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The present day hybrid seed corn industry is based upon the development and utilization of inbred lines in hybrid combination. Current breeding procedures are largely predicated on the assumption that the actions of complementary, dominant or partially dominant favorable genes are responsible for the heterosis observed in the hybrids produced. If heterosis were due in considerable part to nonallelic interactions or to overdominance, the present breeding procedures might be expected to yield superior hybrids but would not be highly efficient.
Since the proposal of complementary growth factors as a genetic explanation of heterosis and the alternative proposal of physiological stimulation due to heterozygosity per se, a considerable body of information bearing on the problem has been accumulated. The results obtained and the interpretation of these results do not permit completely satisfactory conclusions as to the relative importance of the various types of gene action believed to be present. The present research problem is an attempt to gain further evidence on this point by means of a critical evaluation of the relationship between the inbreeding depression of a set of diallel crosses and the combining ability level of the inbred corn lines used as parents.