Co-dominance is a form of inheritance wherein both alleles for the same characteristic are simultaneously and equally expressed in the phenotype.

You may have seen codominance in the world around you when two alleles are expressed equally. For example, when a brown cow mates with a cream coloured cow, this may result in a cow which is brown, but with cream spots, this is because of some of each allele being expressed as the phenotypes. This is codominance.

A classic example of codominance in humans is the AB blood type. People are blood type A if they have antigen A. This allele is designated I^A. In the same manner, people are blood type B if they have an antigen B. People who have alleles (I^A and I^B) produce both antigens A and B. As a result, they are blood type AB. Because the effect of both alleles is observed, we say that the I^A and I^B alleles are codominant.

Practice Questions

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Key Points

• Co-dominance is a condition wherein both alleles for the same characteristic are simultaneously and equally expressed, leading to mixed phenotypes.

Key Terms

codominance:  occurs when two versions, or “alleles,” of the same gene are present in a living thing, and both are expressed

phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

allele – different types of the same gene on a chromosome