Drosophila protein-encoding gene has one intron. When the gene is examined the mutant sites are expected. Since, the gene encoding protein has only one intron, the chance of the mutation being found at the exon would be higher.
The chance of the null mutations to be found in the intron would be comparatively low as there is only one intron in the entire gene. The intron would be continued with specific sequences that are requisite, for the splicing mechanism to happen. But, if there is any mutation at this point of the intron, then it would necessarily end up in changing the entire output of the gene. Though transcription of the gene occurs, translation is not supposed to occur.
If there is any mutation in the gene sequence in the promoter, then it is tough to transcribe. In order to initiate transcription the promoter region is highly significant. Any mutation in this area of the gene would make the gene unavailable, for RNA polymerase to attach to the gene.
The boundary between the intron and exon are highly essential, for the process of splicing to occur. Any change in the sequence at this boundary would hamper the splicing process.
Though, transcription occurs, without any proper processing of the RNA transcript, translation is not likely to happen in the cytoplasm, for the proteins to synthesize.