This investigation aimed to study the type of gene action and heterotic
effects in maize. Seven parental single crosses and their resulted 21 diallel double
crosses (derived In 2000 season) were tested on a field experiment in the
Experimental Farm, Faculty 01 Agriculture, Kalr EI·Sheikh, Tanta University during the
season 2001 in a randomized complete blocks design with three replications. The
studied traits were silking date, plant and ear heights, ear length, ear diameter,
number of rows/ear, number of kemels/row, 100-grain weight and grain yield
ardablfed. The results showed that:         1-     Highly significant differences were obtained among the studied genotypes
(parental crosses and single vs. double crosses). General and specific
combining ability mean squares were highly significant for all studied traits,
except plant height for specific combinlng ability.         2-       Non-additive gene effects played a major role in the inheritance of all studied
characters except for ear length and ear diameter where the additive gene
effects played the Important role.         3-        Single crosses 10 and 129 showed the best general combining ability effect for
grain yield and most agronomic traits, while those for days to 50% silking with
short plants were obtained by single crosses 120 and 124.         4-          Highest values of specific combing ability effects for grain yield were obtained
from the double cross (9 x 10). While nine double crosses exhibited negative
and significant specific combining ability effects toward ear1iness.         5-            Heterosis values as a percentage relative to the mid and high parent ranged
from negative to positive significant values for all studied traits. The highest
heterosis value for grain yield ard.lfed. was obtained from the double cross (9 x
10) only.