Final answer:
A gene with two dominant alleles that are expressed at the same time shows codominance. The MN blood group is an example where both alleles are equally expressed in heterozygotes.
Step-by-step explanation:
When an organism inherits a gene with two dominant alleles that are expressed at the same time, this form of expression is known as codominance. An example of codominance in humans is the MN blood groups. Individuals who are homozygous (for example, LMLM or LNLN) will express only one type of antigen, M or N respectively, on their red blood cells. However, those who are heterozygous (LMLN) express both M and N antigens equally. This means that the presence of both alleles can be detected in the phenotype, unlike cases of complete dominance where only the dominant allele is visibly expressed, or incomplete dominance where a mixture or intermediate form of the trait is observed.