What term describes a situation where neither allele is dominant?

The term that describes a situation where neither allele is dominant is codominance. In codominance, both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that is distinct and represents both alleles simultaneously. This can often be seen in cases like blood type AB, where both the A and B alleles contribute to the phenotype of the individual, displaying characteristics of both without blending.

Genetic drift, on the other hand, refers to random changes in allele frequencies in a population, often due to chance events, and does not involve the dominance relationship between alleles. Mutation is the process through which changes to the DNA sequence occur, leading to new alleles, but does not specifically address dominance. Epistasis involves the interaction between different genes, where the effect of one gene is masked or modified by another gene, and again does not relate to dominance between alleles.