's not going to be our answer. In choice C. G and H. Are next to each other on one end. That's not going to be our answer choice D. They're also don't have any other chromosomes in between them. So you'd expect to see the highest um Excuse me, the lowest frequency of recombination between them. So D. Is also incorrect. So we've now um already just by that one um gene frequency determined that choice A. Is our correct answer. But to be thorough, let's go through and look at the other values here to confirm that this is the correct order here. So when we look at our lowest frequency of recombination here, We're going to see that that occurs between G. And I at only 3%. And when we look at our map, Fergie and I. We see that deed G. And I are located the closest together. G. We know is on an end. So we expect I to be next to it in order. And then if we start from G. With I the closest to it, we'd expect to see um the G. F. Recombination frequency being in between G. I. And G. H. And indeed when we look at that. So G. I. is 3%, G. H. Is 27%. And then when we look at G. F, we see it's 18% it's in between those two. And if we went the other direction we see a similar phenomenon um H. G. Being the largest frequency recombination. We expect to see H. F. Being the smallest. It's 9%. Then H. I. is 24% with HG 27%. So this map is definitely confirmed when we look at all those numbers. And so we see that the image that correctly illustrates or that illustrates the correct gene sequence in the chromosome is choice A. Here and the order G. I. F. H. See you in the next video.

Key Concepts

Genetic Linkage

Recombination Frequency

Two-Point Mapping

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