Q24 - The brains of identical twins

[geverett]

I confidently eliminated B, C, and E. However, I have having a hard time understanding A and D.

[zl7391e]

For D, the first sentence in the stimulus tells us that identical twins have the exact same genetic information. The second sentence tells us that there are cases when only one of the identical twins has schizophrenic.

Suppose two identical twins are called T1 and T2, and they have the same property P, where P denotes the genetic information.

Since T1 and T2 both have P, and we know that only one of T1 and T2 has a disease called S. How can we tell which one has S by looking at P alone and not by some other properties ?

It's almost like asking the question. By looking at the following pairs (T1, P) and (T2, P), tell me which one T1 or T2 but not both has the disease S?

No, we can't (because the property P that they both have is the same thing). We need something else to answer the question. For example, (T1, P, C) and (T2, P, D) where C and D are distinct properties other than P.

Otherwise, we would have a logical contradiction because the only way that T1 has S but T2 does not is due to some factors other than P.

For A, "Genetic susceptibility" is neither mentioned nor defined in the stimulus; thus, it's logically possible that what determines "genetic susceptibility" are factors that combines both genetic information and something else(something other than genetic information such as human's brain size, weight, dietary habits, etc.). Because the stimulus allows this logical possibility that genetic information is not the sole determinant of "genetic susceptibility", it can't prove A false.

I find using symbols easier to explain a question. This may sound very confusing to some. hopefully, it helps a bit. If not, please let me know.

[geverett]

Makes complete sense and seems so obvious now. Thanks!

[rpcuhk]

For D, the first sentence in the stimulus tells us that identical twins have the exact same genetic information. The second sentence tells us that there are cases when only one of the identical twins has schizophrenic.

Suppose two identical twins are called T1 and T2, and they have the same property P, where P denotes the genetic information.

Since T1 and T2 both have P, and we know that only one of T1 and T2 has a disease called S. How can we tell which one has S by looking at P alone and not by some other properties ?

It's almost like asking the question. By looking at the following pairs (T1, P) and (T2, P), tell me which one T1 or T2 but not both has the disease S?

No, we can't (because the property P that they both have is the same thing). We need something else to answer the question. For example, (T1, P, C) and (T2, P, D) where C and D are distinct properties other than P.

Otherwise, we would have a logical contradiction because the only way that T1 has S but T2 does not is due to some factors other than P.

For A, "Genetic susceptibility" is neither mentioned nor defined in the stimulus; thus, it's logically possible that what determines "genetic susceptibility" are factors that combines both genetic information and something else(something other than genetic information such as human's brain size, weight, dietary habits, etc.). Because the stimulus allows this logical possibility that genetic information is not the sole determinant of "genetic susceptibility", it can't prove A false.

I find using symbols easier to explain a question. This may sound very confusing to some. hopefully, it helps a bit. If not, please let me know.

So, would (A) be correct if we replace "a genetic susceptibility " in answer choice (A) with say, "some genetic information"?

(A) seems correct to me because it's too extreme for the stimulus to prove. How can we know people will not develop schizophrenia? The stimulus says that as long as people suffer damage to the brain they might develop schizophrenia.

[wgutx08]

Human diseases are typically due to both genetic and environmental factors. Genetic susceptibility describes the tendency of a person to get certain disease based on his/her genetic information. It has nothing to do with environmental factors (diet, habit, infection etc.) as assumed by zl7391e. In fact, the whole point of having this concept is to differentiate genetic factors from environmental factors and to look at them separately.

Back to your question, no, I don't think A would be correct even if you say "some genetic information" (besides the fact that genetic susceptibility IS "some genetic information").

The stimulus tells us that genetic information alone does not decide if a person will have schizophrenia (which is why D is the ""correct wrong" answer); a certain genetic "makeup" is not sufficient to develop schizophrenia. However, we have no idea if a certain genetic makeup would be necessary for the disease, so A could be true or false, we just don't know.

As an example. Let's say A is true, only ppl with a genetic susceptibility will get schizophrenia -- their genes have to be of a certain type. But this is not enough, you have to have suffered some infection in your infancy as well. All identical twins mentioned in the stimulus have this certain type of genes, but only one of the twins had the infection and thus developed schizophrenia (and a different brain anatomy) later. This scenario is perfectly in accordance with the stimulus right? So A can very well be true and is not the correct answer.

Of course, A can be false too and still does not contradict the stimulus. (Maybe the whole thing has nothing to do with genetics!) So I think you are absolutely right when you say A is "too extreme for the stimulus to prove". But we are not looking for a "not provable" answer, we are looking for an answer that can be proved to be wrong, that contradicts the stimulus.

[PepitoH243]

I don't understand. The author believes that schizophrenia is rather physical than genetic. How can someone develop schizophrenia due to genetic susceptibility?

[MeenaV936]

Because A isn't saying that ONLY genetic susceptibility can determine schizophrenia. Whereas D is saying that ONLY genetic info can determine schizophrenia, and since the twins have identical info and only one had schizophrenia, that means there is no way ONLY genetic info can determine it or not. Always look for the answer that can actually be proven.

[AlexisE386]

(A) means that if someone lacks a genetic susceptibility, then he will not develop S.

This means if someone develops S, then he must have a genetic susceptibility.

Someone may think that since the genes of the twins are identical. If one of them has a genetic susceptibility, then the other must have too. However, only one of them becomes schizophrenic. Therefore, they do not have such a genetic susceptibility. This means someone who does not have a genetic susceptibility could have this disease. So, (A) is false.

However, this reasoning is incorrect.

Having a genetic susceptibility does not mean the person will definitely come down with this disease. Maybe other factors also have to play a role. For instance, maybe one of the twins once hit his head accidentally, which resulted in the damage to the physical structure of his brain. This factor plus the fact that he has a genetic susceptibility led to his disease. Anyone that develops S has a genetic susceptibility, but this susceptibility alone cannot result in this disease.

Therefore, (A) could be true.

(Additionally, the question stem assumes that the statements on which the conclusion is based are all true. It does not assume the conclusion itself is true, and thus there is a possibility that S is caused by the genetic susceptibility plus the physical damage to the brain.)