Recovery of superior homozygous progeny from biparental crosses and backcrosses

Breeders of self-pollinated species are often confronted with the question of how best to incorporate exotic germplasm into breeding populations. The objective of this work was to illustrate those situations in which a breeder might utilize one or more backcrosses to the better parent of a self-pollinating population to increase the probability of recovery of desirable homozygous progeny from the population. The stochastic theory is presented along with probabilities associated with different conditions. The number of cycles of backcrossing needed to recover desirable progeny is a function of three variables specific to a particular pair of parents: the number of allelic differences between parents (N), the proportion (k) of desirable alleles in the better parent, and the number of loci homozygous for desirable alleles by which a progeny must exceed the number in the better parent to be selected (n(d)). In crosses between parents bearing equal or nearly equal numbers of desirable alleles (k close to 0.5), selfing provides the greatest probability of recovering the desired plants. As k approaches 1, the increased probability of recovering desirable alleles by backcrossing to the better parent outweighs the decreased probability of recovering desirable alleles from the worse parent, and backcrossing is indicated. With increasing N, or increasing n(d), more cycles of backcrossing are needed to maximize the probability of recovery of desirable plants.