This study sought to identify the optimal general and specific combiners and the nature of gene action concerning morphological, yield, and fiber traits under the line-by-tester mating design. It utilized six Bt-gene-containing parental lines and three non-Bt testers, along with their 18 F₁ hybrids in cotton (Gossypium hirsutum L.), with experiments held at the Sindh Agriculture University, Tandojam, Pakistan. Eight characteristics, such as days to first flowering, plant height, sympodial branches per plant, bolls per plant, seed cotton yield per plant, fiber strength, and insect mortality (%), sustained scrutiny. The notable disparities in line-by-tester interactions revealed the significance of specific combining ability in hybrids, indicating the role of dominant genes in the manifestation of traits. The substantial mean squares attributed to lines and tester parents confirmed that the general combining ability (GCA) reflects the dominant influence of additive genes on the traits. The GCA estimation indicated the Bt lines IUB-13, FH-901, and Koonj, along with the non-Bt testers Chandi-95 and CRIS-585, emerged as the most effective general combiners for the studied traits. The nine hybrids comprised parental genotypes with high × high, high × low, and low × low GCA effects for earliness, yield, and fiber quality traits.

Keywords: Cotton (G. hirsutum L.), Bt-cotton, line × tester, GCA, SCA, insect mortality, earliness, seed cotton yield and fiber traits

Key findings: Among inbred lines, such as IUB-13, FH-901, and Koonj, and testers Chandi-95 and CRIS-585, become most recognized effective general combiners in cotton (G. hirsutum L.). They should be favorable for use in hybridization programs to develop superior cotton hybrids.