Practice question

Birds employ ZZ-ZW chromosomal mechanism where females heterogametic and males homogametic. Determination relies on dosage of DMRT1 gene on Z; two copies promote testis development while single copy combined with W-linked elements like HINTW and FET1 promotes ovary development. Early gonad decision leads to differential hormones shaping plumage, song and reproductive tract. W chromosome not inert but contributes anti-testis signals. Unlike haplodiploidy or environmental determination, avian sex remains chromosomal yet differs fundamentally from mammalian XX/XY because dosage of Z rather than presence of dominant Y provides instructive signal.