Explain the four ABO phenotypes and six common-model genotype combinations
Three available allele types give six common-model genotypes and four phenotypes.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
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
How many ABO alleles are in one diploid genotype in this model?
What you will learn
- Connect IA IB and Io alleles with ABO phenotypes
- Separate codominance from dominance over Io
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| A phenotype | IAIA or IAIo |
| B phenotype | IBIB or IBIo |
| AB phenotype | IAIB; IA and IB codominant |
| O phenotype | IoIo; Io recessive to IA and IB |
Connect IA IB and Io alleles with ABO phenotypes
The Cambridge model uses three ABO allele forms: IA, IB and Io. These plain-text labels represent I with superscripts A, B and o in conventional notation. A diploid individual has two alleles at the ABO gene, even though three forms are available in the model population. Three available allele types does not mean every individual carries all three.
IA and IB are codominant with each other: IAIB gives the AB phenotype, with both A and B contributions. Each is dominant over Io in the common classroom model. IAIA and IAIo give phenotype A; IBIB and IBIo give phenotype B. IoIo gives O. Thus there are six unordered genotype combinations but four named phenotypes: A, B, AB and O.
Separate codominance from dominance over Io
AB is a phenotype rather than the name of one allele. IAIB is its two-allele genotype. Similarly, O is a phenotype and Io is an allele label; IoIo is the model genotype for that phenotype. A and B phenotypes each leave two common-model genotype possibilities, so the phenotype alone does not tell you whether Io is carried.
The ABO diagram here is a syllabus inheritance model, not a blood test or a complete account of all rare biological variants. Use supplied genotype records when calculating a cross. Do not use classroom phenotype predictions as transfusion guidance or to establish real parentage. The teaching task is to connect the allele key, possible contributions and four phenotype categories under the stated common model.
Sources for this lesson
Pause and explain
Which genotype gives AB in the common model?
Worked example
List the possible genotypes for the four common-model ABO phenotypes.
Show the worked solution
- Use the relationship that IA and IB are codominant with each other and each dominates Io.
- A can be IAIA or IAIo; B can be IBIB or IBIo; both contributions require IAIB for AB, and no A or B contribution requires IoIo for O.
Answer Common model: A → IAIA or IAIo; B → IBIB or IBIo; AB → IAIB; O → IoIo. IA and IB are codominant with one another and each dominant over Io.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
List the common-model genotype possibilities for phenotype A and for phenotype B.
Give me a hint
Each phenotype can include or omit the recessive Io contribution.
Compare my reasoning
- List IAIA and IAIo for A.
- List IBIB and IBIo for B.
A: IAIA or IAIo; B: IBIB or IBIo.
Look for these in your work
- Two correct A possibilities
- Two correct B possibilities
Assign phenotypes to IAIB, IoIo, IAIo and IBIo. State the two different expression relationships used.
Give me a hint
IA and IB are codominant; each is dominant over Io.
Compare my reasoning
- Assign AB to IAIB and O to IoIo.
- Assign A to IAIo and B to IBIo.
IAIB gives AB, IoIo gives O, IAIo gives A and IBIo gives B. IA and IB are codominant with each other, while each is dominant over Io in this model.
Look for these in your work
- Four correct phenotypes
- Correct codominance and dominance distinction
A supplied record names phenotype A but gives no genotype. Explain what you can infer for a cross and why a genotype must not simply be assumed.
Give me a hint
Phenotype A has two common-model possibilities.
Compare my reasoning
- Retain IAIA or IAIo.
- Identify their different possible gamete allele types.
- Request or explicitly condition on the genotype.
The common-model possibilities are IAIA and IAIo. They give different gamete alternatives, so a cross prediction must use a supplied genotype or state separate conditional cases rather than assume IAIA.
Look for these in your work
- Retains two possibilities
- Connects ambiguity to gamete and cross predictions
Common mistakes
- Giving one individual three alleles because three types are available
- Calling AB a single allele
- Assuming phenotype A identifies IAIA rather than IAIo
What can you explain now?
Which genotype gives AB in the common model?
Compare with the explanation
IAIB
Both A and B allele contributions are expressed.
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Make it stick
Without your notes, connect ia ib and io alleles with abo phenotypes and separate codominance from dominance over io. 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.