Infer genotypes in a stated recessive pedigree while keeping ambiguity explicit
Use the stated recessive rule to infer parents, while an unfilled sibling can remain ambiguous.
Exam scope and assessment notes
Core and Extended · 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 genotype gives pigment absence under this rule?
What you will learn
- Use a recessive offspring to infer parental allele contributions
- Keep an unfilled sibling’s two possible genotypes separate
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Filled trait key | Pigment absent, recessive tt |
| I-1 and I-2 | Pigment present; each supplied t; both Tt |
| II-1 | Filled offspring: tt |
| II-2 | Unfilled offspring: TT or Tt |
Use a recessive offspring to infer parental allele contributions
Use this declared autosomal pigment model: T gives pigment when present and tt gives pigment absence. In this pedigree, filled symbols mean pigment absence, the recessive feature. Both parents I-1 and I-2 are unfilled and therefore show pigment. Their offspring II-1 is filled and has no pigment, while II-2 is unfilled and shows pigment. The pedigree is invented and follows the stated normal inheritance rule.
The filled II-1 individual has genotype tt because absence is expressed only without T. One t contribution must come from each parent. Each unfilled parent also shows the dominant pigment feature, so each must contain T. Combining these two deductions gives Tt for both I-1 and I-2. The inference uses both the phenotype rule and the supplied parent-child links, not shading alone.
Keep an unfilled sibling’s two possible genotypes separate
The unfilled sibling II-2 has pigment, so its genotype can be TT or Tt under this rule. Having a recessive sibling does not choose between those possibilities. Both are possible in the parental Tt × Tt square. Write the ambiguity explicitly instead of assigning Tt to every unfilled symbol or claiming all siblings must share one genotype.
For a later offspring under the same independent normal model, the cross predicts three pigment to one absence in phenotypes. The already observed pedigree does not force the next offspring to compensate toward a perfect ratio. These deductions rely on the supplied gene, expression rule, parentage and normal transmission assumptions; the diagram does not independently establish a mechanism for an unspecified real characteristic.
Pause and explain
What are the two pigment-producing parents’ genotypes if they have a tt offspring in this normal model?
Worked example
In the supplied recessive-key pedigree, what can be inferred from two pigment-producing parents and a tt offspring?
Show the worked solution
- A tt child received t from each parent under normal transmission. Each parent must therefore contain t.
- The parents have pigment and thus also contain T: both are Tt. A pigment-producing sibling could be TT or Tt, so retain that uncertainty.
Answer Unfilled pigment-producing parents with a filled tt offspring must each contribute t and also carry T: both are Tt. The unfilled sibling remains TT or Tt.
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.
Use the filled recessive offspring II-1 to determine its genotype and the allele it received from each parent.
Give me a hint
The declared recessive phenotype requires two t alleles.
Compare my reasoning
- Assign tt to II-1.
- Trace one t contribution from each linked parent.
II-1 is tt and received t from I-1 and t from I-2 in this normal model.
Look for these in your work
- Correct tt genotype
- One t contribution from each parent
Both parents show pigment and each supplied t. Determine their genotypes and list possible genotypes for pigment-producing sibling II-2.
Give me a hint
Each parent needs T for its phenotype and t for the observed transmission.
Compare my reasoning
- Combine T and t in each parent.
- Identify pigment-producing offspring genotypes in Tt × Tt.
I-1 and I-2 are both Tt. II-2 can be TT or Tt; its pigment phenotype does not distinguish those possibilities.
Look for these in your work
- Both parental Tt genotypes
- Sibling ambiguity TT or Tt retained
A student predicts the next child must show pigment because one recessive offspring is already present in the pedigree. Correct this under the independent model.
Give me a hint
An observed outcome does not impose balancing on the next event.
Compare my reasoning
- Retain the Tt × Tt cross.
- State the predicted phenotype probabilities.
- Reject compulsory compensation.
The next offspring has pigment probability 3/4 and absence probability 1/4 under the model. The existing recessive offspring does not guarantee the next phenotype or force the pedigree into a perfect ratio.
Look for these in your work
- Correct three-quarter and one-quarter probabilities
- No forced-compensation claim
Common mistakes
- Assigning recessiveness from shading without the key
- Calling every pigment-producing sibling Tt
- Treating the observed pedigree as a compulsory future ratio
What can you explain now?
What are the two pigment-producing parents’ genotypes if they have a tt offspring in this normal model?
Compare with the explanation
Both Tt
Each supplied t to the offspring and also has T for its own phenotype.
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Make it stick
Without your notes, use a recessive offspring to infer parental allele contributions and keep an unfilled sibling’s two possible genotypes separate. 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.