Explain the genetic basis of variation patterns
Relate discontinuous and continuous variation to the number and effect of genes and environmental influence.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers and calculations are self-checked; this is not fieldwork or practical-skills certification.
Before you begin
- Populations contain inherited variation, and alleles can be transmitted from parents to offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What commonly contributes to continuous variation?
What you will learn
- Explain the genetic basis of discontinuous variation
- Explain how many genes and environmental factors can contribute to continuous variation
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Usual discontinuous model | Genes only, with model limits |
| Continuous variation | Genes and environment contribute |
| Line A means | 30 mm / 45 mm: difference 15 mm |
| Line B means | 34 mm / 48 mm: difference 14 mm |
Build the evolutionary explanation
Discontinuous variation is often associated with one or a small number of genes producing distinct categories, although environment can still influence expression. Continuous variation commonly reflects several genes with additive effects and environmental contributions, producing a broad range. The number of loci and the environment together influence the observed distribution.
Interpret data with care
These are common patterns, not absolute rules. Do not infer a specific number of genes from a histogram alone; connect the genetic explanation to evidence or syllabus examples.
Pause and explain
A trait’s distribution widens when plants are grown in less controlled conditions. What does this suggest?
Worked example
Why can many individuals with different genotypes show a continuous range of heights?
Show the worked solution
- Several loci can contribute to the trait.
- Environmental conditions can add further variation.
Answer Multiple genes and environmental influences can combine to produce many intermediate heights.
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.
Explain why a characteristic influenced by many genes may show a continuous distribution.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- Each locus can contribute to the phenotype.
- Many allele combinations create intermediate outcomes.
Several genes can each contribute to the phenotype, producing many possible combinations and intermediate values; environmental effects can broaden the range further.
Look for these in your work
- Population process identified
- Cause and consequence linked
A trait has two clear categories associated with alleles at one locus. What genetic pattern is consistent with this observation?
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- A small number of genes can produce distinct categories.
- The pattern is consistent with discontinuous variation.
It is consistent with discontinuous variation controlled by one or a small number of genes, although the evidence should not be overgeneralised.
Look for these in your work
- Evidence interpreted
- Assumptions stated
A trait’s distribution widens when plants are grown in less controlled conditions. What does this suggest?
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- Environmental variation may contribute.
- Genetic effects may still also matter.
The wider distribution suggests environmental conditions add phenotypic variation; it does not show that genetic variation is absent.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Treating every discontinuous trait as controlled by one gene only
- Ignoring environmental effects on continuous traits
What can you explain now?
What commonly contributes to continuous variation?
Compare with the explanation
Several genes and environmental factors
Their combined effects can produce a range of intermediate phenotypes.
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Make it stick
Why do many-gene traits often show continuous rather than discrete phenotypes?
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.