Distinguish genotype from phenotype using a stated allele key
Allele combinations describe genotype; observed features describe phenotype.
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 is a genotype record in the stated one-gene model?
What you will learn
- Describe genotype in terms of the alleles present
- Describe phenotype as an observable feature
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| TT | One model genotype; pigment present under the rule |
| Tt | Different genotype; same pigment phenotype |
| tt | Genotype giving pigment absence under the rule |
| Observed phenotype | Does not always identify a unique genotype |
Describe genotype in terms of the alleles present
Genotype is the genetic make-up of an organism. When a question follows one gene, state its genotype in terms of the alleles present at that gene. In an original model using T and t, TT, Tt and tt are three possible two-allele genotypes. These symbols do not describe every gene in the organism; they record only the gene specified in the model.
Phenotype means the observable features of an organism. A feature can be seen, measured or detected using an appropriate method; observable does not mean it must be obvious from a photograph. In a stipulated pigment model, the recorded phenotype might be purple pigment present or purple pigment absent. An allele symbol is part of a genotype record, whereas the pigment feature describes a phenotype.
Describe phenotype as an observable feature
Use the supplied expression rule. If the model states that T gives the purple phenotype whenever present and t gives absence only when paired with t, then TT and Tt share the purple phenotype while tt lacks it. The shared phenotype does not make TT and Tt the same genotype. Do not infer which one is present from the purple feature alone under this rule.
Real phenotypes can also depend on environmental conditions and interactions between genes. A simple classroom rule intentionally controls or omits those complications. State the rule before using it, and do not assume capital and lower-case letters have the same biological meaning in every question. The genotype describes the alleles; the phenotype describes the observable outcome under the supplied conditions.
Pause and explain
If TT and Tt both show pigment, what can pigment alone establish?
Worked example
Under the supplied pigment rule, compare the genotypes and phenotypes of TT, Tt and tt.
Show the worked solution
- Read the genotype as the actual allele pair, keeping TT and Tt distinct.
- Apply the rule that T produces pigment whenever present: TT and Tt have pigment; tt lacks it.
Answer Given the original rule T → pigment when present, TT and Tt both show pigment but have different allele combinations. tt shows no pigment. Genotype and phenotype answer different questions.
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.
Classify Tt and purple pigment present as genotype or phenotype, explaining each choice.
Give me a hint
Ask whether the record names alleles or an observable feature.
Compare my reasoning
- Identify T and t as allele symbols.
- Identify pigment presence as the feature observed.
Tt is the genotype at the model gene; purple pigment present is the phenotype.
Look for these in your work
- Tt labelled genotype
- Pigment presence labelled phenotype
Using the rule T gives pigment whenever present, list the phenotypes of TT, Tt and tt and identify which two share a phenotype.
Give me a hint
Apply the rule to every allele combination.
Compare my reasoning
- Check whether T is present in each genotype.
- Assign pigment or no pigment.
- Compare the observable outcomes.
TT and Tt show pigment; tt has no pigment. TT and Tt share the phenotype but differ in genotype.
Look for these in your work
- All three phenotypes correct
- Shared phenotype distinguished from genotype identity
A seedling shows pigment under this model. Give its possible genotypes and explain why a photograph alone cannot choose between them.
Give me a hint
Find all genotypes that produce the observed feature.
Compare my reasoning
- List TT and Tt under the supplied rule.
- Identify the missing allele-level evidence.
TT or Tt. Both produce the same pigment phenotype under the rule, so a photograph of that feature does not establish the second allele.
Look for these in your work
- Lists TT and Tt
- Explains the allele ambiguity
Common mistakes
- Calling the visible feature a genotype
- Assuming the same phenotype proves the same alleles
- Using a letter without reading the supplied expression rule
What can you explain now?
If TT and Tt both show pigment, what can pigment alone establish?
Compare with the explanation
The phenotype, without distinguishing TT from Tt
The same phenotype is compatible with two genotypes under the rule.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, describe genotype in terms of the alleles present and describe phenotype as an observable feature. 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.