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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.
Multiplicity of gamete types within a species would still be considered a form of sexual reproduction.
However, no third gamete is known in multicellular animals.
The gametes produced by an organism are determined by its sex: males produce male gametes (spermatozoa, or sperm, in animals; pollen in plants) while females produce female gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic.
Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience.
the malaria parasite) also engage in sexual reproduction.
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.
Other animals have a sex-determination system as well, such as the ZW sex-determination system in birds, and the X0 sex-determination system in insects.
Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, that sex helps in the removal of disadvantageous traits, and that sex facilitates repair of germ-line DNA.
Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria.
As sexual reproduction developed by way of a long process of evolution, intermediates exist.