Kaplan FL 8 B/B Question Spoiler

[betterfuture]

Could someone please explain what double crossover's product is? What counts as a double cross over?

---

Members don't see this ad.

 

[Marine2MD]

Think of it as a crossover that happens twice in this scenario - A link occurs between AB and ab which is one crossover so there is a swap of the genes there - Abcde and aBCDE. Then another crossover would occur at the points between CD and cd which allows for another swap - AbcDE and aBCde. So you can eliminate A from the start as it is a single swap. B since there is a mutation in the second portion ABdcE instead of ABcdE, C is a triple crossover, which only leaves D.

Basically it is just a swap of alleles at two points as long as there are no mutations.

 

[betterfuture]

So does this count as a double crossover?

 

[theonlytycrane]

It looks like (c) would also work as the "bc" and "e" region are both swapped.

 

[Marine2MD]

Don't look at it as single letter swaps, look at like a segment swap. ABcde and abCDE is the first swap. So the two connected/touched together or however you want to think about it at whatever point. Cut the strands and swap the alleles. Now move further down the chromosome and have another section connect/touch and cut strands, swap alleles again. That is kind of the mechanism but a double crossover is when chromosomes literally crossover each other twice. So the middle portion of the alleles swap the ends remain the same.

No, C is a triple crossover. You have 2 segments from each chromosome. You should only get 1 segment from the opposite chromosome and 2 from the original chromosome.

 

[theonlytycrane]

This is how I'm thinking of (d). I think it's wrong but I'm trying to straighten out my reasoning!

 

[betterfuture]

This is how I think of it. I have numbered the steps so you can follow through. I have labeled the areas where what stays and what gets swapped.

And I followed this thru with @Marine2MD 's method.

 

[theonlytycrane]

Aren't both methods possible? Couldn't a single swap containing regions "bc" occur?

 

[aalamruad]

Aren't both methods possible? Couldn't a single swap containing regions "bc" occur?

A single swap containing only BC couldn't occur. That would require 2 swaps. At a point of crossover, everything on one side of that chromosome is swapped to the homologous chromosome. For BC to be swapped, there would have to be a crossover right after A, which would swap BCDEFGHIJ...etc to the other chromosome, and then you would need another crossover (making it a double crossover) right after C, to swap DEFGHIJ...etc back to the original chromosome, leading to a "net" swap of only BC.