In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F₁, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F₂ generation, and 1000 offspring were counted, with the results shown in the following table. Phenotype Offspring sc s v 314 + + + 280 + s v 150 sc + + 156 sc + v 46 + s + 30 sc s + 10 + + v 14 No determination of sex was made in the data. Determine the sequence of the three genes and the map distances between them.

Video transcript

Hi everyone. Welcome back. Let's look at our next question. It says which of the following statements is true concerning gene crossing over? Well, if we need a little help to recall from our conduct video, what crossing over is we have this little rough diagram here, the two matching chromosomes lined up during meta phase and sometimes they physically cross over each other leading to a swapping of a portion of those chromosomes. So with that kind of image in mind, let's look through our answer choices. We have choice A. The closer the genes are to each other, the more likely they are to cross over. The farther the genes are from each other, the less likely they are to cross over. I see the closer the genes are to each other, the less likely they are to cross over or D. Both A and B. Well, if we again look at this diagram to envision that sort of what's going on physically and crossing over and then imagine a case. So here's our, you know, chromosome here. And if we look at it, imagine a case where we have gene A up here and jean B down here. So they're very far apart. The likelihood of a crossover event occurring somewhere between these. Again, it could be up here down here is higher than if we have a situation where a chromosome looks like this and we have gene A and B right near each other. The likelihood of a crossover event occurring between those two genes is much less likely than when we have this large face here. So with that in mind we can go ahead and look at our answer choices. Choice A is going to be incorrect because it says the closer they are to each other, the more likely they are to cross over. And we've said no, that is not correct. Choice be the farther the jeans are from each other, less likely they are to cross over again, incorrect choice D obviously incorrect since neither A nor B is correct. So here we are at choice. See the closer the genes are to each other, the less likely they are to cross over. And that's correct. Again, with less space between them, there's less likelihood of a crossover event occurring between the two genes. See you in the next video.