Codominance

Alleles are codominant when both alleles are expressed in the phenotype of a heterozygote. Neither is dominant or recessive, so the heterozygote has a third, distinct phenotype that shows the effects of both alleles.Because neither allele is recessive, codominant alleles are written with the same capital letter for the gene and a superscript for each allele, for example C^R and C^W.Example: coat colour in shorthorn cattle. C^RC^R cattle are red, C^WC^W are white, and C^RC^W are roan: their coat has a mixture of red hairs and white hairs, because both alleles are expressed.Worked example: two roan cattle are crossed.Parents: C^RC^W × C^RC^W
Gametes: C^R, C^W and C^R, C^W
Offspring: 1 C^RC^R : 2 C^RC^W : 1 C^WC^W
Phenotypes: 1 red : 2 roan : 1 whiteSo a cross between two heterozygotes gives a 1 : 2 : 1 phenotype ratio, not 3 : 1. Every genotype has its own phenotype, so the genotype can be read directly from the phenotype.Other examples: blood group AB (I^AI^B), where both A and B antigens are made, and sickle cell trait, where a person with the genotype Hb^AHb^S makes both normal and sickle haemoglobin.