These experiments were carried out at El-Baramon Horticulture Res. Station of Horti.Res. Institute , during the two successive summer seasons of 2008 and 2009. Five inbred line selected from Eskandrani cultivars differing in some economical
traits, i.e., number of internodes to the first female flower ,early yield fruit weight and number ),total yield fruit weight and number ,plant vigor and leaf area ,were chosen as original plant materials in this study . In summer season 2008 , the five parents were crossed in the field and their all possible crosses ,without reciprocal , to obtain the experimental materials .The F₁ crosses and their parental lines were planted in the field experiment in the second season 2009 .The objective of this study was to determine the different types of gene effect in terms of general and specific combining abilities (GCA and SCA )regarding some characters viz ,number of internodes to the first female flower ,early yield as fruit number and weight ,total yield as fruit number and weight per plant and leaf area .This knowledge about the sort of genetics would be helpful to the plant breeder for planning a successful breeding program. The obtained results could be summarized as follows : 1- Highly significant differences for general and specific combining abilities for all studied characters were found . 2- The estimated ratio between GCA and SCA mean squares suggested that the additive gene effects have the main role in the inheritance of all studied characters expect the nods of first female flower appearance and sex ratio. 3- Obtained values of GCA for the studied traits in each of studied parental lines showed that the inbred lines P₁ and P₂ are the best of all since it showed significant GCA values for all studied traits, followed by inbred line P₃. 4- A critical examination of data obtained on SCA effects for F₁ hybrids showed that the best crosses were P₁ X P₃, P₂ X P₄, P₂ X P_3, since they showed significant SCA effect values for all evaluated characters. Accordingly, these superior and prospective materials can be used in summer squash improvement through breeding programs.