Codominance

 In a do-dominance relation, the effect of both alleles is equally visible in the phenotype of the heterozygote without being diluted by the presence of the other allele (as in incomplete dominance) or being suppressed by a dominant allele (as in complete dominance).

ABO Blood Group system

The ABO blood group system provides an example. Three alleles, I^A,  I^B , and i determine a person’s blood type. The two of these alleles, I^A,  I^B are codominant to each other, producing an AB blood type in the heterozygote. 

The ABO blood group system is determined by the antigen A and antigen B on the surface of RBC. These surface antigens are groups of interconnected sugars—oligo saccharides. The synthesis of these surface antigens is controlled by two alleles, designated IA and IB, respectively. The i allele is recessive to both IA and IB. 

A person who is homozygous ii has type O blood and does not produce either antigen. A homozygous IAIA or heterozygous IAi individual has type A blood and contains the antigen A. Similarly, a homozygous IBIB or heterozygous IBi individual has type B blood and produces surface antigen B.  A person who is IAIB has the blood type AB and expresses both surface antigens A and B. The phenomenon in which two alleles are both expressed in the heterozygous individual is called codominance. In this case, the IA and IB alleles are codominant to each other.

 

The above cross shows the possible offspring between two parents who are IAi and BI i. The IAi parent makes IA and i gametes, and the IBi parent makes IB and i gametes. These combine to produce IAIB, IAi, IBi, and ii offspring in a 1:1:1:1 ratio. The resulting blood types are AB, A, B, and O, respectively.

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