Explain genetic and environmental variation
Use examples to explain variation caused by genes, environment or their interaction.
Section 17.1 · Variation
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
Can phenotype be affected by both genotype and environment?
What you will learn
- Explain that phenotypic variation can be genetic, environmental or both
- Use examples that distinguish inherited differences from environmental effects
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
Phenotypic variation can result from genetic differences, environmental differences or interactions between them. ABO blood group is primarily genetic; plant height can reflect genotype and growing conditions; a scar results from environmental experience. The same genotype can show different phenotypes under different environments, and different genotypes can respond differently to one environment.
Interpret data with care
Do not force every characteristic into a purely genetic or purely environmental category. State the factors relevant to the example and distinguish inherited allele variation from effects acquired during life.
Pause and explain
A trait varies among individuals and is also affected by nutrition. How should the variation be described?
Worked example
A plant-height difference is observed between seedlings of the same genotype grown with different light levels. Explain the source of variation.
Show the worked solution
- The genotypes are held alike.
- The environmental light condition differs.
Answer The height difference is environmental in this comparison because the seedlings share a genotype but experienced different light conditions.
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.
A population has different flower colours caused by different alleles. Classify the main source of variation.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- The difference follows inherited allele variation.
- This is genetic variation.
The colour differences are genetic variation because they are associated with different inherited alleles.
Look for these in your work
- Population process identified
- Cause and consequence linked
Two genetically similar plants differ in leaf size after growing in different water conditions. Explain the evidence.
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- The water conditions are different.
- Environment can change phenotype without a new allele.
The difference may be environmental because the plants experienced different water availability; the observation alone does not show a genetic difference.
Look for these in your work
- Evidence interpreted
- Assumptions stated
A trait varies among individuals and is also affected by nutrition. How should the variation be described?
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- Both inherited and environmental factors can contribute.
- The observed phenotype can reflect their interaction.
The variation may reflect both genotype and environment. Nutrition can affect phenotype, while inherited differences can also influence the trait or its response to nutrition.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Calling every measurable trait genetically determined
- Treating an environmental effect as an inherited allele
What can you explain now?
Can phenotype be affected by both genotype and environment?
Compare with the explanation
Yes
Many characteristics reflect both inherited information and environmental conditions.
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Make it stick
How can genetic and environmental factors each contribute to phenotype?
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.