Predict the ABO cross between AB and O model genotypes
The stated gamete types give only A or B, with equal model probabilities.
Section 17.4 · Monohybrid inheritance
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
Which allele can an IoIo parent contribute in the model?
What you will learn
- Construct the IAIB crossed with IoIo square
- Calculate A and B phenotype probabilities without predicting AB or O
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Parents | IAIB × IoIo |
| Gametes | IA or IB; and Io |
| Genotypes | IAIo and IBIo equally |
| Phenotypes | A:B = 1:1; AB and O not predicted in this model |
Construct the IAIB crossed with IoIo square
In the common ABO model, an AB parent has genotype IAIB and an O parent has IoIo. The AB parent can contribute IA or IB through a normal gamete, while the O parent contributes Io. Start from these genotypes rather than treating AB or O as single allele names. Repeated Io columns in the square represent the same available allele form.
The equal-position square gives IAIo, IAIo, IBIo and IBIo. IA is dominant over Io, giving phenotype A; IB is dominant over Io, giving B. The predicted genotype ratio IAIo:IBIo is 1:1 and the phenotype ratio A:B is also 1:1. The two ratios have different labels even though their numbers match.
Calculate A and B phenotype probabilities without predicting AB or O
AB offspring are not predicted in this normal stipulated cross, because the O parent cannot contribute IA or IB; IA and IB cannot both be supplied by the one AB-parent gamete. O offspring are also not predicted, because the AB parent cannot contribute Io. The exclusion follows from the supplied gamete types, not from a general belief that offspring cannot have a parental phenotype.
For sixty expected offspring in the model, A and B each have expected count thirty. Real recorded counts need not be exactly equal in a finite sample. This is an original calculation exercise using stated genotypes, not a claim about a particular family or a substitute for biological records. Rare variation and clinical interpretation lie outside the common syllabus model.
Pause and explain
What are the predicted phenotype categories for IAIB × IoIo?
Worked example
Predict common-model blood-group phenotypes for IAIB × IoIo.
Show the worked solution
- The AB parent supplies IA or IB and the O parent supplies Io. Combine to obtain IAIo or IBIo.
- Apply the common expression rule: IAIo is A and IBIo is B. Equal transmission gives A:B = 1:1, with neither AB nor O possible under these assumptions.
Answer IAIB × IoIo: IA or IB combines with Io, giving IAIo or IBIo equally; A:B = 1:1.
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.
Give the gamete types and possible offspring genotypes for IAIB × IoIo.
Give me a hint
Combine one AB-parent allele with the O-parent Io.
Compare my reasoning
- List IA or IB from IAIB.
- List Io from IoIo.
- Combine to form IAIo or IBIo.
Gametes IA or IB, and Io; offspring IAIo or IBIo.
Look for these in your work
- Correct gamete types
- Correct two offspring genotypes
For 100 expected offspring in the same model, calculate A and B counts and give the labelled phenotype ratio.
Give me a hint
Each category has probability one half.
Compare my reasoning
- Calculate 100 × 1/2 for A.
- Use the same probability for B.
Expected A 50 and B 50; phenotype A:B = 1:1.
Look for these in your work
- Correct fifty and fifty
- Expected wording and labelled ratio
Explain why neither AB nor O is predicted from the supplied IAIB and IoIo parents in the normal model.
Give me a hint
Check the required contributions for IAIB and IoIo offspring.
Compare my reasoning
- AB needs IA and IB from separate gamete contributions.
- The O parent supplies only Io.
- O needs Io from both contributions, but the AB parent lacks it.
AB cannot be formed because the O parent supplies neither IA nor IB. O cannot be formed because the AB parent supplies no Io. The supplied gamete types therefore predict only A or B in the common normal model.
Look for these in your work
- Correct exclusion of AB
- Correct exclusion of O
Common mistakes
- Passing IA and IB together through one normal gamete
- Predicting AB without an A or B contribution from the O parent
- Predicting O without Io from the AB parent
What can you explain now?
What are the predicted phenotype categories for IAIB × IoIo?
Compare with the explanation
A and B in a 1:1 ratio
The combinations are IAIo and IBIo.
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Make it stick
Without your notes, construct the iaib crossed with ioio square and calculate a and b phenotype probabilities without predicting ab or o. 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.