In codominance, how are alleles expressed?

In codominance, alleles are expressed such that both traits associated with the alleles are visible in the phenotype of the organism. This means that in a heterozygous individual, where both alleles contribute to the trait, neither allele completely masks the other. As a result, both alleles are expressed equally and distinctly.

An example of codominance can be seen in the ABO blood group system, where individuals with one allele for A blood type and one for B blood type will express both A and B antigens on the surface of their red blood cells, resulting in AB blood type. This illustrates how heterozygous individuals exhibit characteristics of both alleles without one being dominant over the other.

The other responses reflect different genetic principles: if only one allele is expressed, that describes complete dominance, while the expression of only recessive alleles occurs in the absence of dominant alleles, or when only recessive homozygous conditions are met. Similarly, alleles being silenced does not occur in codominance, as both alleles remain active in determining phenotype.