Not all genes have dominant and recessive alleles. Some have alleles that are both expressed together in the heterozygote individuals.

Co-dominant alleles have three phenotypes, one for each genotype.

In humans there are a number of conditions that are co-dominant.

Example

Sickle cell anaemia is a genetic disease which affects the heamoglobin of the red blood cells. Haemoglobin is normally a ball-shaped molecule but the sickle cell allele makes it form long strands. The red blood cell carrying these molecules distorts into characteristic long shapes.

The shape of the haemoglobin molecule is controlled by two alleles:

There are three phenotypes

Normal
Normal individuals have two normal haemoglobin alleles

Sickle cell anaemia, a severe form where all the red blood cells are affected. Sickle cell anaemia patients have two sickle cell alleles in their genotype

Sickle cell trait, a mild condition where 50% of the red blood cells are affected. Sickle cell trait individuals are heterozygotes, having one of each allele.

Symbols for codominant alleles

Because both alleles are expressed in the heterozygote they are considered codominant and both take a capital case letter. An index letter identifies the allele.

Therefore:

Because the heterozygotes have their own phenotype this gives rise to different proportions amongst their offspring compared with crosses between heterozygotes for dominant and recessive alleles.