What term describes a situation where one allele is not completely dominant over another?

Incomplete dominance refers to a genetic situation in which the phenotype of the heterozygote (an organism with two different alleles for a gene) is distinct and intermediate between the phenotypes of the homozygotes (organisms that carry two identical alleles). This means that neither allele completely overshadows the other; instead, their effects blend together in the organism's appearance. For example, if one allele codes for red flowers and another for white flowers, an incompletely dominant interaction might produce offspring with pink flowers. The key characteristic of incomplete dominance is this blending effect, indicating that both alleles contribute to the phenotype in a way that is not purely dominant or recessive.

In this scenario, the other terms do not fit the description of the relationship among the alleles. Complete dominance implies that one allele fully masks the presence of another, co-dominance refers to both alleles being fully expressed individually without blending, and recessive dominance is not a standard term used in genetics. Hence, incomplete dominance accurately captures the nature of the allelic interaction described in the question.