الفهرس 
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Abstract
The main objective of this investigation was to estimate the combining ability effects of miscellaneous genotypes as well as to present information about the nature of gene action for yield and its components and other morphological traits. For this purpose, in the season of 2000, five inbred lines and their possible combination excluding reciprocals were split sawn at Farm of Sakha Agricultural Research Station, Maize Research Section. During 2001 and 2002 growing seasons, the obtained seeds of six populations (P1, P2, F1, F2, BC1 and BC2) for ten crosses were evaluated for the following traits i.e., tasseling date (days) and silking date (days) as earliness traits; plant height (cm), ear height (cm), number of leaves/plant and leaf area index (cm2) as morphological traits and number of kernels/row, number of rows/ear, ear length (cm), ear diameter (cm) and grain yield/plant (gram) as yield and yield component traits. The results demonstrated that both general combining ability (GCA) and specific combining ability (SCA) mean squares were highly significant for all the studied traits in the two years and from the combined data, indicating that both additive and nonadditive gene action were involved in the inheritance of these traits. While, the SCA mean squares were larger in magnitudes than the corresponding values of GCA mean square, indicating that the nonadditive genetic variances play the major role in the inheritance of these traits. Highly significant differences were present among crosses as well as populations within each cross with respect to all the studied traits. Therefore, the comparisons between genotypic means are valid and the partition of this genotypic variance to their components could be made. Furthermore, years, crosses and populations within crosses interacted significantly with years for the earliness, morphological and yield and yield component traits. Also, populations within each cross interacted significantly with years for most crosses for the previous traits. This finding detected that these crosses and their populations gave different performances in different years for the studied traits. The results of six parameters procedure showed that the estimates of mean effects (m), which reflects the contributes due to the overall mean (additive) plus the locus effects (dominance) and the interaction of the fixed loci were found to be highly significant for all the studied traits i.e.; earliness and morphological traits and yield and its attributes in the two years and combined. This indicated the contribution of additive and dominance gene action in the genetic expression of these traits. The dominance gene effects (d) values were higher in magnitude than those of additive gene effects (a) for all the studied traits, revealing that the role of dominance gene action in the inheritance of these traits and the higher frequency of dominant genes in the parental lines, which involved in these crosses. Furthermore, the results showed that most of the studied traits were significantly influenced by one or more types of epistatic gene action, which included additive x additive (aa), dominance x dominance (dd) and additive x dominance (ad) gene effects as appeared in the ten studied crosses for earliness, morphological and yield and its component traits, indicating the role of nonallelic interaction in the genetic expression of the previous traits. Therefore, judging by heterotic values general and specific combining ability as well as genetic variance components, the production of corn hybrids is the best breeding programs for the improvement these traits.