Co-dominant Alleles

When two alleles each display their phenotypes independently in the heterozygote then we refer to these alleles as co-dominant. This differs from incomplete dominance, where the heterozygote has a phenotype that is intermediate between the two homozygotes. Instead, in co-dominance we see both phenotypes displayed.

The classic example is the ABO blood system. There are three main alleles, I^A, I^B, and I^O. The allele I^A is dominant to I^O so that the genotype I^AI^O has the I^A phenotype. Additionally the I^B allele is dominant to I^O so that the genotype I^BI^O has the I^B phenotype.

The I^A and I^B alleles, however, are co-dominant. The I^AI^B genotype has both the I^A and the I^B phenotype: that is, the alleles are manifested independently.

This example provides a good lesson for how co-dominance can occur. This gene codes for a transmembrane protein that is expressed in blood cells. The protein is exposed to the extracellular environment and can react with antibodies. The A form will react with anti-A and the B form will react with anti-B. The I^O allele is a loss-of-function allele and as such does not code a protein that can react with an antibody. See the top row in this figure:

In I^AI^B heterozygotes, both protein forms are expressed and as a result the blood cells can react with both anti-A and anti-B. Therefore, the two alleles are expressed independently and are co-dominant.