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Simulated Comparisons of Single Seed Descent and Bulk Population Breeding Methods¹

Two plant breeding methods generally used in breeding self-pollinated crops, the bulk-population (BP) and the single-seed-descent (SSD) methods, compared by computer simulation to determine which retained the most additive genetic variation after four generations of inbreeding. Various levels of fecundity were simulated in the BP method, and various probabilities of plant survival were used in the SSD method simulation. All simulations were done with a hypothetical crop having seven chromosomes with six loci on each and various degrees of linkage.

In the F₆, the additive genetic variance in the BP method was smaller than that in the SSD method; this difference was attributed to losses in genetic variability in BP during generation advance. Within BP a considerable proportion of the total additive variance originated from within-line variance, as opposed to that which originated from between-line variance. When the inbreeding coefficient was larger than 0.9, the difference between within-line and between-line variances was considerable. In the SSD method, all additive variance must originate from between-line variance.

Fecundity affected the genetic variability in BP, whereas the probability of individual plant survival was important in SSD. When the standard deviation of fecundity was greater than 25 seeds per plant, progeny from 75% of the original F₂ plants were not represented in the population after four generations of advance by BP. About the same number of lines were lost after four generations when seedling survival dropped below 70% in each generation of advance by SSD. Linkage had little effect on additive genetic variance in either system, unless the two lines of the original cross had considerably different sets of alleles.

Key Words: Selection • Intergenotypic competition • Gene frequencies • Linkage • Breeding value, • Genetic variability.

² Research geneticist, SEA, USDA, Pullman, WA 99164; bio-systems analyst, College of Forestry, Wildlife and Range Sciences, Univ. of Idaho, Moscow, ID 83843; statistician, Statistical Services; and biological aide, Dep. of Agronomy and Soils, Washington State Univ., Pullman, WA 99164, respectively.

Received for publication November 10, 1980.