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Other animals have different sex-determination systems, such as the ZW system in birds, the X0 system in insects, and various environmental systems, for example in crustaceans.Fungi may also have more complex allelic mating systems, with sexes not accurately described as male, female, or hermaphroditic.
the origin of chromosomal sex determination may have been fairly early in eukaryotes (see Evolution of anisogamy).
For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes.
Among humans and other mammals, males typically carry XY chromosomes, whereas females typically carry XX chromosomes, which are a part of the XY sex-determination system.
The defining characteristic of sexual reproduction in eukaryotes is the difference between the gametes and the binary nature of fertilization.
Multiplicity of gamete types within a species would still be considered a form of sexual reproduction.