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MCAT Content / Mendelian Concepts / Complete Dominance

Complete dominance

Topic: Mendelian Concepts

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



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