The seven parents and their diallel crosses in the F₂ generations with the 21 F₃ selected families for earliness were sown under normal and late planting (heat stress) conditions to obtain some information on the nature of the genetic system controlling heading date and yearly changes from contemporaneous populations. The results showed that both additive and non-additive genetic variances were highly significant. The results exhibited the presence of partial dominance for earliness under all conditions. A remarkable shift in the order of dominance characterized the first season. An exceptional case was the earliest under both conditions. Heritabilities in narrow sense were similar and rather high in magnitude under favourable and stress conditions. Highly significant differences were observed between environments, among  genotypes and their interactions for all studied traits. The response to selection measured as deviation from the means of F₂ populations ranged from -8.09 to -0.90% and from  -8.57 to -2.49% under favourable and stressed conditions, respectively. The earliest F₃ selected families showed consistent earliness in the two environments, whereas some crosses indicated inconsistent responses, reflecting    the  presence of G x E interactions. A correlated response in each of grain yield/plant and 1000-grain weight was decreased under both conditions, the first trait was more affected than the second. It is concluded that the direct selection for earliness under late sowing date is expected to be more effective than  indirect selection.