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The term that describes a condition where both alleles are fully expressed is codominance. In codominance, neither allele is dominant over the other, which means that each allele contributes equally and visibly to the phenotype of the organism. A classic example of codominance is seen in certain blood types, such as AB blood type, where both A and B alleles are expressed simultaneously, resulting in an individual displaying characteristics of both.

In contrast, incomplete dominance refers to a situation where the phenotype of heterozygotes is intermediate between the phenotypes of the two homozygotes. This results in a blending of traits rather than the full expression of both alleles, as seen in flower color where red and white parents produce offspring with pink flowers.

Homozygosity describes a genetic condition where an individual has identical alleles for a trait and does not specifically refer to the expression of two different alleles.

Polygenic inheritance involves multiple genes influencing a trait, often resulting in a wide range of phenotypes, and does not specifically refer to the expression of alleles in a single trait.

Thus, the concept of codominance uniquely identifies the scenario where both alleles manifest in the phenotype without one overshadowing the other.