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.
Sex comprises the arrangements that enable sexual reproduction, and has evolved alongside the reproduction system, starting with similar gametes (isogamy) and progressing to systems that have different gamete types, such as those involving a large female gamete (ovum) and a small male gamete (sperm).
In complex organisms, the sex organs are the parts that are involved in the production and exchange of gametes in sexual reproduction.
Some bacteria use conjugation to transfer genetic material between cells; while not the same as sexual reproduction, this also results in the mixture of genetic traits.
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.