Definition
Genera is the plural form of genus, the biological classification rank between family and species. In the taxonomic hierarchy, organisms are classified from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, species. The domestic horse (Equus caballus) belongs to the genus Equus within the family Equidae.
The Genus Equus
The genus Equus includes all living members of the horse family: the domestic horse (E. caballus), domestic donkey (E. asinus), African wild ass (E. africanus), onager (E. hemionus), kiang (E. kiang), Grevy’s zebra (E. grevyi), plains zebra (E. quagga), and mountain zebra (E. zebra). All Equus species share a common ancestor that lived approximately four to five million years ago, originating in North America before spreading to other continents. The genus is the only surviving genus of the once-diverse family Equidae.
Hybridization Within the Genus
Members of different species within Equus can interbreed, producing hybrids that are typically sterile or have reduced fertility. The most common hybrid is the mule, produced by crossing a broodmare side of the cross with a male donkey (jack). A hinny results from crossing a female donkey (jenny) with a stallion. Zebroids are hybrids between zebras and horses or donkeys. These crosses are possible because all Equus species share sufficient genomic similarity despite differences in chromosome number — the horse has 64 chromosomes, the donkey has 62, and their hybrid the mule has 63. A foal born of such a cross inherits an odd number of chromosomes, which disrupts the pairing required for gamete production and produces infertility.
Taxonomic Context
Understanding the genus level of classification clarifies which animals are truly related to the horse versus superficially similar. The structural markers that define the genus that define Equus — a single functional toe, high-crowned teeth for grass grinding, and a large fermentation chamber — distinguish genus members from other large mammals regardless of behavioral or geographic similarity.
Further reading: Genus on Wikipedia; Taxonomy at Britannica.