A type of reproduction that combines the genetic material of two gametes (such as a sperm or egg cell or fungal spores). The gametes have an haploid genome (with a single set of chromosomes, the product of a meiotic division) and combines with one another to produce a zygote (diploid). Note that gametes may come from two organisms or from a single organism in the case of self-fertilizing hermaphrodites, e.g. C. elegans, or self-fertilization in plants. Note also that sexual reproduction may be seen as the regular alternation, in the life cycle of haplontic, diplontic and diplohaplontic organisms, of meiosis and fertilization which provides for the production offspring. In diplontic organisms there is a life cycle in which the products of meiosis behave directly as gametes, fusing to form a zygote from which the diploid, or sexually reproductive polyploid, adult organism will develop. In diplohaplontic organisms a haploid phase (gametophyte) exists in the life cycle between meiosis and fertilization (e.g. higher plants, many algae and Fungi); the products of meiosis are spores that develop as haploid individuals from which haploid gametes develop to form a diploid zygote; diplohaplontic organisms show an alternation of haploid and diploid generations. In haplontic organisms meiosis occurs in the zygote, giving rise to four haploid cells (e.g. many algae and protozoa), only the zygote is diploid and this may form a resistant spore, tiding organisms over hard times.

http://purl.obolibrary.org/obo/GO_0019953

https://github.com/geneontology/go-ontology/issues/22929

http://purl.obolibrary.org/obo/GO_0022414