Codominance occurs when two different alleles of a gene are both expressed in the phenotype of a heterozygote. Neither allele is recessive, so the heterozygote shows the characteristics of both alleles, rather than a blend.Because neither allele is dominant, capital and lower-case letters are not used. Instead, the gene is shown by a capital letter with the alleles written as superscripts, for example CR and CW.Example 1: coat colour in shorthorn cattle. The allele CR codes for red hair and CW for white hair.• CRCR: red coat
• CWCW: white coat
• CRCW: roan coat, with a mixture of red hairs and white hairsThe roan animal is not pink: each hair is either red or white, showing that both alleles are expressed.
Example 2: sickle cell anaemia. The alleles HbA (normal haemoglobin) and HbS (sickle haemoglobin) are codominant. A heterozygote, HbAHbS, makes both types of haemoglobin and has sickle cell trait. They are usually healthy but have some resistance to malaria.With codominance there are three phenotypes for three genotypes, so the genotype of every individual can be worked out from its phenotype. This is different from complete dominance, where the homozygous dominant and heterozygous individuals look the same.
• CWCW: white coat
• CRCW: roan coat, with a mixture of red hairs and white hairsThe roan animal is not pink: each hair is either red or white, showing that both alleles are expressed.
Example 2: sickle cell anaemia. The alleles HbA (normal haemoglobin) and HbS (sickle haemoglobin) are codominant. A heterozygote, HbAHbS, makes both types of haemoglobin and has sickle cell trait. They are usually healthy but have some resistance to malaria.With codominance there are three phenotypes for three genotypes, so the genotype of every individual can be worked out from its phenotype. This is different from complete dominance, where the homozygous dominant and heterozygous individuals look the same.