Use a test cross to investigate an unknown dominant-phenotype genotype
The recessive partner’s fixed contribution makes the unknown allele contribution informative.
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 genotypes can fit the unknown pigment-producing parent under this key?
What you will learn
- Compare an unknown dominant-phenotype parent with a recessive homozygote
- Use a recessive offspring to identify a heterozygote in the stated model
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Unknown pigment parent | TT or Tt under the supplied rule |
| Known partner | tt; supplies t only |
| Predicted TT case | Only Tt offspring; all possible outcomes show pigment |
| Predicted Tt case | Tt and tt equally; a tt record identifies Tt in the normal model |
Compare an unknown dominant-phenotype parent with a recessive homozygote
A test cross investigates the unknown genotype of an individual showing the dominant phenotype. In the original pigment model, an unknown pigment-producing plant can be TT or Tt. Cross it with a known recessive homozygous tt plant. That partner contributes only t at the gene, making any T or t supplied by the unknown parent informative about its allele pair.
If the unknown parent is TT, its normal gametes carry T and every possible offspring genotype is Tt, showing pigment. If it is Tt, its normal gametes can carry T or t; the square gives equal Tt and tt positions. Under the standard assumptions, the heterozygote case predicts pigment present to absent as 1:1. Keep predicted possible outcomes separate from a particular observed sample.
Use a recessive offspring to identify a heterozygote in the stated model
An observed pigment-absent tt offspring receives t from the known tt partner and t from the unknown parent. The unknown parent also has T because it shows pigment under the supplied complete-dominance rule. These two facts identify it as Tt in the normal one-gene model. The inference depends on the stated trait rule, parentage, accurate records and normal allele transmission.
An observed finite sample showing only pigment does not logically prove TT: a Tt parent can by chance supply T to all the recorded offspring. A larger suitable sample with no recessive outcome can support the TT hypothesis, but uncertainty remains unless the task supplies stronger evidence. The test-cross method predicts contrasting distributions and uses informative offspring; it does not turn a small all-dominant record into a guarantee.
Pause and explain
Which partner makes the normal test cross informative here?
Worked example
How can an unknown pigment-producing T_ parent be investigated using a known tt partner?
Show the worked solution
- Consider both possible pigment-parent genotypes. TT supplies only T and gives Tt; Tt supplies T or t and gives Tt or tt.
- If a normal-model tt offspring is observed, the unknown parent contributed t while also having T for pigment. This identifies Tt under the stated assumptions.
Answer Unknown pigment parent × known tt: TT predicts only Tt; Tt predicts Tt and tt equally. A tt offspring identifies the unknown parent as Tt within the stated normal model.
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.
Compare the possible offspring from TT × tt and Tt × tt. Give genotypes and the pigment phenotype predictions.
Give me a hint
Use the known partner’s t contribution in both cases.
Compare my reasoning
- Combine T from TT with t to give Tt only.
- Combine T or t from Tt with t.
- Apply the pigment expression rule.
TT × tt predicts only Tt, all possible offspring showing pigment. Tt × tt predicts Tt and tt in a 1:1 ratio, with pigment present:absent = 1:1.
Look for these in your work
- Correct two genotype patterns
- Correct contrasting phenotype predictions
An unknown pigment-producing plant crossed with known tt gives at least one pigment-absent offspring. Infer the unknown genotype under the stated normal model.
Give me a hint
The absent offspring is tt and needs a t contribution from each parent.
Compare my reasoning
- Identify tt for the absent offspring.
- Trace t to the unknown parent.
- Combine that with T required for its own pigment phenotype.
The unknown parent is Tt: it shows pigment through T and supplied t to the tt offspring.
Look for these in your work
- Correct Tt inference
- Explains phenotype and offspring contribution
Eight recorded offspring all show pigment after an unknown pigment parent is crossed with tt. Explain why this observation alone does not prove the unknown parent is TT.
Give me a hint
A heterozygote can transmit T in each recorded event by chance.
Compare my reasoning
- Compare the prediction from TT with the possible finite record from Tt.
- Separate an all-pigment sample from all possible outcomes.
Both TT and Tt are compatible with this finite all-pigment record. TT predicts only pigment outcomes, but a Tt parent can by chance supply T in all eight events; absence of a recorded recessive outcome alone is not proof of TT.
Look for these in your work
- Keeps both possibilities compatible
- Distinguishes finite record from predicted possibilities
Common mistakes
- Crossing the unknown parent with another unknown dominant phenotype
- Confusing predicted all-possible outcomes with a finite observed record
- Calling a dominant-phenotype unknown parent tt
What can you explain now?
Which partner makes the normal test cross informative here?
Compare with the explanation
A known tt recessive homozygote
It contributes t only, exposing the unknown parent’s contribution.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, compare an unknown dominant-phenotype parent with a recessive homozygote and use a recessive offspring to identify a heterozygote in the stated model. 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.