A relationship among alleles where both alleles contribute to the phenotype of the heterozygote is

A relationship among alleles where both alleles contribute to the phenotype of the heterozygote is codominance.

This is a phenomenon when both alleles are fully expressed. As a result, offspring will have a phenotype that is neither dominant nor recessive, it is combination of both. For example, if one allele contributes to white color of flower, another allele contributes to red color, the offspring (if we cross those two flowers) will have pink flowers (both phenotypes expressed).