A diallel analysis of eight Iranian bread wheat (Triticum aestivum L.) cultivars was conducted to estimate genetic parameters and their combining ability for yield and yield components. The parental lines and their FI hybrids were evaluated in a randomized complete block design with three replications. Combining ability effects and genetic parameters were estimated by Griffmg's Method H; Model I and Jinks-Hayman procedure; respectively. The variance of GCA was significant for spike length; spikelets/spike; grains/spike; grain weight; yield/plant and harvest index. The ratio of GCA to SCA mean squares indicated that there were large additive effects for all the traits; except for number of fertile tillers; yield/plant and biological yield. Thebest combiner for fertile tillers; spike length; yield/spike and yield/plant was "Arvand" cultivar. The degree of dominance indicated that in controlling yield/spike; seed weight; yield/plant; overdominance effects and in controlling the other traits; partial dominance effects of genes were important. It was implied that the recessive alleles had more contribution in increasing spikelets/spike; grains/spike and seed weight. The narrow-sense heritability estimates were low for yield/plant (4%); biological yield (14%) and harvest index (37%). However; for the other traits; it was more than 50%.
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