BRAINCAKECodominance
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 CR and CW.
Example: coat colour in shorthorn cattle. CRCR cattle are red, CWCW are white, and CRCW are roan: their coat has a mixture of red hairs and white hairs, because both alleles are expressed.
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