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Effects and Control of Genetic Drift in the Autotetraploid Population¹

The effects and the potential for control of random genetic drift were investigated theoretically for an outcrossing autotetraploid population. With reference to breeding acrop such as alfalfa [Medicago sativa (L.)], two situations were considered. First the variance effective population sizes were determined for maintaining a nearly constant phenotype over generations of seed increase assuming additive, monoplex dominance, duplex dominance and recessive types of gene action at a locus. The interaction of gene frequency and type of gene action on the potential change in population phenotype with random genetic drift was shown. For a given confidence level and a range of tolerable variation in phenotypic mean, the population sizes needed with the non-additive types of gene action were up to three times the population size with additive gene action. Second the variance effective population sizes were determined for keeping the gene frequency in a population above a critical frequency. A critical frequency was defined as either the minimum gene frequency where the desirable gene was recoverable in a breeding population or the minimum frequency where agronomic value was maximized. In this case variance effective population sizes up to near 3000 were indicated to protect some gene frequencies. These results indicated that for the moderate population sizes of 50 to 300, often used in an isolation seed increase, the changes in gene frequencies from generation to generation at all loci not fixed maybe substantial. The actual change in population phenotype for a given change in gene frequency is dependent on gene frequency and type of genic action at a locus. These random changes in population phenotype are expected to be more important for traits controlled with nonadditive gene action by one or only a few genes. For genetic studies using multiple generations the results of this study indicated that for a seed increase either a polycross by hand or the use of 1000 or more plants in an isolation are necessary to be certain of a meaningful interpretable relationship between parents and offspring.

Key Words: Medicago sativa L. • Alfalfa • Selection • Seed Increase • Autopolyploid

² Research geneticist, USDA-ARS, College of Agriculture, Rm 323A, Univ. of Nevada, Reno, NV 89557.

Received for publication March 18, 1985.