Pattern of inheritance in which both alleles contribute to the phenotype of the organism

The answer is codominance.

A codominance is a pattern of inheritance in which both alleles contribute to the phenotype of the organism, i. e. both alleles are expressed. For example, there are alleles for red flowers and alleles for white flowers. If a plant has both allele (it is a heterozygote), then its flowers will be neither red, nor white, nor pink, but red and white. Hence, both alleles for red flowers and white flowers contribute to the phenotype.