Describe continuous variation using a measured range of body lengths
Intermediate measurements form a range; a small sample need not contain every value.
Section B17.1 · Variation
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
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
Which required example is continuous?
What you will learn
- Identify a continuous phenotype range
- Interpret measured extremes without inventing categories
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Feature | Pod body length in mm |
| Sample extremes | 42 mm and 76 mm |
| Sample range | 76 − 42 = 34 mm |
| Missing value | No record is not proof of impossibility |
Identify a continuous phenotype range
Continuous variation gives a range of phenotypes between two extremes, rather than a few separated categories with no intermediate phenotypes. Body length and body mass are the required examples. A body-length measurement may lie between the shortest and longest recorded individuals. The biological feature is continuous even when a measuring instrument records only whole millimetres or when only a small number of individuals are sampled.
The original pod records span 42 mm to 76 mm. Intermediate lengths include 45, 50, 58, 63 and 70 mm. The sample range is the largest recorded value minus the smallest: seventy-six minus forty-two gives thirty-four millimetres. The range describes this sample’s spread. It does not establish the shortest and longest lengths possible in the species or identify a genetic cause for the spread.
Interpret measured extremes without inventing categories
The dot plot places observations on a numerical scale. Positions correspond to the lengths rather than arbitrary category names. A gap at a particular number does not show that this biological length is impossible: the sample may simply contain no individual recorded at that value. Rounding measurements can also make a continuous underlying feature look like a list of discrete recorded numbers.
Do not use invented labels such as short and long to decide that the underlying feature has become discontinuous. Those labels are categories imposed by the observer on a length scale, and intermediate lengths are still possible. Record the actual measurement rule, units and sample boundaries. The same reasoning applies to body mass, but mass and length need their own units and cannot be combined into one numerical range.
Sources for this lesson
Pause and explain
A small length sample has no record at 54 mm. What follows?
Worked example
Find the range of the supplied pod lengths and explain why missing 54 mm does not prove discontinuity.
Show the worked solution
- Use the extremes 42 mm and 76 mm: 76 − 42 = 34 mm.
- A continuous feature permits intermediate values; twenty sampled records and whole-mm reporting do not require every value to appear.
Answer The recorded range is 34 mm. No 54 mm record does not make body length a discontinuous biological feature.
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.
Use the supplied shortest 42 mm and longest 76 mm pod records to calculate the sample range.
Give me a hint
Range subtracts the minimum from the maximum.
Compare my reasoning
- Identify the two extremes in the same unit.
- Calculate 76 − 42.
The sample range is 34 mm.
Look for these in your work
- Correct subtraction
- Millimetres and sample scope
A second same-species sample has recorded lengths 51, 53, 56, 62 and 67 mm. Find its range and explain the absence of 60 mm.
Give me a hint
Distinguish sample values from possible biological values.
Compare my reasoning
- Subtract 51 from 67 to obtain the sampled spread.
- A five-individual sample need not contain each intermediate measured value.
The range is 16 mm; no 60 mm observation does not prove that 60 mm is biologically impossible.
Look for these in your work
- Range 16 mm
- Finite-sample explanation
A researcher calls lengths below 60 mm short and all others long, then claims the original feature is discontinuous. Correct the claim.
Give me a hint
Classifying a continuous measurement does not change the underlying scale.
Compare my reasoning
- The short/long boundary is an observer’s rule.
- Lengths on either side and between measured values remain possible on the biological scale.
The researcher has categorised a continuous length feature. The imposed two-label rule does not remove intermediate body lengths.
Look for these in your work
- Observer-imposed categories
- Underlying continuous range
Common mistakes
- Confusing instrument rounding with biological categories
- Treating sample extremes as species-wide limits
- Adding length and mass into one range
What can you explain now?
A small length sample has no record at 54 mm. What follows?
Compare with the explanation
The sample lacks that recorded value
Sampling and rounding do not establish biological impossibility.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, identify a continuous phenotype range and interpret measured extremes without inventing categories. 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.