Here's why:

* Codominance occurs when both alleles of a gene are expressed equally in the heterozygote. This means the phenotype of the heterozygote is a combination of both alleles, rather than one allele masking the other (like in dominant-recessive inheritance).

* Example: In the ABO blood group system, the alleles for A and B are codominant. A person with both the A and B alleles will have AB blood type, expressing both A and B antigens on their red blood cells.

Let me know if you'd like more examples or a deeper explanation!