Final answer:
Codominance is the correct answer because it describes the expression of both alleles in a heterozygous individual, resulting in a phenotype where both traits are simultaneously and equally expressed, as in the MN blood groups.
Step-by-step explanation:
The expression of both alleles for a trait in a heterozygous individual illustrates codominance. In codominance, both alleles are simultaneously expressed, and each contributes to the phenotype of the organism. A classic example of codominance occurs with the MN blood groups in humans, where individuals with both M and N alleles (LMLN) will express both M and N antigens on their red blood cells. This is different from incomplete dominance, where the heterozygous phenotype is intermediate between the two homozygous ones, such as red and white colors blending to produce pink flowers in certain plants. Also, while codominance and multiple alleles can be related, as seen in the human ABO blood group system, codominance specifically refers to the equal expression of both alleles in a heterozygote.