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Changes in Gene Frequency During Mass, Modified Ear-To-Row, and S₁ Selection: A Simulation Study¹

Computer simulation studies have indicated that S₁ recurrent selection is more efficient than mass or modified ear-to-row selection for improving corn (Zea mays L.) populations (Choo and Kannenberg, 1979). The present study extends the comparison of mass, modified ear-torow, and S₁ selection in terms of genetic drift and changes of gene frequency. Severe genetic drift occurred in S₁ selection under high (5%) selection intensity. The desirable genes lost during S₁ selection included both major (A = 2) and minor (A = 1) genes, with some losses occurring during the very early cycles of selection. Fewer and only minor genes were lost during mass selection. Gene losses under modified ear-to-row selection were intermediate. In all cases, genetic drift resulted is losses of alleles of 0.1 initial frec~uency, with losses being less severe under complete domtnance than under additive gene action. The loss of desirable genes depended almost entirely on their initial frequency and not on the magnitude of their individual effects. The results indicate that if 20 or fewer S₁ lines are recombined in each cycle, then another germplasm pool should be maintained as a source of extra genetic variability. S₁ selection brought the fastest change of gene frequency regardless of the gene effect and the initial frequency. When gene frequency exceeded 0.5, the rate of increase in gene frequency during S₁ selection was similar under the additive and the complete dominance models, but in mass or modified ear-to-row selection the rate of increase was much slower under the complete dominance model.

Key Words: Computer simulation • Selection limit • Population improvement method • Corn breeding • Genetic drift • Gene losses

² Former research associate (now research scientist, Research Station, Agriculture Canada, P. O. Box 1210, Charlottetown, P. E. I. CIA 7M8) and professor, Crop Science Dep., Univ. of Guelph, Guelph, Ontario, Canada N1G 2W1.

Received for publication November 6, 1978.