Next Step Full Length 1 - Bio/Biochem Q24

[maikelm]

I'm reviewing my FL 1, and I'm confused on Question 24 of the Bio/Biochem section.

Q) A follow-up study by the researchers showed that the Y chromosome experiences a much higher mutation rate than the X chromosome. This is likely due to the fact that:
A) the X chromosome encodes more proteins responsible for genetic repair than the Y chromosome.
B) the Y chromosome is much smaller and therefore more susceptible to mutation.
C) the Y chromosome goes through many more divisions than the X chromosome in a typical population.
D) the Y chromosome has significantly fewer genes, making mutations more evident.

Now, I thought it was D because having less genes on the Y chromosome will allow the mutations to have phenotypical changes, because it will be more dramatic. However, NextStep claims most of the DNA on the Y chromosome is junk DNA "therefore, mutation of the Y chromosome is actually relatively unlikely to have a phenotypic effect." Is this true? I never heard of this.

The answer is C. The explanation by NextStep is: "The Y chromosome is inherited through sperm, which undergo many divisions during gametogenesis. Each cellular division makes mutation more likely. Additionally, sperm develop in the testes, which exist under unusually oxidative conditions; this also promotes mutation. The X chromosome undergoes fewer total replications in the population due to the limited number of replicative rounds during oogenesis. Additionally, females have crossing-over events to restore function to damaged X chromosomes." Don't males and females undergo the same process of meiosis? I know females are born with a set amount of ovums, so is that what NextStep is hinting at?

Can someone explain to me what's going on with this question?
Thanks

@NextStepTutor_1 @NextStepTutor_2 @NextStepTutor_3

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[NextStepTutor_1]

I'm reviewing my FL 1, and I'm confused on Question 24 of the Bio/Biochem section.

Q) A follow-up study by the researchers showed that the Y chromosome experiences a much higher mutation rate than the X chromosome. This is likely due to the fact that:
A) the X chromosome encodes more proteins responsible for genetic repair than the Y chromosome.
B) the Y chromosome is much smaller and therefore more susceptible to mutation.
C) the Y chromosome goes through many more divisions than the X chromosome in a typical population.
D) the Y chromosome has significantly fewer genes, making mutations more evident.

Now, I thought it was D because having less genes on the Y chromosome will allow the mutations to have phenotypical changes, because it will be more dramatic. However, NextStep claims most of the DNA on the Y chromosome is junk DNA "therefore, mutation of the Y chromosome is actually relatively unlikely to have a phenotypic effect." Is this true? I never heard of this.

The answer is C. The explanation by NextStep is: "The Y chromosome is inherited through sperm, which undergo many divisions during gametogenesis. Each cellular division makes mutation more likely. Additionally, sperm develop in the testes, which exist under unusually oxidative conditions; this also promotes mutation. The X chromosome undergoes fewer total replications in the population due to the limited number of replicative rounds during oogenesis. Additionally, females have crossing-over events to restore function to damaged X chromosomes." Don't males and females undergo the same process of meiosis? I know females are born with a set amount of ovums, so is that what NextStep is hinting at?

Can someone explain to me what's going on with this question?
Thanks

@NextStepTutor_1 @NextStepTutor_2 @NextStepTutor_3

Hi mikeman1994,

Excellent question! This is an interesting question that I think provides a good example of how to approach questions that ask you to assess hypothetical explanations. For any explanation to be correct, it has to both (1) be empirically consistent with the passage and/or general scientific information and (2) logically link empirical information to the hypothesis proposed in the question stem. Or, to be more concise, you're looking for an answer choice that is right and makes sense.

For some questions like this you can easily knock out some answer choices by looking for clearly false statements, but that's not true here. So you have to assess whether the answer choices provide a plausible explanation for mutation rate. This last part is important. You have to match the logic for the correct answer choice directly with the wording of the question. We're not talking about the severity of mutations, how phenotypically obvious they are, or anything else -- just the rate.

The other key to this Q is that you can think of mutations as being something that occur at some fixed rate per # of replications or due to external sources of damage. This may be a slight simplification but it's appropriate for the MCAT. More replications = more mutations, and more damage to a specific set of genes = more mutations.

With this in mind...

A is out because it doesn't actually differentiate between X and Y chromosomes. Although the X chromosome may encode more proteins responsible for genetic repair, those proteins act throughout the genome, not in a chromosome-specific way. Also, we're primarily interested in how many mutations occur, not how well they get repaired, so something about genetic repair would only be plausible as an absolute last resort.

B is out b/c size has nothing to do w/ susceptibility to mutation on a molecular level. Even if you did try to build up an argument along the lines of "the Y chromosome is small, so it contributes to a small proportion of all the mutations to the genome..." it would point in the opposite direction of this answer choice.

C is actually right because the details of spermatogenesis imply that the Y chromosome is replicated many, many, many times in the population. To reply to your substantive question, yes, this has to do w/ the differences between oogenesis & spermatogenesis. A female is born with something on the order of ~1 million eggs, which sounds like a lot, but a male of reproductive age can produce hundreds of millions of sperm cells per day. So on a population level a tremendous amount of replication happens in the Y chromosome. Remember our logic from above that more replication = more mutation.

D is out for several reasons. The simplest reason is that for the purposes of this question, we don't care about how evident the mutations are -- that's not what the question is asking for. Also, in response to your question, the phenotypic effect of mutations is not a property of chromosome size. Proteins encoded from all of the different chromosomes work together in ridiculously complex ways to generate a phenotype, so the relationship is a lot more complicated than this answer choice would imply.

Hope this helps, and best of luck!