Codominance

Term used in genetics. If alleles are equally involved in the expression of a phenotype, this inheritance pattern is referred to as codominant. Both alleles are fully implemented as a phenotype, instead of partially as in the case of intermediate inheritance or incomplete dominance. A good example of codominance is the human blood group.

The blood group is determined by the antigens on the surface of the erythrocytes. These are coded in turn by different alleles. Dominant alleles code for the two most common blood groups A and B. If a person has alleles A and B simultaneously, they produce both antigens at the same time and in the same quantity. Therefore, a person with the AG genotype also has the AB phenotype (and the AB blood group).

In blood group 0, there are no antigens on the cell surface of the red blood cells. The allele for O is recessive, and therefore there are two inheritance patterns for the blood groups in the ABO system:

• Codominance for A and B
• Dominant-recessive: A or B paired with the O allele.