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- Thread starter fa21212
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Hi @fa21212 -
Thanks for the questions! Let me take them one by one.
First, regarding the ends vs. middle question, think about what happens if a recombination affects the very end of the chromosome – that is, if the last set of genes/codons is “switched.” For this to happen, a single recombination must have taken place (or, technically speaking, an odd number of recombinations, but we could break those down into a set of double recombination events + a single recombination). In contrast, consider what happens if there’s only a limited area of recombination in the middle (loosely speaking) of the chromosome – that is, if the area of “switching” starts somewhere and then switches back, so that the ends of the chromosomes are the way they were before. For this to happen, we need a double recombination. I’ve attached an image showing what we mean.
I think the point of confusion might have been interpreting “end” and “middle” as statements about where in the chromosome a given type of recombination is likely to be initiated – I’m not aware of any evidence of that either – whereas instead the idea has more to do with how recombinant segments affecting the ends vs. just an area in the middle are generated.
Re: answer choice D, which talks about arms, it's important to distinguish between the arms (i.e., the p and q arms, separated by the centromere), and strands. (Although it's worth noting that in our experience, 3- and 4-strand double crossovers are extremely unlikely to come up on the MCAT.) In any case, we'll review this answer choice with the FL team and make any necessary updates to improve its clarity.
Thanks for the questions! Let me take them one by one.
First, regarding the ends vs. middle question, think about what happens if a recombination affects the very end of the chromosome – that is, if the last set of genes/codons is “switched.” For this to happen, a single recombination must have taken place (or, technically speaking, an odd number of recombinations, but we could break those down into a set of double recombination events + a single recombination). In contrast, consider what happens if there’s only a limited area of recombination in the middle (loosely speaking) of the chromosome – that is, if the area of “switching” starts somewhere and then switches back, so that the ends of the chromosomes are the way they were before. For this to happen, we need a double recombination. I’ve attached an image showing what we mean.
I think the point of confusion might have been interpreting “end” and “middle” as statements about where in the chromosome a given type of recombination is likely to be initiated – I’m not aware of any evidence of that either – whereas instead the idea has more to do with how recombinant segments affecting the ends vs. just an area in the middle are generated.
Re: answer choice D, which talks about arms, it's important to distinguish between the arms (i.e., the p and q arms, separated by the centromere), and strands. (Although it's worth noting that in our experience, 3- and 4-strand double crossovers are extremely unlikely to come up on the MCAT.) In any case, we'll review this answer choice with the FL team and make any necessary updates to improve its clarity.
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Hi @fa21212 -
Yes, that's absolutely correct about 4-strand double crossover events...again, though, keep in mind that they're pretty unlikely to come up on the MCAT. A good undergraduate genetics textbook will definitely contain more in-depth information than you'll need to apply. On one hand, there's nothing wrong with studying the material in more depth than is strictly speaking necessary (if you have the time to do so w/out taking away from other studying/practice), but on the other hand, you do want to develop instincts for the level of depth that the MCAT is going to be looking for. A careful review of all of the AAMC materials—as well as realistic third-party materials—may be helpful for calibration purposes.
Yes, that's absolutely correct about 4-strand double crossover events...again, though, keep in mind that they're pretty unlikely to come up on the MCAT. A good undergraduate genetics textbook will definitely contain more in-depth information than you'll need to apply. On one hand, there's nothing wrong with studying the material in more depth than is strictly speaking necessary (if you have the time to do so w/out taking away from other studying/practice), but on the other hand, you do want to develop instincts for the level of depth that the MCAT is going to be looking for. A careful review of all of the AAMC materials—as well as realistic third-party materials—may be helpful for calibration purposes.