A genetic trait might be caused by incomplete dominance if:

A genetic trait might be caused by incomplete dominance if the alleles of both parents do not mask one another. SO this means that if a white flower will be mated with the red flower, and there is no dominant allele, then it will not express any of their colors but it will lead to the third phenotype which is pink flower offspring.

A form of intermediate inheritance which is called as incomplete dominance in which one allele is not express completely from it's paired allele. This then would result in a third phenotype. It usually occurs in traits like eye color as well as skin color.