Final answer:
The unexpected restoration of the wild-type phenotype in fruit flies, despite a retaining nonsense mutation, suggests a genetic suppression mechanism, probably due to a second mutation in a tRNA gene mitigating the effects of the first mutation.
Step-by-step explanation:
Mutation Analysis in Drosophila
A forward mutation that caused fruit flies to have green eyes was reversed to a wild-type phenotype by a second exposure to UV radiation. Despite the reversal, DNA analysis showed that the first mutation, a nonsense mutation, was still present, and a second mutation within a tRNA gene was found. This suggests the existence of a genetic compensation or suppression mechanism where the second mutation in the tRNA gene suppressed the effects of the initial nonsense mutation, allowing the wild-type phenotype to be restored. This concept is an example of genetic intricacy where alterations in one part of the genetic code can influence or negate the effects of mutations in another, showing the plasticity and robustness of genetic systems to maintain functionality in the face of mutational pressure.