> Having genetic differences between males and females is mostly a bird and mammal thing, at least among vertebrates.
Also: the configuration and function of sex chromosomes is not consistent even within mammals. There are a number of species - primarily rodents - with unusual sex-determining systems, like species with XX/X0 (i.e. where males have an unpaired X chromosome) or even X0/X0.
Important plot element in Jurassic Park!