SIMULATION OF BACKCROSS BREEDING METHOD .1. EFFECTS OF HERITABILITY AND GENE NUMBER ON FIXATION OF DESIRED ALLELES

The potential of backcross breeding for improvement of a pure line by addition of favorable alleles from a donor line was investigated by computer simulation coupled with quantitative genetics theory. Attention was focused on the effects of heritability (among individual F2 plants) and of the number of genes of which the allele present in the donor line is more favorable than the one present in the recipient. The specific programs investigated were ones involving a large amount of effort, a minimum of 1000 pollinations/backcross generation and selection among families in 3 or more generations. The criterion employed for effectiveness was the probability of fixation in the product of the program of favorable alleles derived from the donor line. Effectiveness was greater when heritability was greater but the effect of increased heritability was not as great as expected. Number of favorable alleles available from the donor line had greater impact, particularly when success was defined in terms of probability that all the available favorable slleles would be transferred from donor to recipient. Success, so defined, was limited to 1 allele in the case of the least laborious of the 3 programs studied and no more than 5 in the case of the most costly. When success was measured in terms of percent improvement in the selected trait, substantial change was possible with heritability as low as 15% and favorable alleles available in the range from 1-16. Additive effects and independent assortment were assumed for genes simulated.