Lesson focus: variationThe differences between living things of the same kind, such as different heights in a class. describes differences among individuals and can arise from inherited alleleOne version of a gene, such as the version for blue eyes or for brown eyes., environmental conditions and interactionWhen one factor changes how much another factor matters, such as light only helping growth when there is enough water. between them.
Classify sources of variation and explain why most biological traits cannot be labelled simply inherited or environmental.
Build on earlier learning
Adapting to changePopulation responses to change depend on differences among individuals. This lesson examines where those differences come from.
You will be successful when you can…
- Define variation within a species.
- Identify inherited and environmental contributions.
- Explain gene–environment interaction.
- Avoid deterministic claims about complex traits.
Words that carry the science
- variation
- Differences in characteristics among individuals of the same species.
- genotype
- The alleles an organism carries, at one or more genetic loci.
- phenotype
- An observable or measurable characteristic produced by genotype and environment.
- allele
- An alternative version of a gene or genetic sequence.
- mutation
- A change in DNA sequence that can create new genetic variation.
- interaction
- A situation in which the effect of one factor depends on another factor.
Treat phenotype as the outcome of genes, environment and interaction
01Inherited variation begins with DNA differences
Sexual reproduction combines alleleOne version of a gene, such as the version for blue eyes or for brown eyes. from two parents through meiosis and fertilisation. mutationA random change in the DNA of a gene, which may produce a new allele. creates new DNA variants, while recombination and independent assortment create new allele combinations.
Inherited does not mean unchanged by the environment. A genetic difference may alter a range of possible phenotypeThe features you can see or measure in a living thing, such as height or flower colour. rather than specify one fixed outcome.
02Environmental conditions alter phenotype
Diet, exercise, disease, light, temperature, education and experience can influence measurable traits. Scars are strongly environmental, while blood group is strongly genetic under normal conditions.
Many important traits, including height, body mass, crop yield and behaviour, involve many genes and environments. Categories are useful starting points but not a complete causal model.
03Genes and environment interact
A genotype may produce different phenotypes in different environments, and environmental effects may differ among genotypeThe pair of gene versions (alleles) a living thing has for a feature, such as Ff.. For example, plant varieties can respond differently to the same drought treatment.
To investigate causes, use sufficiently large samples, standardised measurements and designs that separate relatedness from shared environment where possible.
Variation in a population
Variation is the differences between individuals of the same species. It can be caused by the genes you inherit, by your environment, or by both. Measure a sample of people and plot the results.
Controls
Values are illustrative (random samples). Bigger samples give a smoother, more reliable pattern.
Classify a phenotype carefully
Two genetically different seedlings grow to different heights under both low and high light.
- Height is the phenotype measured.
- Genetic differences may contribute because the seedlings differ genetically.
- Light is an environmental factor that can alter growth.
- If varieties respond by different amounts, the data show a genotype–environment interactionWhen one factor changes how much another factor matters, such as light only helping growth when there is enough water..
How to make the reasoning visible: A good explanation can include genetic contribution, environmental contribution and interaction at the same time.
Interpret a common-environment data set
Mean heights are illustrative values after six weeks.
| Plant variety | Low light / cm | High light / cm | Response to light | Inference |
|---|---|---|---|---|
| A | 12 | 20 | +8 | Genotype and environment contribute |
| B | 16 | 22 | +6 | Different baseline and response |
| C | 10 | 25 | +15 | Strong response in this environment |
| Clones of A | 12–13 | 19–21 | Similar within treatment | Small residual environmental variation |
What evidence supports an environmental effect?
What evidence supports inherited differences?
Check your understanding
Answer from memory, then check against the suggested answer.
What is a phenotype?
What creates new alleles?
Why is height not purely inherited?
Identical twins differ slightly in a measured trait. Does this prove genes are unimportant?
Explain why heritability is a population statistic rather than a fixed property of a trait.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Plants of one species grow to different heights.
Explain the genetic causes of this variation.
(2)Explain the environmental causes and how they combine with genes.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., give a genetic mechanism and an environmental mechanism, then connect them in the phenotype.