Practice question

ZW chromosome system defines female heterogamety found predominantly in birds, some reptiles, fish and Lepidoptera butterflies where females are ZW and males ZZ. In birds two copies of DMRT1 on Z promote testis formation while single copy plus W-linked factors HINTW and FET1 promote ovary. This system evolved independently from mammalian XY multiple times, demonstrating convergent evolution of sex chromosomes where heterogametic sex differs. Identification of ZW in birds contrasts with mammalian XX/XY and Drosophila XY but X:A counting, illustrating evolutionary diversity of mechanisms achieving sexual dimorphism.