Complete dominance is a condition wherein the allele regarded as dominant completely masks the effect of the allele that is recessive. This is the opposite of codominance.

For example, people with blood type A have antigen A. This allele is designated I^A. In the same manner, people with blood type B have antigen B, and an I^B allele is assigned. The third allele is I. If a person has I^A and i alleles, the person will have blood type A. Notice that it does not make any difference whether a person has two I^A alleles or one I^A and one i allele. In both cases, the person is blood type A. Because I^A masks i, we say that I^A is dominant to i. Likewise, a person with I^B and i alleles will have a blood type B because I^B completely masks i.

Consequently, both alleles must be recessive for the recessive allele to be expressed. People who have two i alleles (recessive alleles) do not produce either A or B antigens. Instead, they have type O blood.

The following table summarizes the expression of blood type.

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Key Points

• Complete dominance is a condition wherein the dominant allele completely masks the effect of the recessive allele; consequently, both alleles must be recessive for the recessive allele to be expressed.

Key Terms

complete dominance: a condition that occurs when one allele or “version” of a gene completely masks another.

allele: different types of the same gene on a chromosome

recessive: is the term to describe the allele which is not expressed as a phenotype

dominant: the genotype is the stronger of the two alleles and is expressed

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