A half diallel set of crosses involving seven parents were used to study heterosis and combining ability in the F₁ generation as well as the nature of gene action controlling seed yield and its contributing traits in both F₁ and F₂ generations.  Results indicated that the expression of heterosis varied with the crosses as also with characters investigated. The maximum significant positive heterobeltiosis was observed for branches/plant (52.9%) followed by seed yield/plant (38.0%), capsules/plant (33.6%), capsule length (19.0%), 1000-seed weight (18.6%) and plant height (12.1%). Analysis of variance for combining ability indicated that general (GCA) and specific (SCA) combining ability variances were highly significant for all studied traits, suggesting the presence of both additive and non- additive (dominance) gene effects involved in the expression of these traits. General and specific combining ability effects were frequently significant among parents and crosses for the investigated traits. Superior parents were identified for particular characters. None of the parents appeared to be good general combiner for all traits together. The good combiner parents for seed yield/plant were also high combiner for two or more of yield contributing characters. Estimates of GCA effects showed that EUL92 (P4) and EXM90 (P5) were the best general combiner parents for earliness and low position of the first capsule. However, the local cultivar Giza 32 (P1) and Mutant 48 (P3) were the best general combiner parents for seed yield and two or more yield components especially number of capsules per plant. Moreover, the favorable cross combinations that indicated highly significant positive SCA effects for seed yield and some of its main components were derived from the P1 and P3 parents. Estimates of the type of gene action confirmed the importance of both additive and non- additive (dominance) gene effects in the inheritance of the studied characters in both F₁ and F₂ generations. However, the dominance components were larger in magnitude than additive ones for most investigated traits in the F₁ and vise versa in the F₂ generation. The mean degree of dominance as estimated by the ratio (H₁/D)^0.5 revealed the existence of overdominance for all traits in the F₁ except capsule length, and partial dominance in the F₂ generation except seed yield/plant. The negative and positive genes were unequally distributed in the parents for all traits in both generations. Heritability in narrow sense (Hn) was found to be low for all characters except capsule length in the F₁ generation. The presence of overdominance and low heritability estimates recorded for most traits in the F₁, suggested the possibility using of cross breeding. High values of heritability were recorded for all the investigated traits in the F₂ generation, indicating that the genetic variance associated with those traits was mostly due to additive effect of genes.