Complimentary 1-hour tutoring consultation Schedule Now

Complimentary 1-hour tutoring consultation
Schedule Now

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.


Practice Questions

Khan Academy

Collagen Mutations Result in Osteogenesis Imperfecta

Inheritance patterns in flowering plants

MCAT Official Prep (AAMC)


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



Billing Information
We had trouble validating your card. It's possible your card provider is preventing us from charging the card. Please contact your card provider or customer support.
{{ cardForm.errors.get('number') }}
{{ registerForm.errors.get('zip') }}
{{ registerForm.errors.get('coupon') }}
Tax: {{ taxAmount(selectedPlan) | currency spark.currencySymbol }}

Total Price Including Tax: {{ priceWithTax(selectedPlan) | currency spark.currencySymbol }} / {{ selectedPlan.interval | capitalize }}