Plan a reproducible investigation of discontinuous pea phenotypes
Define categories, sample without selecting outcomes and report unclassified specimens.
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 genes, alleles, genotype and phenotype from the inheritance pathway. Use the stated organism, units and sample; the records in these lessons are invented.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What should happen to an unclassifiable damaged specimen?
What you will learn
- Use a supplied phenotype key to tally specimens
- Report exclusions and limits of categorical evidence
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Categories | Round / wrinkled under the supplied key |
| Valid sample | 25: round 15 / wrinkled 10 |
| Unclassified example | 3 damaged; 28 examined in total |
| Evidence limit | Counts alone do not establish the parental cross |
Use a supplied phenotype key to tally specimens
To investigate discontinuous variation, choose a same-species feature with a defined supplied key, such as mature pea seed shape. State the categories round and wrinkled, the specimen stage and the decision features from the key. Use clearly identified safe specimens or suitable supplied photographs. The classification is an observation under that key; it does not automatically establish a dominant allele, the parent genotypes or a universal rule for damaged seeds.
Choose specimens from a stated frame without deliberately selecting only round seeds or a desired ratio. Tally each valid specimen once and include a total check. If a seed is damaged, immature or not classifiable using the key, record that fact separately and explain how it affects the valid-record denominator. Removing unclear records silently makes the reader unable to evaluate the sampling and decision method.
Report exclusions and limits of categorical evidence
Suppose a sample has twenty-five clearly classifiable seeds: fifteen round and ten wrinkled. The valid phenotype percentages are sixty and forty per cent, and the observed count ratio is three to two. These numbers describe the sample. Without a supplied parental cross and inheritance rule, they do not establish the expected offspring probability or prove a deviation from a predicted Mendelian ratio.
Use labelled separated bars or a category table to describe the results. Compare more samples using explicit denominators rather than the larger raw count alone. A sample with more round records can still have a smaller round percentage if it contains many more seeds in total. Written classification planning and invented records practise scientific reasoning; supervision, actual observation accuracy and practical competence remain separate assessment work.
Pause and explain
Do observed phenotype counts alone establish parental genotypes?
Worked example
Describe fifteen round and ten wrinkled valid seeds in a twenty-five-seed sample.
Show the worked solution
- Check the total 15 + 10 = 25.
- Calculate 15/25 × 100 = 60% and 10/25 × 100 = 40%; simplify 15:10 to 3:2.
Answer The valid sample is 60% round and 40% wrinkled, with observed ratio 3:2. A parental cross is not established.
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.
Calculate round and wrinkled percentages from 15 and 10 valid records.
Give me a hint
Use the 25-record denominator.
Compare my reasoning
- 15/25 × 100 = 60%.
- 10/25 × 100 = 40%.
Round 60%, wrinkled 40% of the valid sample.
Look for these in your work
- Total 25
- Both percentages labelled
Three damaged seeds accompany those 25 valid records. Report examined, valid and unclassified totals and the denominator for the stated phenotype percentages.
Give me a hint
Examined and classified totals can differ.
Compare my reasoning
- 25 + 3 = 28 examined.
- Only 25 have valid supplied-key phenotypes.
28 examined, 25 validly classified, 3 unclassified; the given 60%/40% phenotype percentages use 25.
Look for these in your work
- All three totals
- Explicit valid-record denominator
Sample X has 15 round out of 25, while sample Y has 24 round out of 50. Which has the larger round proportion, and can the two raw counts settle it?
Give me a hint
Calculate both percentages first.
Compare my reasoning
- X: 15/25 = 60%.
- Y: 24/50 = 48%.
X has the larger round proportion, 60% versus 48%, despite fewer round records. Raw counts alone cannot settle a comparison with different totals.
Look for these in your work
- Both sample denominators
- Proportion distinguished from count
Common mistakes
- Choosing only specimens with a desired phenotype
- Hiding unclassified records
- Inferring parental genotypes from one observed count ratio
What can you explain now?
Do observed phenotype counts alone establish parental genotypes?
Compare with the explanation
No; a cross and inheritance evidence are missing
A descriptive sample is not a complete genetic investigation.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, use a supplied phenotype key to tally specimens and report exclusions and limits of categorical evidence. Keep units, the stated sample and the limits of the evidence clear.
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.