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

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.­