Explanation: While the transgenic fly is expected to have normal transcriptional activity per gene, and a normal biological clock, however, the transgenic fly can have red eyes and per transcriptional activity can be reported as light emissions. Transcription of the transgene results in production of the enzyme luciferase, which acts on luciferin substrate. Therefore, in summary, transgenic per-luc flies will produce light in a pattern that reflects the transcriptional activity of the period gene.
