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See genus (mathematics) for the use of the term in mathematics.
See genus (music) for the use of the term in music.

In biology, a genus (plural genera) is a grouping in the classification of living organisms having one or more related and morphologically similar species. In the common binomial nomenclature, the name of an organism is composed of two parts: its genus (always capitalized) and a species modifier. An example is Homo sapiens, the name for the human species which belongs to the genus Homo. See scientific classification for more details of this system.

The type genus of a taxon is usually the first genus to be named and described. Families, and in plants all taxa up to division, are named after the type genus. The genus and these higher taxa are typified by a specimen that shows the characteristics of the genus. The specimen used to describe this species is preserved as the holotype and designated as a generitype in a zoological museum or a herbarium to be available for further study.

A genus name in one kingdom is allowed to bear the same name as a genus or other taxon name in another kingdom. For instance, Anura is a genus of plants as well as the order of frogs; Aotus is both a pea and a monkey; Oenanthe and Oenanthe are genera of birds and plants respectively, as are Prunella and Prunella. It is, however, not allowed for two genera within the same kingdom to have the same name. This explains why the Duck-billed Platypus is called Ornithorhynchus; although Platypus was originally chosen for it, the name had already been given to the ambrosia beetle, an invertebrate. Invertebrates are in the same kingdom, Animalia as the platypus, so could not be used again for a different animal.

See also: Linnaean taxonomy, cladistics