Interpret categorical variation records with denominators and graph choices
Phenotype frequencies describe the sample; category codes are not a measurement scale.
Section B17.1 · Variation
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 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 do separated ABO bars represent?
What you will learn
- Read labelled category counts and percentages
- Choose a graph that preserves the feature type
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Counts | A 11 / B 7 / AB 4 / O 3 |
| Total | 25 invented records |
| Percentages | 44% / 28% / 16% / 12% |
| Graph | Named separated bars, not length intervals |
Read labelled category counts and percentages
A categorical variation record needs a clear feature and key. The second invented ABO sample contains twenty-five phenotypes: A eleven, B seven, AB four and O three. Add the counts before interpreting percentages. Eleven plus seven plus four plus three equals twenty-five. The categories describe phenotypes in the common model, not body measurements or all possible allele-pair genotypes.
Divide by the checked total: A is forty-four per cent, B twenty-eight, AB sixteen and O twelve. The percentages sum to one hundred. A is the most frequent sample phenotype with eleven records. Reordering the four bars does not change the biology or counts, because the categories are names rather than a continuous numerical scale. Gaps between bars help show their separate category status.
Choose a graph that preserves the feature type
Do not calculate an arithmetic mean of arbitrary category codes and report it as an intermediate blood group. If A is coded one and O four, the codes name categories; they do not make O four times the biological amount of A. Compare those bars with the adjoining numerical intervals of a length histogram. In the length case, position on the horizontal axis has a measured order and interval width; in the ABO case, names do not have that quantitative meaning.
Different sample percentages may reflect chance sampling, selection methods or population differences; the two invented charts alone cannot identify which explanation applies. A larger raw A count is not necessarily a larger A fraction when totals differ. Neither chart identifies the IAIA versus IAIo split among A records, proves parentage or supplies clinical information. Written interpretation develops evidence reasoning while teacher and practical review remain separate.
Sources for this lesson
Pause and explain
What is 11 A records out of 25 as a percentage?
Worked example
Find the most frequent phenotype and its percentage in the 25-record ABO sample.
Show the worked solution
- Compare labelled counts 11, 7, 4 and 3; the largest is A with 11.
- Calculate 11/25 × 100 = 44%.
Answer A is the most frequent phenotype in this invented sample, representing 44%; population genotypes are not identified.
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.
Check the total of A 11, B 7, AB 4 and O 3 and find the O percentage.
Give me a hint
Use all valid categories in the denominator.
Compare my reasoning
- 11 + 7 + 4 + 3 = 25.
- 3/25 × 100 = 12%.
Total 25; O is 12% of the invented sample.
Look for these in your work
- Total check
- O percentage with sample scope
Find the B and AB percentages and verify all four sample percentages.
Give me a hint
Divide by 25 for each category.
Compare my reasoning
- B = 7/25 × 100 = 28%; AB = 4/25 × 100 = 16%.
- 44 + 28 + 16 + 12 = 100%.
B 28%, AB 16%; together with A 44% and O 12%, the total is 100%.
Look for these in your work
- Correct B and AB percentages
- Total verification
Choose and justify graph forms for the ABO sample and the four equal-width pod-length intervals.
Give me a hint
Match named categories or continuous intervals to the horizontal scale.
Compare my reasoning
- ABO has separate named categories, suited to separated labelled bars.
- Pod lengths have adjoining measured intervals, suited to an equal-width histogram with the stated numerical axis.
Use a categorical bar chart for ABO and an adjoining equal-width interval histogram for lengths. Do not imply that ABO names are a continuous measurement scale.
Look for these in your work
- Both graph choices
- Feature-type justification
Common mistakes
- Averaging category codes as a new phenotype
- Comparing counts without totals
- Confusing phenotype categories with genotype counts
What can you explain now?
What is 11 A records out of 25 as a percentage?
Compare with the explanation
44%
11 divided by 25 times 100 gives 44.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, read labelled category counts and percentages and choose a graph that preserves the feature type. 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.