This study was carried out at the Sakha agricultural research station, cotton research institute, agriculture research center (ARC), Egypt during the three growing seasons 2015, 2016 and 2017. The genetic material used in the present study consists of twelve genotypes, four of them as testers (male parents) and eight genotypes as female parents. In 2015 season the males and female parents were crossed according to line x tester design to produce 32 F1 crosses. The twelve genotypes and their 32 F1 crosses were evaluated in two years 2016 and 2017 in a randomized complete block design with replications. The results obtained could be summarized as follows:
1- The mean squares for genotypes (parents and crosses) exhibited highly significant for all the studied traits in two years and their combined.
2- Highly significant mean square values were detected for lines, testers, line x testers and their interactions with years for all the studied traits.
3- The parent G.94 and Pima S6, also tester G.86 had the highest and desirable means for all the studied traits in each year and their combined.
4- The crosses G.86 (G.89 x G.86), G.93 x Suven and G.86 x Suvin were the best for most the studied traits.
5- The best desirable general combining ability effects for seed cotton yield/ plant, lint yield/ plant, boll weight, number of bolls/ plant and lint index were found in the parents’ Suvin, G89x G86 and TNB, meanwhile for favorite lint percentage were Suvin, G.96 and pima S6.
6- The cross G.86 x (G.89 x G.86) expressed high significant specific combining ability effects for seed cotton yield, lint percentage, boll weight and number of bolls/ plant in each year and their combined.
7- The highly significant positive heterotic effects relative to better were found in the crosses G.86 x Suvin, Kar x TNB, G.93 x Suvin and G.93 x TNB for all the studied traits for each year and their combined and it could be utilized in prospective cotton breeding programs for improving the studied traits.
Keywords: General and specific combining ability – heterosis- gene action – additive- line x tester