It is another dominance relationship in which the phenotype of heterozygote is not equal to that of either homozygote.
The phenotypic appearance of heterozygote, sometimes, exceeds the phenotypic measurement for both homozygous parents. Such heterozygotes are described as overdominant.
Crosses between homozygous individuals which are poor in yield produce offspring that are better yielders (over dominant) than both parents.
In Drosophila, the white-eyed gene (w) in heterozygous condition (w+/w) produces more pigments than both the white (ww) and wild type (w+w+) homozygotes.