A technique used in plant breeding to introduce a desirable gene into a cultivated variety. Unlike the * pedigree method of plant breeding, the aim of a backcrossing programme is not to create entirely new varieties but to modify existing varieties. The cultivated variety (the recurrent parent) is crossed with the donor parent, which may be quite useless agriculturally except for its possession of one particularly valued gene (e.g. for disease resistance). The progeny from this cross, which contain 50% of the donor genetic material, are screened for the character and those possessing disease resistance are crossed back to the recurrent parent. The progeny of this cross - the first backcross generation (B1) - now contain 25% of the genetic material of the donor. The plants are again screened, the resistant plants again backcrossed to the recurrent parent, and this process is repeated until about the seventh or eighth backcross generation, by which time less than 0.25% of the donor genetic material remains. At this stage the B7 or B8 generation plants are selfed (crossed with each other) to produce plants homozygous for disease resistance; these may be identified by a * test cross . The process described above assumes the allele for disease resistance is dominant. If it were recessive then it is necessary to alternate backcrossing with selfing of the backcross generations. Backcrossing is more efficient with self-pollinating species but the method is not necessarily limited to self pollinators. |
