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.
MCAT Official Prep (AAMC)
- 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.
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