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.

Question: Which one of the following is an assumption required by the argument?
Options:
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.
Answer: "B"