Abstract
Six inbred lines of watermelon (Citrullus lanatus. thumb) were crossed to produce 15 F₁ hybrids through a half diallel design mating design. General combining ability (G.C.A.) estimates, specific combining ability (S.C.A.), and heterosis effects were studied for No. of days to maturity, No. of fruits per plant, total yield / plant, rind thickness, average fruit weight, fruit shape index and TSS.  The analysis of variance of combining ability showed importance of both additive and non-additive gene influences in the inheritance of studied traits. Effects of GCA estimates revealed that inbred line P6 (3MWS53) had the lowest negative value with non-significant effect for No. of days to maturity, also, this inbred line had the highest positive values with significant effects for average fruit weight and total yield / plant. inbred lines P2 (3MWS55) and P5 (96604005) had significant positive values for TSS. The crosses (P1xP2, P1xP4, P3xP4 and P3xP6 had negative SCA values with significant effects for No. of days to maturity, cross P2xP4 had the highest significant value SCA for average of total yield /plant and fruit weight. Heterotic effect revealed different results for all studied traits and mostly better in cases of crossing. All crosses revealed the best heterotic effect for total yield / plant and No. of fruits / plant.