Abstract
The experiment was performed at the Experimental Farm of Agronomy Department , Faculty of Agric. Al-Azhar Univ. Naser City, Cairo, Egypt during the two following growing seasons 2016/2017 and 2017/2018. Eight parents of bread wheat (Triticum aestivum , L.) illustrate a wide range of diversity for some agronomic traits were selected for the study namely; Gemmiza 11, Giza 168, Sids 14, Masr 2, Sids 12, Sakha 95, Masr 1 and Sakha 94 were crossed at 2016/2017 growing season in one direction diallel cross . The genetic analysis (variance, combining ability, heterosis, and type of gene actions) of grain yield / plant and some related traits were evaluated at 2017/2018. Results indicated that variances of genotypes, parents and crosses were highly significant for all evaluated traits. The variances connected to general and specific combining abilities were established be highly significant for all evaluated traits, except number of kernels /spike for GCA .The estimates of GCA/SCA were less than unity recommended the greater amount of non- additive gene actions in determining the performance of all traits. The crosses (P1xP2), (P2xP6); (P3xP5), (P3xP6) and (P5xP6) recorded greatest mean values for grain yield/plant.P6 and P8 were considered as the best general combiners for grain yield /plant . The best crosses for SCA and heterosis effect for grain yield / plant recorded for P1xP2, P3xP8 and P5xP6 where be revealed highly significant and positive SCA and high percent heterosis making them hybrid promising in breeding programs for crop improvement . Hayman and Morley –Jones testing showed that the ratio of (H1/D)1/2 is larger than one for all the studied traits, so over dominance is participating in the genetic of these traits .Estimates of percentage of negative and positive genes (H2/4H1) in the parents ranged from 0.18 for plant height to 0.23 for number of kernels /spike and grain yield /plant hence, negative and positive alleles are consistent distributed in this traits. Heritability in broad-sense were high values detected for all the studied traits, indicated that these traits are more genetic, while narrow sense heritability were low for no. of spikes/plant, spike length and grain yield / plant , so the role of additive part is low. Significant differences were observe for additive (a) effect for all the studied traits in Morley Jones method, Also , dominance (b) part was significant for all the studied traits, appropriately these traits are controlled by both dominance and additive type of gene actions. As (b2) and (b3) were highly significant for all the studied traits, thus epistasis is participating in their genetics. As the part(b1) was significant for all the evaluated traits, with the exception of plant height and 1000-kernel weight, so dominance genes were attributable to directional dominance. Significant (b2) part for all traits showing imbalance of gene giving out for these traits. Significant (b3) part for all traits showed residual dominance effect (b3) outcome from additive × additive, additive × dominance and dominance × dominance interactions .
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