If a trait statistically has a normal (gaussian, bell-shaped curve) distribution of its phenotypical forms it is probable that it is conditioned by polygenic inheritance (quantitative inheritance).
In quantitative inheritance the effects of several genes add to others making it possible to represent the trait variation of a given population in a gaussian curve with the heterozygous genotypes in the center, i.e., appearing in larger number, and the homozygous in the extremities.