#10 - In an experiment, scientists changed a single gene in

[Administrator]

Complete Question Explanation

Assumption. The correct answer choice is (B)

The scientists' experiment involves a comparison between two types of cloned flies. One type had an altered gene and lacked ultraviolet vision; the other type sustained no genetic alterations and had normal vision. From this observation the scientists concluded that flies without ultraviolet vision must have some damage to that particular gene.

This argument is suspect. If the two types of flies are otherwise genetically identical save for the damaged gene in the flies that lacked UV vision, the scientists' experiment only establishes that the gene in question is necessary for the formation of ultraviolet cells:

• Damaged Gene UV
  
  UV →No Genetic Damage

This observation does not, however, establish that all flies without UV vision must have some damage to their gene: such a conclusion is a Mistaken Reversal of the proper conditional relationship between the flies' genetic makeup and their vision. It is perfectly possible that some flies without ultraviolet vision had damage to a different gene (or genes), suffered from a horrible disease, or stared at the sun for too long. Either way, the scientists' experiment does not prove that the altered gene was sufficient for the production of UV vision cells.

An observant test-taker will also notice that causal and conditional reasoning overlap in this argument. The conclusion only proves that the altered gene is a necessary cause for the formation of UV vision, giving us no reason to suspect it is the only cause.

An Assumption question here will almost certainly ask you to provide a Defender Assumption, and you would be wise to formulate it before looking at the five answer choices. If the central weakness of the argument is that other genes may also play a role in the formation of UV vision cells, the Defender Assumption would probably be that they do not. Indeed, the argument relies upon the unstated assumption that the altered gene is the only gene required for the formation of such cells.

Answer choice (A): While some understanding of the relationship between genes and vision in flies may be a necessary assumption in this argument, good understanding is not. This answer choice is incorrect.

Answer choice (B): This is the correct answer choice. To conclude that the lack of UV vision results from damage to the gene in question, the author must have assumed that no other gene could play a role in the formation of such vision. In other words, the author assumes that the damaged gene is not merely necessary for UV vision, but is also sufficient for its formation. Try the Assumption Negation technique: if the phenomenon of UV vision is a function of multiple genes, then the absence of such vision can mean that any one of these genes may have been damaged — not necessarily the ones altered by the scientists. Because this would weaken the argument, answer choice (B) is correct.

Answer choice (C): It is entirely unnecessary that UV vision is found in all species of flies — only the ones examined by the scientists. This answer choice goes too far, and is therefore incorrect.

Answer choice (D): Whether the gene change had other effects on the flies is irrelevant to the conclusion. Even if it did, this would not undermine the proposition that flies without UV vision must have sustained damage to that gene. This answer choice is incorrect.

Answer choice (E): If UV vision were an environmentally influenced trait, then the lack of UV vision may have nothing to do with the flies' genetic makeup. Since this answer choice weakens the argument, it is incorrect.

[mariahenain]

Dear PS,

I'm having some trouble wrapping my head around the explanation given. I got this question right, simply by process of elimination but I want to be sure I can spot SN in the future. My thought process was to narrow the two contenders down to B and C. I figured C wasn't absolutely necessary to be true, and B had negative language so I chose it because I recognized this as a Defender Assumption.

I guess my questions are: First, what are the SN indicators outside of the conclusion? I now see in the conclusion "must have some damage..." but I can't see it anywhere else and thus I didn't diagram this question. I'm having some trouble understanding the diagramming of the stimulus above as well.

Secondly, how does one attack a question with both conditional and causal reasoning?

Any help given or general advice for questions of this type would be greatly appreciated!

[nicholaspavic]

Hi Maria,

The phrase "even though" acts a counterpremise indicator. Aside from that, I see no indicators other than the conclusion's laguage which you already noted.

With respect to your second question, in this stimulus, conditional logic is being employed to try to establish causality so think of it as cause effect. Try to see how conditional logic can capture that relationship quite well because

(cause) Damaged Gene (effect) UV.

Also, remember there are a variety of ways to strenghten causality, one of which is to eliminate alternative causes. That's what Answer (B) is is seeking to do. It's eliminating an avenue of attack that would undermine the conclusion.

But most importantly, never forget The Assumption Negation Technique(TM). It is the most important weapon in your quiver when attacking an Assumption question.

It sounds like you correctly narrowed it down to two choices and at that point all you have to do is negate them to see if they attack the conclusion: "Thus, scientists have shown that flies of this species lacking ultraviolet vision must have some damage to this gene."

In negating Answer B "[Another] gene in the flies in the experiment is required for the formation of the ultraviolet vision cells," that statement directly attacks the conclusion while Answer Option (C) become irrelevant.

Thanks for the great question!

[silent7706]

Hello,

The Central Assumption of Basic Causal Conclusions in the LRB states that "When an LSAT speaker concludes that one occurrence definitively caused another, that speaker also assumes that the stated cause is the only possible cause of the effect and that consequently the stated cause will always produce the effect."

Applying the Central Assumption of Basic Causal Conclusions literally helped me predicted (B). I just want to make sure I did not mistakenly apply the principle here.

Thanks in advance.

[Erik Shum]

Hi Silent,

The stimulus does assume that there is only one possible cause to the effect and, therefore, when the flies lack UV vision, that damage to that gene must have taken place.

That assumption provides you with almost everything you need to arrive at the correction answer choice (B). Applying the Central Assumption is absolutely helpful for identifying the assumption made in this stimulus.

[crispycrispr]

I gotta say, this question is where being a biology major really pays off! Most experiments manipulating genes operate on this very basic frame of hypothesis--to find whether a gene is necessary or not, you knock it out, e.g., via CRISPR technology, but to find whether a gene is sufficient, you have to do a "knock-in" approach.

[Girlpower]

I read this as saying that there could be multiple necessary conditions, so I ruled out B. Even if there were other genes required, I don't see how that destroys the conclusion. You could have Gene x and Gene z required, and you could still need both.

[Dana D]

Hey Girlpower,

What are the other necessary assumptions you identified from the stimulus? The argument suggests there is only one required for ultraviolet vision - the damaged gene in question. Answer choice (B) is correct because it solidifies that this is the case - if not for the damage to this gene, the flies would have ultraviolet vision, because (B), no other gene is required for the formation of the ultraviolet cells.