In an experiment, scientists changed a single gene in cloned flies of

Question

In an experiment, scientists changed a single gene in cloned flies of a certain species. These cloned flies lacked the eye cells that give flies ultraviolet vision, even though cloned siblings with unaltered, otherwise identical genes had normal vision. Thus, scientists have shown that flies of this species lacking ultraviolet vision must have some damage to this gene.

Which one of the following is an assumption required by the argument?

(A) The relationship between genes and vision in flies is well understood.
(B) No other gene in the flies in the experiment is required for the formation of the ultraviolet vision cells.
(C) Ultraviolet vision is a trait found in all species of flies.
(D) The gene change had no effect on the flies other than the lack of ultraviolet vision cells.
(E) Ultraviolet vision is an environmentally influenced trait in the species of flies in the experiment.

Bunuel · Accepted Answer

EXPLANATION FROM Fox LSAT

This argument assumes that any  one  cause of blindness must be the  only  cause of blindness, which is stupid and wrong. I can make this more obvious by offering a parallel argument to the argument made here. Ready?

“I jammed an icepick into both eyes of a human research subject, and the subject was thereafter blind. All other study subjects, who did not have an icepick jammed into both eyes, could still see perfectly. Therefore anyone who is blind must have had an icepick jammed into their eyes.”

That’s the exact same logic behind the given argument, and it’s patently ridiculous. The assumption  required  by this argument is that if anything causes blindness, it must be present in every single person who is blind.

Another example of this same flaw would be this:

“Exposure to radiation from a nuclear meltdown causes cancer. Therefore anybody with cancer must have been exposed to radiation from a nuclear meltdown.” Here, the assumption is, “If something causes cancer, then that thing is present in every cancer sufferer.”

Since we’re asked for a necessary assumption, we’re going to pick the answer choice that,  if untrue , would cause the argument to fail. That’s the definition of an assumption “required” by the argument.

A) What? No. This is meaningless. There’s no way this can be a necessary part of the argument. Even if it’s not well understood, well, that’s the reason why the scientists are studying it! No way.

B) This is closer, but I don’t think it’s necessary. If you negate B you get, “Some other gene is required for the formation of ultraviolet vision cells.” If you tried to use that as an attack against the speaker, the speaker might say, “Well obviously there are many genes required to create any trait, but my point is that damage to this one gene is a contributing factor to every case of ultraviolet blindness.”

C) No way. “All species of flies” are not at all relevant. The only thing that’s relevant is the species actually being discussed.

D) No, the gene change could have also made the flies paralyzed, in addition to making them blind. If that were true, it would do nothing to ruin the idea that this gene defect must be present in all cases of ultraviolet blindness. No way.

E) This answer starts off as a weakener, so it can’t be something required by the argument. If you negate this answer, it actually strengthens the argument (by eliminating environmental factors as a potential cause of ultraviolet blindness.) So this is out.

I dislike all five answer choices, but I seriously hate A, C, D, and E and only mildly dislike B. I can see how “no other gene is involved” is a strengthener. I can see how if you negate B, you get “some other gene is involved,” and that’s a threat to the idea that  every  fly with ultraviolet blindness must have a defect in one  specific  gene. I still don’t think this is “ necessary ” in the strictest sense of the word, but it’s the only conceivable answer choice. So our answer has to be B. Tough question, because it doesn’t fit neatly into the “necessary assumption” mold.

The answer is B.

Bunuel · Answer

EXPLANATION FROM POWER SCORE

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.

winterschool · Answer

In an experiment, scientists changed a single gene in cloned flies of a certain species. These cloned flies lacked the eye cells that give flies ultraviolet vision, even though cloned siblings with unaltered, otherwise identical genes had normal vision. Thus, scientists have shown that flies of this species lacking ultraviolet vision must have some damage to this gene.

Which one of the following is an assumption required by the argument?

(A) The relationship between genes and vision in flies is well understood.  Incorrect

cant be an assumption

(B) No other gene in the flies in the experiment is required for the formation of the ultraviolet vision cells.  Incorrect

but lacking ultraviolet vision can be for some other side effects occurred by the changes in gene

(C) Ultraviolet vision is a trait found in all species of flies.  Incorrect

additional info

(D)  The gene change had no effect on the flies other than the lack of ultraviolet vision cells.   Correct

so, the gene change is the only reason for lacking ultraviolet vision

(E) Ultraviolet vision is an environmentally influenced trait in the species of flies in the experiment.  Incorrect

additional info

VerbalBot · Answer

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In an experiment, scientists changed a single gene in cloned flies of

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In an experiment, scientists changed a single gene in cloned flies of

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