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Effect of Reciprocal Recurrent Selection for Yield on Isozyme Polymorphisms in Maize (Zea mays L.)¹

Nine isozyme polymorphisms were used to monitor the genetic effects of two cycles of reciprocal recurrent selection of a population of maize (Zea mays L.). The results indicate that RRS had little effect on gene or genotypic frequencies; the few changes that occurred could be ascribed to random genetic drift associated with restriction of population size (to about N = 30) at the time of selection of lines for advance to the next cycle of RRS. Studies of the joint distribution of alleles at pairs of loci revealed two further effects of random genetic drift at the time of selection: (i) generation of sporadic linkage disequilibrium or gametic unbalance; and (ii) loss of rare recombinants between closely-linked loci, thereby restricting the release of variability through recombination. Thus, random events played a major role in the behavior of the linkage blocks marked by these isozyme loci. Implications concerning RRS as a scheme of maize improvement are discussed.

Key Words: Electrophoresis • Population structure • Genetic drift • Linkage equilibrium

² Formerly Research Assistant (now Lecturer, Department of Biology, University of York, York, England), and Professor of Genetics, University of California, Davis.

Received for publication May 8, 1971.