Hybrid, a plant or animal whose parents belong to two different breeds, varieties, or species. In some cases the parents may even belong to different genera (groups of related species). Plant hybrids are constantly being produced in nature. Hybrids are also produced experimentally in the interests of scientific research, and by plant and animal breeders for economic purposes. Some kinds of organisms, however, will not produce offspring when crossbred with another species. Some hybrids cannot produce offspring. The mule, a cross between a male ass and a mare, is an example.
Hybrid varieties of plants and animals are of economic value because the hybrid is nearly always more vigorous, larger, and more fertile than either of its parents. Also, double-cross hybrids (offspring of two hybrids) have a far greater number of traits from which plant or livestock breeders can select those they wish to reproduce.
Since the offspring of double-cross hybrids do not always resemble their parents, plant breeders use vegetative propagation to maintain desirable traits. Most garden perennial flowers and shrubs, as well as fruit trees, are hybrids, and are propagated in this way.