Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Arnel R. Hallauer


The development of inbred lines for potential use as parent seed stocks for stable, high-performance hybrids is the major objective of maize (Zea mays L.) breeding programs. Inbreeding in maize is accompanied by a reduction in the mean phenotypic value for most traits. This genetic phenomenon is known as inbreeding depression;No empirical evidence was available to determine whether newer greater yielding maize inbred lines are less affected by inbreeding depression than older inbred lines. Two groups of maize inbred lines were used in this study. One group included six inbred lines released before the 1960s (B14A, B37, L289, L317, M14, and WF9). The other group included six lines released after the 1970s (B73, B75, B76, B77, B79, and B84). Nineteen generations within each of the inbred groups representing nine levels of inbreeding (from 0% to 100% homozygosity at 12.5% intervals) were evaluated at five Iowa environments in 1988 and 1989 to determine if the inbreeding depression rate of new lines had changed for 15 plant and ear traits;The rate of inbreeding depression remained unchanged for the older and newer groups of lines for nine of the 15 traits, but rate of inbreeding depression was less for tassel-branch number, ear-leaf width, number of ears per plot, ear length, ear diameter, and cob diameter in the newer group of lines. The effects of inbreeding in the two groups of lines studied were a reduction in the mean for all traits except days-to-anthesis and dropped ears. The reduction in the mean with inbreeding was negatively and linearly correlated with the coefficient of inbreeding for all traits except days-to-anthesis which was positively and linearly correlated with the coefficient of inbreeding. Percentage of dropped ears was unchanged with inbreeding. The additive genetic model explained the majority of the variation among the inbreeding level means. The reduced rate of inbreeding depression in the group of newer lines for six of 15 traits evaluated suggest that this group is either segregating at fewer loci than the group of older lines or the favorable allele frequency has increased beyond 0.5.



Digital Repository @ Iowa State University,

Copyright Owner

Felix M. San Vicente



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142 pages