Changes in Quantitative Traits Associated with Inbreeding in a Synthetic Variety of Maize1

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Abstract
Inbreeding studies were initiated in a synthetic variety of maize (Zea mays L.), ‘Iowa Stiff Stalk Synthetic’ (BSSS), in 1962 by selfing 250 unseIected S0 plants. One genotype of the zygotic array for each S0 plant was maintained by single‐seed descent for seven generations of selfing. In the S7 generation, 248 lines from the original 250 S0 plants were available, and a bulk increase was made for each line. Five experiments were conducted in 1969 and 1970 to evaluate the agronomic changes associated with inbreeding (by selfing) for 10 quantitative traits: plant and ear height, leaf width, kernel‐row number, ear length and diameter, cob diameter, kernel depth, yield, and days to silk. Multiple‐range tests of the combined means showed that most of the differences among generations S0 through S4 were significant. Except for ear diameter, kernel depth, and yield, the differences among generations S5, S6, and S7 were not significant. Linear and quadratic models were fitted for the generation means for each trait on ercentage homozygosity. The regression analyses showed that the relation between the mean performance and levels of inbreeding in BSSS can be described by a genetic model based on the cumulative effects of loci with dominance for most traits; i.e. inbreeding depression resulted from an increase in the frequency of homozygous recessive deleterious loci. In BSSS, inbreeding to homozygosity would, on the average, reduce yield by 45 q/ha, decrease plant height by 48 cm, and increase days to silking by 4.6 days.

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