Offspring of parents that differ in genetically determined traits (seegenetics). The parents may be of two different species, genera, or (rarely) families. The terms mongrel and crossbreed refer usually to animals or plants resulting from a cross between two races, breeds, strains, or varieties of the same species. Because of basic biological incompatibilities, sterile hybrids (those that cannot produce living young) such as the mule (a hybrid between a jackass and a mare) commonly result from crosses between species. Some species hybrids, however, are fertile and can be sources for the formation of new species. Many economically or aesthetically important cultivated plants (e.g., bananas, coffee, peanuts, dahlias, roses, bread wheats, alfalfa, etc.) originated through natural or artificially induced hybridization. Hybridization is important biologically because it increases necessary genetic variation within a species.