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Monte Carlo Studies of Selection and Inbreeding in Swine. III. Gene Frequency¹

U.S. Department of Agriculture, Iowa State University, Ames, and University of Illinois, Urbana

Abstract

These findings clearly demonstrate that although selection was based on an index of phenotypic values computed from the entire genome, independent loci responded separately with changes in gene frequency. This response depended on selection pressure, initial gene frequency, model of gene action and effective population number. For example, at loci controlled by the additive model, gene frequency of the favored allele rose quickly, with larger increases in the larger-sized lines. A similar trend was noted at loci under control of dominance models. Where the overdominance models prevailed and selection was operative, gene frequency of 0.5 was maintained. At other loci where more complicated models were employed, gene frequency changes were more subtle, depending on the various factors enumerated.

Significant differences were noted among replicates of each population in regressions of gene frequency on generation number, for each section of the genome. Clearly demonstrated was the low predictability of changes in gene frequency for a particular small segment of a population over a period of only a few generations.

¹ Appreciation is expressed to Professors J. L. Lush and A. W. Nordskog of Iowa State University, Ames, and L. A. Swiger of Ohio State University, Columbus, for useful suggestations on the manuscript.

² A.S.R.D., A.R.S., U.S.D.A., Beltsville, Maryland 20705.

³ Department of Animal Science, Iowa State University, Ames.

⁴ Department of Animal Science, University of Illinois, Urbana.

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