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Biological classification. It ranks below family and above species, consisting of structurally or phylogenetically (see phylogenetic tree) related species or a single species exhibiting unusual differences. For example, the species of roses collectively form the genus Rosa and those of horses, donkeys, and zebras form the genus Equus. The genus name, capitalized and usually italicized, is the first word of a scientific name in the system of binomial nomenclature.
This entry comes from Encyclopædia Britannica Concise. For the full entry on genus, visit Britannica.com.