Abstract
The aim of this study was to investigate the relationship between specific combining ability, genetic diversity of parents and heterosis over better parent effects. This research, having eighteen F1 crosses derived from crossing between six lines and three testers, was conducted in order to estimate combining ability, to determine the nature of gene action and heterosis for yield and fiber quality traits and to detect the appropriate crosses for cotton breeding program. The experiment was conducted on randomized complete block design with three replications. The analyses of variance showed significant differences among the genotypes, parents (lines and testers) and crosses for all the studied traits. Estimates of both general and specific combining ability effects were significant for most traits, indicating the importance of both additive and non-additive gene effects for these traits. While, specific was higher than general combining ability variances, for all traits, showing non-additive gene action controlling and therefore, heterosis breeding may be rewarding. The heterosis value varied from cross to cross and from trait to trait. This dissimilarity coefficient was ranged from 3.234 between Giza 85 and Giza 80 to 71.002 between Giza 96 and 10229. Association between heterosis over better parent and specific combining ability was positive and significant for all the studied traits except lint yield / plant. No correlations were found between SCA and GD for all the studied traits. Similarly, heterosis effects was negatively significantly correlated with GD only in the case of boll weight, lint yield / plant and uniformity ratio %, while showed positive and significant correlation for fiber strength and micronaire value. Four crosses showed both positive and significant heterosis and specific combining ability for most yield traits. The parents of these crosses belong to different clusters. Crossing diverse parents could produce high heterotic performance in hybri
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