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 19th November and 3rd January at the Experimental Farm of the Faculty of Agriculture, Zagazig University 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 3rd and 4th crosses and grain yield/plant in the 2nd and 3rd 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 1st and 2nd crosses on normal sowing date and the 3rd cross on both sowing dates; days to maturity in the 1st and 4th on normal sowing date and the 3rd cross on both sowing dates, No. of tillers/plant in the 3rd cross on both sowing dates and the 2nd and 4th crosses on normal sowing date; No. of spiklets/spike in all crosses on both sowing dates except the 1st and 2nd crosses on normal sowing date; No. of grains/spike in the 2nd cross on both sowing dates, the 4th cross on normal sowing and the 1st cross on late sowing date; weight of grains/spike in the 2nd cross on both sowing dates and the 3rd on normal sowing, 1st and 4th crosses on late sowing date and grain yield/plant in the 1st on both sowing dates and the 3rd and 4th crosses on late sowing date.