An interesting mutation is discovered in a species of wasp where the DNA sequence of the genome is unknown. In order to try to determine the molecular nature of this mutation, a geneticist treats mutant wasps with the chemical 5-bromouracil, a chemical mutagen that induces both types of transition mutations at high frequencies, and hydroxylamine, which induces only GC-to-AT transitions. Neither chemical mutagen caused the back mutation or reversion frequencies to increase beyond the low spontaneous values. Which of these types of mutations is the least likely to be the original wasp mutation