What type of inheritance involves both alleles being expressed in the phenotype?

The correct answer is codominance. In codominance, both alleles in a gene pair are fully expressed, resulting in a phenotype that clearly shows both traits simultaneously. This is distinct from other types of inheritance. For instance, in incomplete dominance, the phenotype is a blend of the two alleles, rather than the distinct expression of both. In complete dominance, one allele completely masks the effect of the other, resulting in a singular phenotype that reflects the dominant allele alone. Polygenic inheritance, on the other hand, involves multiple genes influencing a trait, leading to a continuous spectrum of phenotypes rather than the distinct expression of alleles. Thus, codominance is characterized by the visible expression of both alleles in the phenotype, making it the correct answer.