GMAT Critical Reasoning (CR) | In an experiment, scientists changed

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
...

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