Explain codominance as both alleles contributing to a heterozygous phenotype
Both allele contributions form the heterozygous phenotype under the supplied rule.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the genotype R1R2 under the supplied marker key?
What you will learn
- Identify both allele contributions in a supplied codominant model
- Distinguish codominance from complete dominance and an intermediate phenotype
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| R1R1 | Marker one only |
| R1R2 | Both marker one and marker two |
| R2R2 | Marker two only |
| Rule comparison | Codominance is distinct from a single intermediate category |
Identify both allele contributions in a supplied codominant model
Codominance is a situation in which both alleles in a heterozygous organism contribute to the phenotype. In this original marker model, R1 produces marker one and R2 produces marker two. R1R1 has marker one only, R2R2 marker two only, and R1R2 has both detectable markers. The two allele labels refer to versions of one gene, not two unrelated genes.
The heterozygote is not assigned only one of the two homozygous features. Nor does the model replace the two contributions with a single intermediate marker. Both supplied contributions are detectable in the phenotype. The key makes this interpretation explicit; a colour name or a capital letter alone cannot establish the expression pattern.
Distinguish codominance from complete dominance and an intermediate phenotype
Compare the earlier complete-dominance pigment key, where Tt shows the same pigment category as TT. In that model the recessive absence feature is not expressed in Tt, although t remains present and transmissible. In the codominant marker key the heterozygote forms a third observable category, both markers. The same one-gene combination method can be used, but phenotype grouping changes with the expression rule.
A real characteristic may require more detail than this original teaching model. For a question, identify the supplied allele key, which markers or features are measured, and how the heterozygote is described. Do not infer genotype from an unspecified blended colour or claim that every heterozygote for every gene is codominant. ABO blood groups provide a required separate example later in the sequence.
Pause and explain
Which feature demonstrates codominance in this model?
Worked example
Classify R1R1, R1R2 and R2R2 when R1 and R2 produce distinct detectable markers codominantly.
Show the worked solution
- Apply each allele’s contribution: R1 specifies marker one and R2 specifies marker two.
- The homozygotes contribute their respective single marker type; the heterozygote contributes both types, rather than losing one contribution or producing a single intermediate category.
Answer Original key: R1R1 → marker one; R2R2 → marker two; R1R2 → both markers. The heterozygote has both contributions, rather than one replacing the other.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Assign phenotypes to R1R1, R1R2 and R2R2 using the original marker key.
Give me a hint
Apply the rule to each allele pair.
Compare my reasoning
- Assign marker one to R1R1.
- Assign both markers to R1R2.
- Assign marker two to R2R2.
R1R1: marker one only; R1R2: both markers; R2R2: marker two only.
Look for these in your work
- Correct homozygous categories
- Both contributions in heterozygote
Compare Tt under complete pigment dominance with R1R2 under the codominant marker rule. Explain why their heterozygous status does not require the same phenotype grouping.
Give me a hint
Heterozygosity describes alleles; the key describes their expression.
Compare my reasoning
- Identify two different alleles in both.
- Apply the separate expression rules.
Both are heterozygous. Tt shows the T-associated pigment category under complete dominance, whereas R1R2 shows both supplied markers under codominance. Phenotype grouping depends on the expression rule.
Look for these in your work
- Both heterozygous
- Correct distinction between expression rules
A diagram shows an intermediate colour without a genetic or marker key. Explain why that alone does not establish codominance.
Give me a hint
The required evidence is both allele contributions, not any colour change.
Compare my reasoning
- Identify the missing expression key.
- Distinguish two detectable contributions from one intermediate category.
An intermediate colour alone does not establish that both allele contributions are separately detectable. The allele and phenotype key is needed to identify a codominant pattern in the supplied model.
Look for these in your work
- Rejects inference from unspecified intermediate colour
- Requests expression or marker evidence
Common mistakes
- Calling a single intermediate feature proof of codominance
- Treating R1 and R2 as unrelated genes
- Applying the earlier dominant/recessive key without checking the new rule
What can you explain now?
Which feature demonstrates codominance in this model?
Compare with the explanation
Both marker contributions are detectable in the heterozygote
The two alleles both contribute to the phenotype.
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Make it stick
Without your notes, identify both allele contributions in a supplied codominant model and distinguish codominance from complete dominance and an intermediate phenotype. Use the supplied allele key and state what the evidence can establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.