By the end of this lesson
- Define codominance.
- Use appropriate allele notation.
- Solve simple codominant inheritance problems.
BIG QUESTION
What happens when neither allele masks the other?
The heterozygote expresses both allele products, as in human ABO blood group AB.
Both allele products are expressed
In codominanceWhen both alleles of a gene are expressed in the phenotype of a heterozygote, as in the AB blood group., both alleles in a heterozygote make a detectable contribution to phenotype. Neither is described as recessive to the other.
Codominance differs from incomplete dominance: both products are present rather than necessarily blending into an intermediate.
ABO blood groups provide an example
The Iᴬ and Iᴮ alleles are codominant, so genotype IᴬIᴮ gives blood group AB and both A and B antigens are present.
The i allele is recessive to Iᴬ and Iᴮ. ABO inheritance therefore includes both codominance and dominance relationships.
Notation must preserve allele identity
Use superscripts or clearly defined symbols, such as Cᴿ and Cᵂ, rather than upper- and lower-case notation implying dominance.
List parental genotypes, gametes and offspring combinations before stating probabilities.
Allele interactions
Heterozygote phenotype distinguishes the patterns.
| Pattern | Heterozygote | Example model |
|---|---|---|
| Complete dominance | Matches dominant homozygote. | Aa shows A trait. |
| Codominance | Both allele products detectable. | IᴬIᴮ gives AB. |
| Incomplete dominance | Often intermediate phenotype. | Red × white gives pink model. |
| Recessive expression | Seen without dominant allele. | aa. |
Language that earns precision
- codominance
- Both alleles in a heterozygote contribute detectably to phenotype.
- antigen
- Molecule recognised by the immune system; ABO antigens occur on red-cell surfaces.
- ABO blood group
- Blood classification determined by A and B red-cell antigens.
- incomplete dominance
- Heterozygote has an intermediate phenotype in the stated model.
Common misconceptions
Claim: Codominance means the alleles blend permanently.
Correction: Both remain distinct and can be inherited separately.
Claim: AB is a new allele.
Correction: It is the phenotype produced by genotype IᴬIᴮ.
Claim: Uppercase always means dominant in codominance.
Correction: Use defined superscripts because neither codominant allele masks the other.
Match the answer to the command word
Define superscripted allele symbols before the cross.
Translate every offspring genotype into the requested phenotype.
Recall, apply, then justify
Explain codominance using blood group AB.
Hint
Genotype and both antigens.
Suggested marking points
- Iᴬ and Iᴮ are codominant alleles.
- A person with genotype IᴬIᴮ expresses both alleles.
- Both A and B antigens are present, producing blood group AB.
A parent with genotype Iᴬi and a parent with genotype Iᴮi have a child. State the four possible blood-group phenotypes.
Hint
Combine A/i gametes with B/i gametes.
Suggested marking points
- IᴬIᴮ gives blood group AB.
- Iᴬi gives blood group A.
- Iᴮi gives blood group B.
- ii gives blood group O.
Explain why an AB parent and an O parent cannot naturally have an O child in the standard ABO model.
Planning hint
List the gametes each parent can form.
What a strong design includes
- The AB parent has genotype IᴬIᴮ.
- That parent forms Iᴬ or Iᴮ gametes, not i gametes.
- The O parent has genotype ii.
- The O parent forms only i gametes.
- Children can be Iᴬi or Iᴮi.
- No child receives two i alleles, so ii/O is not produced.