Question about the location of alleles before recombination

[davcro]

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My question stems from a figure in the answer key for BR Bio part 2 (pg 365). Click the link below to view it:

http://min.us/luVNmWxJJ1UAo

The figure shows two chromosomes whose chromatids have different alleles. These chromosomes are recombining so they must be in prophase 1 of meiosis. Shouldn't that be impossible? I thought chromosomes were composed of identical chromatids before crossing over. This is because one chromatid was replicated from the other during S-phase.

 

[MedPR]

My question stems from a figure in the answer key for BR Bio part 2 (pg 365). Click the link below to view it:

http://min.us/luVNmWxJJ1UAo

The figure shows two chromosomes whose chromatids have different alleles. These chromosomes are recombining so they must be in prophase 1 of meiosis. Shouldn't that be impossible? I thought chromosomes were composed of identical chromatids before crossing over. This is because one chromatid was replicated from the other during S-phase.

I agree with you, this doesn't seem possible. No offense to you, but I'm generally inclined to think that the book is right and I'm wrong when I disagree with it. I'm interested to see what the others have to say.

 

[davcro]

I agree with you, this doesn't seem possible. No offense to you, but I'm generally inclined to think that the book is right and I'm wrong when I disagree with it. I'm interested to see what the others have to say.

Same here, the authors know way more about bio than I do. But if the diagram is correct, than my entire understanding of meiosis is wrong.

Our context here is Prophase I, just before recombination. At this point chromosomes must be composed of two identical chromatids. Why? Because one chromatid was replicated from the other. They remain joined together after replication.

The diagram in the book shows a chromosome composed of different chromatids, one from the father and one from the mother. This is impossible! For this to occur, chromosomes must split into two chromatids after replication. Then the chromatids randomly join together to form chromosomes before recombination.

 

[davcro]

I made a quick diagram to explain my argument:

This is wrong

This is right

 

[MedPR]

Yea I'm in the same boat as you. I hope someone can answer!

 

[gettheleadout]

It says the crossover is experienced between a pair of sister chromatids and a pair of non-sister chromatids (these are the B and b mismatched alleles.) How a pair of non-sister chromatids comes about before crossover, I don't know, so basically I agree with you guys. If however, the situation proposed in the question is possible, then the question makes sense and it isn't an issue of the crossover they want you to do.

 

[MT Headed]

Meiotic crossover doesn't always go according to plan. A lot of the original TBR page is covered up, but it almost sounds like they are trying to set up a case of chromosomal translocation

http://en.wikipedia.org/wiki/Chromosomal_translocation

This can lead to a fully functional adult that produces non viable children half the time. They hit the concept of Philadelphia chromosome pretty hard in my genetic class

http://en.wikipedia.org/wiki/Philadelphia_chromosome

But this is mighty hard core genetics and way beyond the scope of the MCAT.

 

[MedPR]

Ok so this question/explanation are an exception, or rare case/detail in genetics? But typically crossing over occurs in prophase 1. And for the MCAT, unless explained in a passage, we can assume prophase 1 for crossing over, right?

 

[MT Headed]

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