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The Effects of Genetic Diversity and Performance of Inbred Lines Per Se on Hybrid Vigor in Swine¹

University of Minnesota

Abstract

The performance of 2332 crossbred pigs has been compared to the average of the respective parental lines. The lines involved were developed by flexible systems of inbreeding and selection with selection based on performance. A close agreement was found between genetic diversity and the amount of heterosis expressed. Genetic diversity was found to be as important in continuous crosses as in single crosses; full utilization of the heterotic stimulus requires the use of crossbred females in a continuous crossing program. Rotation crosses were found to be superior to crisscross crosses of equivalent genetic width.

Crosses were found to be less variable in performance than inbred parental lines. Factors contributing to this condition were enumerated. The concept was set forth that lines of divergent background developed under intense selection for the same characters of economic importance will fix different alleles of relatively high production potential; when the lines are then crossed the alleles may produce various genotypes which lead to high performance because, relatively speaking only the alleles of high production potential of the multiple allelic series have been retained in the inbred lines so developed.

Preweaning characteristics exhibited greater heterosis than post-weaning characters with a distinct effect on prenatal survival being evident. In general these results illustrate the opposite effects of inbreeding and crossbreeding and support the concept that the effects of inbreeding are progressively more severe as the stage of determination is moved toward the beginning of embryonic life.

The relationship of the performance of inbred lines per se to the performance of their crosses was determined by holding genetic diversity constant and comparing each line and each line cross to an impartial standard. This analysis revealed that the better performing lines produce the better performing crosses. These results are in agreement with genetic expectations; they indicate that the more effective gene combinations in the respective lines also contribute the more effective gene combinations in line crosses. These findings strengthen the case for performance testing as the basis for selection.

The results of this study suggest three genetic essentials for obtaining maximum heterosis in swine crosses: (1) full utilization of genetic diversity, (2) the use of stock from high performing lines, (3) the use of stock from lines that have been sufficiently purged and purified to insure the regular transmission of effective gene combinations from one generation to another.

¹ Paper No. 2973, Scientific Journal Series of the Minnesota Agricultural Experiment Station in cooperation with the Regional Swine Breeding Laboratory, Ames, Iowa, Bureau of Animal Industry, U. S. Department of Agriculture. This paper is condensed from a thesis submitted by the senior author to the Graduate Faculty of the University of Minnesota in partial fulfillment of the requirements for the degree of doctor of philosophy.