Why do humans rely on XY chromosomes to determine sex, while birds use ZW—and many reptiles use neither, letting temperature decide instead? Sex determination is remarkably diverse across the animal kingdom. In genotypic sex determination (GSD), sex is encoded in the genome: XX individuals are female and XY males in humans and most mammals, while in the ZW system, found in birds and some reptiles, males are ZZ and females ZW.

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