opagation and discard plants that are inferior for that trait. By doing so, plant breeders can select and reselect for the trait through successive generations, shifting the population in the desired direction. HybridizationHybridization involves crossing plants of different strains or types to join in the progeny the desireable traits of both parents. Undesirable traits also enter the combination, however, so hybridization is usually followed by several generations of selection. This allows breeders to discard undesirable plants, choosing for further propagation only those plants with the desired combination of traits.Backcrossing is a common variation of hybridization. This technique is often used to transfer into a desirable variety a beneficial trait from an otherwise undesirable parent. First the hybrid between the two parents is made, then the hybrid is crossed with the desirable parent. The progeny from this backcrossing normally segregate widely, with individual plants showing a mixture of the characteristics of both parents. By continued backcrossing and selection the plant breeder concentrates the qualities desired, and, if all goes well, in six or seven generations the variety once again breeds true but now exhibits its new trait. Backcrossing is valuable for adding single gene characteristics to crop plants, particularly for resistance to specific insects and diseases.When desirable charcteristics are fully developed in a hybrid plant, and the plant can be propagated asexually by budding, grafting, or cloning, then no further selection is necessary. A hybrid apple, for example, is propagated by grafting, so all resultant plants are identical. Hybrids are often more vigorous than either parent. This phenomenon is called hybrid vigor and has been widely used by plant breeders to increase crop yielders. Hybrid seeds have helped to double U.S. corn yields since the 1940s, and almost all the...
