What term describes a situation in which both alleles of a gene contribute to the phenotype of the organism?

The situation where both alleles of a gene contribute to the phenotype of an organism is best described by the term co-dominance. In co-dominance, neither allele is dominant or recessive, and both are expressed equally in the phenotype. This results in traits that are a mixture of both alleles. A common example of co-dominance can be observed in blood type inheritance, specifically in individuals with type AB blood, where both the A and B alleles are fully expressed, leading to the presence of both antigens on the surface of red blood cells.

In contrast, the term "dominant" refers to an allele that can mask the expression of another allele, while "incomplete dominance" describes a situation where the phenotype is a blend of the two alleles, resulting in a third phenotype that is different from both parental traits. "Multiple alleles" refers to the presence of more than two alleles for a particular gene within a population, but it doesn't specifically address the interaction of alleles in terms of phenotype expression. Therefore, co-dominance accurately captures the concept of both alleles contributing distinctly to the observable characteristics of the organism.