Which genetic phenomenon involves the expression of both alleles in a heterozygous condition?

The genetic phenomenon that involves the expression of both alleles in a heterozygous condition is codominance. In codominance, neither allele is recessive, and both contribute to the phenotype of the organism. A classic example of codominance can be seen in certain blood types, such as the ABO blood group system, where an individual with one allele for A and one for B expresses both antigens on the surface of their red blood cells, resulting in the AB blood type. This contrasts with incomplete dominance, where the heterozygous phenotype is a blend of the two alleles, rather than a distinct representation of both. Monohybrid inheritance refers to genetic crosses involving one trait, and pleiotropy describes a single gene affecting multiple traits, thus not applicable in this case. Codominance specifically highlights the scenario where both alleles are expressed equally and visibly in the phenotype, making it the correct answer to the question.