Which phenomenon could involve two different alleles being expressed simultaneously?

The correct choice highlights codominance, a genetic phenomenon where both alleles are expressed simultaneously in the phenotype of a heterozygous individual. In codominance, neither allele completely masks the other, leading to a situation where both traits are fully displayed. A classic example of this is found in the ABO blood group system, where individuals with one allele for A blood type and one for B blood type express the AB blood type, exhibiting traits from both alleles at the same time.

In contrast, incomplete dominance results in a blending of traits, where the phenotype of heterozygotes is an intermediate form, but not the display of both alleles distinctly. Heterozygous expression refers to a condition where an individual carries two different alleles, but it does not specifically describe the simultaneous expression of both traits. Simple dominance typically pertains to a scenario where one allele completely masks the effect of another, allowing only the dominant trait to appear in the phenotype.