Practice question

In Drosophila alternative splicing hierarchy defaults to male mode when Sex-lethal inactive. Absence of Sxl allows transcripts of Sxl itself to retain exon 3 with premature termination producing no protein, transformer pre-mRNA splices to truncated nonfunctional male form, and doublesex retains male-specific exon producing DsxM driving male differentiation and courtship behaviors. This default splicing requires no activator, occurring constitutively unless Sxl redirects spliceosome by binding intronic repressor elements. Hence XY males display male splicing pathway as ground state, while females actively divert splicing via Sxl and Tra to female isoforms.