Drosophila females homozygous for the third chromosomal genes pin...

Question

Drosophila females homozygous for the third chromosomal genes pink and ebony (the same genes from Problem 16) were crossed with males homozygous for the second chromosomal gene dumpy. Because these genes are recessive, all offspring were wild type (normal). F₁ females were testcrossed to triply recessive males. If we assume that the two linked genes, pink and ebony, are 20 mu apart, predict the results of this cross. If the reciprocal cross were made (F₁ males—where no crossing over occurs—with triply recessive females), how would the results vary, if at all?

Accepted Answer

Hey, everyone. Let's take a look at this practice problem together. What might be the order of the jeans and the linkage map? If the recombination frequency between genes A and B is 6% between A and C is 14% and between B and C is 20%. OK. Recall that recombination frequency is not an exact measurement of distance, but it's an approximation. And that one recombination frequency equals one map unit. And we use map units to help us draw out linkage map. Also recall that genes that are further apart from each other have a higher R F or recombination frequency and genes closer together have a lower recombination frequency. So I'll draw a line to represent a chromosome and the distance of this line is going to be equal to the highest percentage of our recombination frequency because again, that means they'll be the furthest apart. So in our Question, is the furthest apart at 20 map units. And that means that they will be the furthest away from each other. So our gene order will either start or end with B or C. Remember though we can't technically tell which gene goes first because recombination frequency doesn't tell us that. But I'm going to put B here And I'll put, see here. Well, using this information, we can already deduce the answer, but we're going to go ahead and finish the order of jeans for practice. So the distance from B to C is map units. Now, we're told the distance from A to B A 6% or six map units and A two C is 14 map units. So that means that A will be closer to B than it is to see. So I'm gonna put a right here And this distance is going to be six map units And this distance is going to be 14 map units. And so a diagram of our genes and their order would look like the following. So for us, our order of genes is B A C and that is option D. So D would be the answer. However, though, remember, we don't know whether it's B or C that is the start of the genes on the chromosome. And so technically, our genes could also appear in the order C A B, but that's not an option. So D is our answer. Alright, everyone, I hope you found this helpful and I'll see you for the next practice problem.

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