PrepTest 122, Section 1, Question 10
In an experiment, scientists changed a single gene(50 more words not shown)
Which one of the following(7 more words not shown)
- AThe relationship between(8 more words not shown)
- BNo other gene(16 more words not shown)
- CUltraviolet vision is(8 more words not shown)
- DThe gene change(14 more words not shown)
- EUltraviolet vision is(12 more words not shown)
Reading the argument
That is one way to lose ultraviolet vision. Is it the only way?
Changing this gene left the flies without the cells needed for ultraviolet vision. Their otherwise genetically identical siblings had normal vision. That is evidence that this gene matters. But the conclusion goes further: any fly of this species lacking ultraviolet vision must have damage to this particular gene.
What if a different gene is also needed? A fly could have an intact copy of the gene the scientists studied but damage to another gene involved in building those cells. It would then lack ultraviolet vision for a different reason. Showing that one change can produce a problem doesn't show that every instance of the problem comes from that change.
What we’re looking for
The necessary-assumption question asks what the argument needs to rule out. I expect an answer ruling out another route to losing ultraviolet vision. We don't need a complete explanation of every biological detail; we need to protect the claim that a fly without this vision must have damage to this gene.
Working the answers
- A
“Well understood” is vague and much broader than the argument requires. Scientists might establish the relevant connection without understanding every aspect of genes and vision. Even extensive understanding wouldn't, by itself, rule out another way to lose ultraviolet vision.
- BCorrect
If another gene were required to form these cells, damage to that other gene could prevent their formation while leaving the gene studied here intact. That would give us the alternative explanation the conclusion excludes.
B closes off that specific route. It doesn't by itself prove that there are no other possible causes, such as a nongene-related failure. But the argument does need this particular competing genetic explanation to be unavailable. A necessary assumption can protect part of an argument without completing the whole proof.
- C
The conclusion concerns flies of this species. Another species might have ultraviolet vision or might lack it entirely. We don't need a claim about all fly species to evaluate the proposed explanation for this one.
- D
The gene change could also affect some other trait. Perhaps the altered flies differ in a way unrelated to vision. That wouldn't establish another cause of their vision loss, and the argument doesn't need the gene to have only one effect. Keep “other effects of this cause” separate from “other causes of this effect.”
- E
The argument doesn't require environmental influence. If anything, an environmental route to losing ultraviolet vision could create another explanation the conclusion would have to address. E doesn't rule out an alternative cause or supply a claim the genetic conclusion depends on.
More Necessary Assumption questions
Learn the LSAT our way
Our platform drills Necessary Assumption questions at your level and adjusts as you improve.