Aforward mutation resulting in a green-eyed phenotype was generated in fruit flies by uv exposure. a second uv exposure, however, reversed that mutation as measured by the return of the wild-type phenotype. a researcher conducted a dna analysis of these mutations. to her surprise, it revealed that the original "green-eye" mutation, which she identified as a nonsense mutation, was still retained in the now normal organism. moreover, she identified a second mutation within a second gene, which was a trna gene. this researcher found evidence of a(n)