This investigation aimed to study mean performance, genotypic variances, types of gene action and heritability for earliness, yield and components in four bread wheat crosses using six populations i.e. (P₁, P₂, F₁, F₂, BC₁ and BC₂) under different thermal conditions. Wheat crosses populations were sown on 19^th November and 3^rd January at the Experimental Farm, Faculty of Agriculture, Zagazig University, Egypt in a randomized complete block design in 2017/2018 season. Scaling test provide evidence of non-allelic interaction in controlling all studied characters in the four crosses on both sowing dates except No. of spiklets/spike in the 3^rd and 4^th crosses and grain yield/plant in the 2^nd and 3^rd crosses on normal sowing date, indicated the presence of epistasis and the digenic model proved to be satisfactory in explaining the inheritance of the previous characters in the corresponding crosses. Narrow-sense heritability estimates recorded high values (>50%) for days to heading in the 1^st and 2^nd crosses on normal sowing date and the 3^rd cross on both sowing dates; days to maturity in the 1^st and 4^th  on normal sowing date and the 3^rd cross on both sowing dates, No. of tillers/plant in the 3^rd cross on both sowing dates and the 2^nd and 4^th crosses on normal sowing date; No. of spiklets/spike in all crosses on both sowing dates except the 1^st and 2^nd crosses on normal sowing date; No. of grains/spike in the 2^nd cross on both sowing dates, the 4^th cross on normal sowing and the 1^st cross on late sowing date; weight of grains/spike in the 2^nd cross on both sowing dates and the 3^rd on normal sowing date, 1^st and 4^th crosses on late sowing date and grain yield/plant in the 1^st  cross on both sowing dates and the 3^rd and 4^th crosses on late sowing date.