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Effects of Inbreeding by Selfing and Full-sib Mating In a Maize Population¹

Random noninbred full-sib families (generation 1) out of diploid Syn. O.P., [composite of 36 open-pollinated maize (Zea mays L.] varieties were inbred on an ear-to-row basis through 7 generations of sib-mating (SBF₂ to SBF₈) and 4 generations of selfing (SLF₂ to SLF₅) evaluate the effects of inbreeding and the correlations between generations. Generations 1, SBF₂, SBF₃, SBF₄, SLF₂ and SLF₃ of 60 families were studied in 1970 and all 12 generations of 42 families in 1971. Characters studied were grain yield, plant height, ear height, percent moisture at harvest, percent oil, and kernel weight.

All characters except percent moisture showed significant inbreeding depression. Inbreeding depression, when significant, was linearly related to coefficient of inbreeding (F), except for percent oil, which showed significant quadratic effects. The linear rate of inbreeding depression for plant height, ear height, and grain yield was greater under selling than under sib-mating. Generation SBF₄ was significantly higher in grain yield than SLF₂ (both with F = 0.5) in 1970. Significant generation x family interactions were found for all characters.

Genetic correlations between generations were lower for characters that showed greater inbreeding depression and were lowest for yield. Correlations of generation 1 means with estimates of regression on level of homozygosity in partially inbred generations were generally negative, but significant only for ear height. Correlations of generation 1 means with predicted means of homozygous lines suggest that selection among noninbred families should result in a response in the desired direction in homozygous lines ultimately to be obtained. For yield, however, these correlations were so low that they indicated that yield of noninbred families is of little value as a predictor either of rate of inbreeding depression or of yield of homozygous lines (as lines) ultimately obtained.

Key Words: Corn breeding • Quantitative corn genetics • Generation means analysis • Genetic correlation • Selection

² Assistant Professor of Agronomy and Statistics, University of Kentucky; and Professor of Plant Genetics, University of Illinois, respectively.

Received for publication February 9, 1974.

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