The Backcross Method

Abstract The backcross method is a breeding method for producing a new cultivar that replicates the genotype of an already superior cultivar with a targeted trait upgrade. It is the most widely utilized breeding method, and is effective for both self- and cross-pollinated crop species. The backcross method will always result in the population structure of the recurrent parent regardless of mating system and genome structure as long as adequate population sizes to discourage genetic drift are utilized. A relatively short time period is needed to reach project culmination, usually 4–5 generations. The backcross method is based fundamentally on the Mendelian test cross to study the mode of inheritance of a given trait. If the trait being transferred is primarily dominant over the allele(s) present in the recurrent parent, the method reduces to a simple algorithm: select for the trait under transfer; cross to the recurrent parent, then repeat. If the trait being transferred is recessive, however, it is necessary to insert regular inbreeding steps, usually self-pollination, to force the appearance of recessive homozygotes that may be selected for further backcrosses. Progress towards the recovery of the genome of the recurrent parent is estimated as R = iH2ns √ VP where R is the response to selection, and i is the selection intensity measured in standard deviations from the mean. In cases where two different species are sexually compatible, specific phenotypes may be excised from one species and moved into the other. The backcross method has also been used extensively to achieve the desired combination of nuclear and cytoplasmic genotypes by employing the parent with the desired cytotype as the female recurrent parent. If two or more desirable traits are identified in distinct populations, there are three alternative courses that could be taken, all incorporating the power of the backcross method: (1) pyramid the desired traits into one donor population, then embark on a backcross program; (2) conduct two or more independent, parallel backcross programs, then combine the desired traits by recurrent selection or F1 hybrid populations; and (3) conduct two or more independent backcross programs in series. The “modified backcross method” was first developed for fruit tree breeding to achieve a partial alleviation of the problems of unwanted segregation and inbreeding depression, and this method is broadly applicable to large-sized cross-pollinated woody perennial crop species. Concurrent backcross programs for introgression of resistance genes to different races of a pathogen may be performed with a single recurrent parent, and the resulting “isolines” are then bulked to create a broadly-resistant multiline cultivar. The backcross method is replete with points at which molecular markers may be inserted and used to enhance the efficiency of a given program. Such markers are extremely powerful because they are employed basically as tags for chromosome segments, and not for any particular function per se. MAS is an effective strategy to pyramid genes by backcross concurrently instead of sequentially. Genetic transformation has been advanced as a means to achieve targeted trait transfer more quickly and accurately than backcross. The backcross method may, however, be more cost-effective than transformation, and the socio-political stigma of GMO organisms is averted.