Abstract
If a set of traits which can be easily selected are represented by A genes, and another set of traits which are desired, but not easily selected are by B genes, a combination of an intensive selecton for A genes and a recurrent backcross of a donor of B genes may be of plant breeders' interest. To study the feasibility of such a breeding scheme, two genotypes of parents are herewith designated as AAbb and aaBB. To obtain the aimed type AABB a combinatin of backcross and selection can be practiced in such a way as to backcross aaBB to AAbb for introducing B genes, while restoring initial number of A genes through repeated selection. In the present simulation model, eight pairs of chromosomes are assumed, where A1 and b1 (i=1-8) are located in i-th pair of chromosomes in one of the parents, and a1 and B1 in the corresponding chromosomes of the other. Genetic value is assumed as 2 for AA and BB, 1 for Aa and Bb, and 0 for aa and bb. Three intensities of linkage in repulsion phase are assumed uniformly over the independent eigrht pairs of loci. To obtain AABB each of the backcross of aaBB to AAbb was followed by the selection for A under a heritability of O.8. One, two and three backcrosses were tested. Initial population size was set at 400 for each scheme which was repeated ten times. In each test the progress toward AABB was measured in terms of the genetic value defined above. From the results, it seemed possible to transfer nearly 90 percent of the aimed B genes by three backcrosses under the recombination values of 0.3 or O.5. In case of two backcrosses, about 70 percent of the desired B genes were introduced even under the recombination value of O.3 over eight pairs of loci. The progress was attained with simultaneous selection for A genes. And the number of A genes ranged from 61 to 70 percent and 55 to 63 percent, for two and three backcrosses, respectively. Despite the repeated selection for A genes, the progress for restoring all A genes seemed difficult after a single backcross. Particularly, an increase in the frequency of B genes and that of A seemed exclusive for each other, when the recombination value between A and b is O.1. A combination of repeated selection for visible traits (A genes) and recurrent backcrosses of a donor of traits desired, but not easily selected (B genes) seemed effective to obtain interim lines. In such lines, desired traits from both of the parents are partially combined depending on the intensity of the linkages between the two groups of genes. Initially aimed type AABB can be bred out through the secondary step of crossing the interim lines. Since they may have a high frequency of B genes, selection for A genes should be the main objective in the second step.
