Practice question

Drosophila sex determination is fixed early during syncytial blastoderm stage before cellularization around nuclear cycle 12-14. X-linked numerator genes transcribed from both maternal and zygotic genomes are counted; high X:A ratio in XX activates early promoter SxlPe producing burst of Sxl protein that then auto-regulates maintenance promoter SxlPm through alternative splicing. This early decision locks sexual fate cell-autonomously in all somatic lineages. Unlike mammalian gonadal determination at week seven, fly determination precedes gastrulation, ensuring consistent sexual identity independent of hormones and long before morphological gonad differentiation becomes visible.