Co- Dominance

When the two alleles neither show dominant or recessive relationship nor show intermediate condition but both of them express themselves simultaneously in the heterozygous condition, then this condition is called co-dominance.

Example: Different types of RBC cells determine ABO blood groups. This blood group is controlled by gene I. It contains a glycoprotein substance called an antigen.

It is present in the plasma membrane of RBC.

The antigen protrudes out from the surface of the plasma membrane.

• Blood group A contains A antigen, blood group B contains B antigen and blood group AB contains both antigen ( antigen A & antigen B) and, blood group O contains no antigen.

• Four blood groups: A, B, Ab & O

• Three different alleles: IA, IB, and i.

• In the case of a human being (diploid organism) possible genotypes are IAIA, IAi, IBi, IBIB, IAIB, ii. The alleles IA& IB, are completely dominant over i.

• When an individual contains the IAi allele the blood group will be A and when the individual contains the IBi allele blood group will be B but when IAIB both are present then the blood group becomes AB because of codominance.