Not all animals have gender based on the x and y chromosome system humans have. This is due to divergent evolution. Many animals by default have female offspring and it's the presence of something that causes male. For example many amphibians and reptiles have gender based on temperature. If the eggs reach a certain temperature in the nest then they become male, otherwise they are female. In hymenoptera (bees, wasps, ants, etc), though, the default is male and fertilization leads to females. Females have 2 different chromosomes while males have only 1 single chromosome. Birds have an entirely different system too.
Sorry for the long post, I'm a genetics student and that caught my eye!
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applesdontpee,
Not all animals have gender based on the x and y chromosome system humans have. This is due to divergent evolution. Many animals by default have female offspring and it's the presence of something that causes male. For example many amphibians and reptiles have gender based on temperature. If the eggs reach a certain temperature in the nest then they become male, otherwise they are female. In hymenoptera (bees, wasps, ants, etc), though, the default is male and fertilization leads to females. Females have 2 different chromosomes while males have only 1 single chromosome. Birds have an entirely different system too.
Sorry for the long post, I'm a genetics student and that caught my eye!