Codominance

What is Codominance?

A situation in which both alleles of a gene are expressed in the phenotype of a heterozygous individual, so that the heterozygote shows the effects of both alleles rather than the effect of only one. Codominance differs from complete dominance, where the heterozygote shows only the dominant allele's effect, and from incomplete dominance, where the heterozygote shows an intermediate phenotype rather than both. The ABO blood group system is the required Cambridge O Level Biology example: the alleles for A and B are codominant, so a person carrying one of each has blood group AB.

This definition is part of the Inheritance chapter in Cambridge O Level Biology.

Concepts related to Codominance

• abo blood group
• heterozygous
• dominant allele
• phenotype

Common mistakes with Codominance

• “Codominance means the alleles blend.” Blending is incomplete dominance. Codominance means both alleles are expressed: a person with genotype IAIB has both A and B antigens, not something halfway. Write “Both alleles are expressed in the heterozygote.”

Examiner tips on Codominance

• Notation matters here. With complete dominance, T and t are ideal, because the case of the letter records which allele is dominant. With codominance there is no dominant partner to record, so alleles are written as the same base letter with different superscripts — IA and IB in the ABO system. Using A and a for two codominant alleles states something false about them before you have written a single line of the cross.