Practice question

Testis determination in mammals absolutely requires Sox9, an HMG-box transcription factor functioning downstream of Sry. Sry expressed briefly in XY supporting precursors binds testis-specific enhancer TESCO together with Sf1 to trigger Sox9 transcriptional surge. Sox9 then establishes feed-forward loops with Fgf9 and prostaglandin D2, autoregulates, represses ovarian Wnt4/β-catenin pathway, activates Dhh, Amh, Sox8 and extracellular matrix genes driving testis cord formation. Loss-of-function XY gonads develop as ovaries, while ectopic Sox9 in XX gonads induces complete testes without Sry, proving central indispensability.