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Desirable traits for crop species include:
Plant improvement is practiced worldwide and is extremely improtant for ensuring food security and developing practices of sustainable agriculture
Classical plant breeding uses interbreeding (crossing) of closely or distantly related species to produce new cops with desirable properties. Plants are crossed to introduce traits/genes from one species into a new genetic background. For example a mildew resistant pea may be crossed with a high-yielding but susceptible pea, the goal of the cross would be to introduce the mildew resistance without losing the high-yield characteristics. Classical breeding relies on homologous recombinationGenetic recombination is a general category for a number of processes involving DNA studied in classical genetics, population genetics, molecular biology, and evolutionary biology. Crossing over Main article Chromosomal crossover The most important and we of two genomes to generate genetic diversity. It also makes use of a number of molecular techniques to generate diversity and produce plants that would not exist in nature.
The Yecoro wheataestivum ''T. aethiopicum ''T. araraticum ''T. boeoticum ''T. carthlicum ''T. compactum ''T. dicoccon ''T. durum ''T. ispahanicum ''T. karamyschevii ''T. militinae T. monococcum ''T. polonicum T. spelta ''T. timopheevii ''T. trunciale ''T. turanicum ''T. (right) cultivar is sensitive to salinity, plants resulting from a hybrid cross with cultivar W4910 (left) show greater tolerance to high salinity
When distantly related species are crossed plant breeders make use of a number of plant tissue culture techniques to produce progeny from other wise fruitless mating. Interspecific and intergeneric hybrids are produced from a cross of related species or genera that do not normally sexually reproduce with each other. The cereal triticale, is a wheat and rye hybrid. The first generation created from the cross was sterile, so the cell division inhibitor colchicine was used to double the number of chromosomes in the cell. Cells with an uneven number of chromosomes are sterile.
Failure to produce a hybrid may be due to pre- or post fertilization incompatibility. If fertilization is possible between two species or genera, the hybrid embryo aborts before maturation. When the cross is incompatible after fertilization the resulting embryo resulting from an interspecific or intergeneric cross can be rescued and cultured to produce a whole plant. This technique has been used to produce new rice for Africa, an interspecific cross of Asian rice (Otyza sativa) and African rice (Otyza glaberrima).
Chemical mutagens like EMS and DMSO, radiation and transposons are used to generate mutants with desirable traits to be bred with other cultivars. Classical plant breeders also generate genetic diversity within a species by exploiting a process called somaclonal variation. Somaclonal variation occurs in plants produced from tissue culture, particularly plants derived from callus.
When a desirable trait has been bred into a species, a number of crosses to the favoured parent are made to make the new plant as similar as the parent as possible. Returning to the example of the mildew resistant pea being crossed with a high-yielding but susceptible pea, to make the mildew resistant progeny of the cross most like the high-yielding parent, the progeny will be crossed back to that parent for several generations. This process removes most of the genetic contribution of the mildew resistant parent. Classical breeding is therefore a cyclical process.
It should be noted that the breeder does not know exactly what genes have been introduced to the new cultivars, and some scientists argue that plants produced by classical breeding methods should undergo the same safety testing regime as genetically modified plants.
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