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.
_________________

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