Geneticists call alleles that are both expressed in the phenotype when they are together codominant.

Here's why:

* Codominance: Both alleles contribute equally to the phenotype. This means that instead of one allele masking the other, both alleles are expressed simultaneously.

* Example: The ABO blood group system. A person with the genotype AB will have both A and B antigens present on their red blood cells. This is because both the A allele and the B allele are codominant.

Let me know if you have any other questions about genetics!