Infer genotypes in a stated dominant pedigree using a recessive offspring
A declared dominant-key pedigree can resolve Tt from a recessive offspring and known partner.
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
What does an unfilled individual mean in this pedigree?
What you will learn
- Apply the dominant trait key to parents and offspring
- Use known gamete contributions to resolve a dominant genotype
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Filled trait key | Pigment present: TT or Tt before further evidence |
| Unfilled I-2 and II-1 | Pigment absent: tt |
| I-1 after offspring evidence | T for pigment and transmitted t: Tt |
| Filled II-2 with tt parent | Must be Tt, not TT |
Apply the dominant trait key to parents and offspring
Use the same T/t pigment gene with a different shading key: in this pedigree, filled means pigment present, the dominant feature. Parent I-1 is filled and parent I-2 is unfilled. Offspring II-1 is unfilled and II-2 is filled. Square and circle shapes follow the supplied male/female symbol key, but the gene is autosomal and the phenotype rule is the same for either shape.
The unfilled I-2 parent has genotype tt, since pigment absence requires no T. Unfilled offspring II-1 is also tt. That offspring received t from each parent, so the filled I-1 parent must have t as well as T for its own pigment phenotype. I-1 is therefore Tt, not TT. This resolves a previously ambiguous dominant phenotype by using the linked offspring evidence.
Use known gamete contributions to resolve a dominant genotype
Filled offspring II-2 must receive t from the tt parent I-2. To show pigment it must receive T from I-1, making its genotype Tt. It cannot be TT in this normal model because the tt parent supplies no T. If only II-2 had been observed, without the recessive II-1 record, I-1 could still have been TT or Tt; the exact evidence supporting the deduction matters.
The resulting Tt × tt parental cross predicts pigment present to absent as 1:1. It does not require an alternating sequence or a perfectly balanced finite pedigree. Nor does filled always mean dominant in other diagrams: compare this key with the preceding recessive-key example. Read the current trait key, expression rule and family lines each time rather than carrying shading assumptions between pedigrees.
Sources for this lesson
Pause and explain
Why must filled I-1 be Tt when its linked offspring is tt?
Worked example
A pigment-producing I-1 with a tt partner has a tt child. Infer I-1 and a pigment-producing sibling.
Show the worked solution
- The tt child establishes that I-1 supplied t. Its pigment phenotype also establishes T, giving Tt.
- The pigment-producing sibling receives t from the tt partner and must receive T for pigment, giving Tt rather than TT.
Answer Filled I-1 with unfilled tt partner has an unfilled tt offspring: I-1 must be Tt. A filled offspring with that tt parent is Tt, not 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.
In the declared dominant-key pedigree, assign the genotypes of unfilled parent I-2 and unfilled offspring II-1.
Give me a hint
Unfilled means pigment absent under this key.
Compare my reasoning
- Apply absence requires tt to I-2.
- Apply the same rule to II-1.
I-2 is tt and II-1 is tt in this normal model.
Look for these in your work
- Correct parent tt
- Correct offspring tt
Filled I-1 shows pigment and supplied t to II-1. Determine I-1 and filled offspring II-2, whose other parent is tt.
Give me a hint
Use both phenotype and parental contribution evidence.
Compare my reasoning
- Assign Tt to I-1.
- Use t from I-2 for II-2.
- Add T to produce II-2’s pigment phenotype.
I-1 is Tt. II-2 is Tt because it receives t from its tt parent and T for the dominant phenotype from I-1.
Look for these in your work
- Correct I-1 Tt
- Correct II-2 Tt with source explanation
If the pedigree recorded only the filled offspring II-2 and the same parents, explain which genotypes could fit filled I-1 and why the unfilled II-1 record resolves the uncertainty.
Give me a hint
A filled parent could be TT or Tt until t transmission is established.
Compare my reasoning
- List TT and Tt from I-1’s phenotype alone.
- Use the tt offspring to establish a t contribution.
Without II-1, I-1 could be TT or Tt. The tt offspring II-1 establishes that I-1 supplied t; combined with its pigment phenotype, this identifies Tt under the stated normal model.
Look for these in your work
- Retains TT or Tt before extra evidence
- Explains why tt offspring resolves I-1 as Tt
Common mistakes
- Carrying the previous diagram’s shading meaning into this one
- Calling every dominant phenotype TT
- Ignoring the known tt parent when assigning a filled offspring genotype
What can you explain now?
Why must filled I-1 be Tt when its linked offspring is tt?
Compare with the explanation
It carries T for its own phenotype and supplied t to that offspring
Both pieces of evidence identify its allele pair.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, apply the dominant trait key to parents and offspring and use known gamete contributions to resolve a dominant genotype. 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.