A term used by molecular biologists to describe the results of genetic engineering. It is an unfortunate choice of words, which is best avoided, because any form of breeding constitutes genetic modification. Genetic source of resistance
Mendelian breeders working with single-gene resistances must first find a genetic source of resistance, usually in a wild progenitor. This concept is now so pervasive that many believe the only way to breed for horizontal resistance is to first find a source of resistance. This is incorrect, as polygenic characters cannot be transferred in the way that single-gene characters are transferred, either by back-crossing or by genetic engineering. Breeding for horizontal resistance involves changing gene frequencies, and this can be achieved with susceptible parents using recurrent mass selection , provided that the genetic base is wide enough. Genetic uniformity
A genetically uniform population in which all the individuals are identical. Such a population lacks genetic flexibility in the sense that it cannot respond to selection pressures. For example, if a genetically uniform host population has too little horizontal resistance, it cannot gain more resistance, because all the individuals have an equal level of resistance, and are equally parasitised. No individual has a reproductive advantage over any other individual and, consequently, there will be no change in the level of resistance in the next generation. See also: Genetic flexibility.
Genetic uniformity means that all the individuals within a population are identical in one or more of their inherited attributes. Modern crops are typically uniform because they are cultivated as pure lines, hybrid varieties, or clones. A genetically uniform population has genetic inflexibility and this is desirable in agriculture because it ensures that valuable characteristics will not be lost. See also: genetic diversity.
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