Recessive alleles produced and selected in an inbred line usually produce an array of pleiotropic effects involving vigor characteristics both as heterozygotes and homozygotes in the homozygous background of the inbred line, and as heterozygotes in the heterozygous background of a hybrid combination. The vigor effects may be positive or negative for a particular characteristic. In general, the effects of an allele on vigor characteristics in the homozygous background of the inbred line are of greater magnitude than in the heterozygous background of a hybrid combination. Indeed, many visible recessive alleles may be selected in the first generation as heterozygotes since plants heterozygous for a mutant recessive allele may flower earlier, or later, or be somewhat larger or taller, etc. (alleles are produced by treating inbred material with a mutagen before meiosis and using the pollen as a screen to screen out undesirable types of mutations, i.e., pollen from treated inbred plants is used on silks of untreated inbred plants). Of the twelve alleles under investigation (in inbred line Oh43), ten alleles as homozygotes -/- yield less than the normal +/+ homozygotes. Four of the alleles as heterozygotes yield more than the normal +/+ homozygotes. In no case do the heterozygotes yield less than the normal +/+ homozygotes. In the four cases where the heterozygotes yield more than the normal +/+ homozygotes the recessive homozygotes yield less. The ora2-13 (orange endosperm-case 13) allele is a case in point. The ora2-13/ora2-13 homozygote yields 15.5% less and the +/ora2-13 heterozygote 8.6% more than the +/+ homozygote (grams of grain per plant for +/+ = 79.4, +/- = 86.2**, -/- = 67.1**). In this case the heterozygote flowers earlier and the homozygote later than the normal +/+ homozygote (days to silk for +/+ = 85.5, +/- = 83.2**, -/- = 88.3**; days to pollen shed for +/+ = 84.0, +/- = 82.2**, -/- = 87.6**). From the information available it seems that single locus heterosis is controlled by the pleiotropic effects of recessive alleles.
E. J. Dollinger
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