What you need to know
- 3.20(i)phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. results from genotypeThe alleles an organism has for a gene. interacting with the environment.
- 3.21multiple allelesWhen a gene has more than two alleles in a population, such as the ABO blood group gene., polygenicDescribes a characteristic controlled by many genes, often giving continuous variation. inheritance and continuous variation.
By the end, you should be able to…
- Distinguish multiple alleles from multiple geneA sequence of DNA bases that codes for a polypeptide or a functional RNA..
- Explain how polygenic inheritance can produce continuous variation.
- Interpret comparisons across genotypes and environments.
Build your understanding
A phenotype develops in an environment
phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. means observable characteristics, including physiology as well as appearance. It results from genotypeThe alleles an organism has for a gene. interacting with environmental conditions. Genotype can affect which proteins are made; environmental factors can affect their expression, activity and the resources available for growth.
For example, plant height can depend on alleleA different version of the same gene, found at the same locus. at many loci as well as light, water and mineral supply. Genetically identical plants can grow differently in different conditions. A difference between environments does not necessarily mean the DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. sequence changed.
Multiple alleles: one locus, several alternatives
A gene can have more than two alleles in a populationAll the individuals of one species living in one area at the same time., but a typical diploidHaving two sets of chromosomes (2n), one from each parent. individual carries at most two alleles at that autosomal locusThe position of a gene on a chromosome.. multiple allelesWhen a gene has more than two alleles in a population, such as the ABO blood group gene. refers to the population’s alternatives at one gene, not to several geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. influencing a trait.
In the conventional ABO model, Iᴬ, Iᴮ and i are three alleles. Iᴬ and Iᴮ are codominant, and each is dominantAn allele that is expressed in the phenotype whenever it is present. to i. IᴬIᴮ gives group AB; Iᴬi gives A; Iᴮi gives B; ii gives O. These are discrete categories, demonstrating that multiple alleles do not automatically produce continuous variation.
Polygenic inheritance: contributions from several loci
A polygenicDescribes a characteristic controlled by many genes, often giving continuous variation. characteristic is influenced by more than one gene. In a simple additive model, different numbers of contributing alleles generate several phenotypic values. More contributing loci produce more possible combinations, often with many intermediate values and fewer extremes.
Environmental effects can further blur boundaries between genetic values, producing continuous variation such as a range of heights. A roughly bell-shaped distribution can arise, but not every polygenic trait or sampled population must follow a perfect normal curve. A histogram by itself does not prove a particular inheritance mechanism.
Separate genotype effects from environmental effects
In an illustrative model, plant genotype A averages 20 cm in low light and 32 cm in high light; genotype B averages 24 cm and 28 cm. Light changes height within each genotype, while the genotypes differ within each environment. The response to light differs too: A increases by 12 cm and B by 4 cm.
Use replicates and report variation, not only means. Randomly assign genetically comparable plants to conditions and control water, mineral supply, age and measurement method. A common environment helps compare genotypes, but one environment cannot describe every possible response.
NOT: a change caused only by the environment is not a change in the DNA sequence.
- Phenotype can have genetic and environmental causes
- The same environment can still give different phenotypes if genotypes differ
- Name the factor and the feature
- Do not call an environmental effect an adaptation acquired on purpose
Two traps this diagram blocks
- Do not say the environment mutates the gene whenever the phenotype changes.
- Do not claim genotype is irrelevant because the environment also matters.
From the diagram alone
Question
Explain how two plants with the same genotype can have different phenotypes. [3]
Show a model answer
Phenotype depends on the environment as well as the alleles. If one plant receives less light, or a different mineral supply, gene expression or growth changes. The alleles themselves need not change.
| Term | What varies? | Example |
|---|---|---|
| Multiple alleles | Versions at one locus in a population | ABO alleles |
| Polygenic inheritance | Alleles at several loci | Genetic contributions to height |
| Environmental effect | Conditions during development or life | Light affecting plant growth |
Make your reasoning visible
Say “in the population”
More than two alleleA different version of the same gene, found at the same locus. can exist in a populationAll the individuals of one species living in one area at the same time.; this does not give an ordinary diploidHaving two sets of chromosomes (2n), one from each parent. individual three alleles at the same locusThe position of a gene on a chromosome..
Keep the terms separate
multiple allelesWhen a gene has more than two alleles in a population, such as the ABO blood group gene.: one gene. polygenicDescribes a characteristic controlled by many genes, often giving continuous variation.: several geneA sequence of DNA bases that codes for a polypeptide or a functional RNA.. Environmental effects can act alongside either.
Use paired comparisons
Compare the same genotypeThe alleles an organism has for a gene. across environments, then different genotypes within the same environment.
Key explanations
- Genotype is the alleles present. phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. is the observable feature.
- An example is the same plant genotype grown in full light or deep shade, giving different height or chlorophyll.
- A difference between genotypes can remain hidden if the environment is not the one that reveals it.
Avoid this misconception
The claim: Continuous variation means the environment is the only cause.
The correction: Many genes and environmental effects can both contribute to a continuous range.
Why are plant heights a smooth range, not a few sizes?
Some characteristics come in a few clear categories, such as ABO blood groups. Others, such as height, form a continuous range. This model shows how several genes and the environment together can produce that range.
What the model shows
A made-up plant whose height depends on a number of genes you choose, each adding the same amount, plus some variation from its surroundings. Each run grows a sample of plants from a cross between tall and short parents and plots their heights as a histogram.
What to look for
How many separate bars there are with one gene, how that number grows with more genes, how rare the tallest plants are, and what happens to the bars when the environment varies.
What it tells us
More genes give more height classes, with most plants in the middle. Environmental variation smooths the classes into continuous variation. Both genes and environment contribute.
Be careful what you claim
The plant and its numbers are invented, and every gene has the same small effect. A bell-shaped histogram does not on its own tell you how many genes are involved or how much is due to the environment.
Try it in the model: start with 1 gene and no environmental variation and count the bars. Then add genes one at a time.
Genes, environment and continuous variation
A fictional plant’s height is set by a number of genes, each adding 2 cm, and by its surroundings. Grow an F2 sample, then add genes and environmental variation and watch a few separate heights become a continuous range.
What you see
- A histogram of the heights of a fictional F2 population of plants, in 1 cm bars.
- Small markers under the axis at each genetic class: the height a plant with 0, 1, 2 … contributing alleles would have in average conditions.
- Readouts for the number of genetic classes, the mean and standard deviation, and the expected share in the tallest class.
How to use it
- Choose how many genes affect height (1 to 6) and how much the environment varies (standard deviation, cm).
- Choose a sample size and take new samples.
- Answer the questions, including the lesson’s genotype-by-light data.
What it shows
- How polygenic inheritance gives many intermediate classes and rare extremes (3.21).
- How environmental variation blurs those classes into continuous variation (3.20(i)).
- Why a histogram on its own does not prove how a characteristic is inherited.
Limits of the model
- Fictional plant and numbers: 10 cm with no contributing alleles, and 2 cm for each one.
- Every gene adds the same amount, genes are unlinked and there is no dominance or gene–environment interaction in the histogram. Real traits are less tidy.
- Multiple alleles (one gene, such as ABO) are a different idea and give discrete groups; they are not modelled here.
Guided tour · step 2 of 7
- Keep Environmental variation (SD) at 0 cm.
- Raise Genes affecting height to 3 or more.
n genes give 2n + 1 genetic classes. Most plants have a middle number of contributing alleles; the extremes need every allele to be the same, so they are rare.
Genes and environment
Sample
What the histogram shows
Parents: pure-breeding tall × pure-breeding short. The F1 are all heterozygous at every gene; the F2 shown here are their offspring, so each allele position is a contributing allele with probability ½.
Questions
With 3 genes, what fraction of the F2 has all six contributing alleles (the tallest class)?
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Distinguish multiple alleles from polygenic inheritance.
Show suggested answer
Multiple alleles are alternative versions of one gene in a population. Polygenic inheritance involves several genes contributing to a characteristic.
Practice question
Retrieval and explanation · suggested answer
In the conventional ABO model, cross Iᴬi with Iᴮi. Give possible offspring groups and proportions.
Show suggested answer
IᴬIᴮ, Iᴬi, Iᴮi and ii occur with probability 1/4 each: AB, A, B and O. These are probabilities across many offspring, not a guaranteed family pattern.
Practice question
Retrieval and explanation · suggested answer
Using the plant-height model, explain why both genotype and environment matter.
Show suggested answer
Each genotype changes height with light, showing an environmental effect. Genotypes differ in the same light conditions, supporting a genetic contribution; their responses also differ.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A population has a continuous distribution of mass. Does this alone prove that mass is polygenic?
Show a hint
An observed pattern does not identify all its causes.
Explore the reasoning
No. Environmental variation and measurement effects may contribute, and the distribution alone does not reveal the number of genes. Controlled comparisons and genetic evidence would be needed.
Recall this lesson
Lesson recall questions
Question 1 of 5: Describe, in order, how an epigenetic mark can persist after cell division.
Genotype, environment and epigenetics
Practice question
Retrieval and explanation · suggested answer
Describe, in order, how an epigenetic mark can persist after cell division.
Lesson: 3C 3: Controlling Gene Expression
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Quantitative genetics
3C · Interactions Between Genes and the EnvironmentNHGRI uses height as an example of a polygenic trait. Researchers studying such traits look beyond a single dominant or recessive allele. Human height is measured on a…Specification 3.21Read & use the scienceClose article
Why this matters
NHGRI uses height as an example of a polygenic trait. Researchers studying such traits look beyond a single dominant or recessive allele. Human height is measured on a continuous scale. Many loci can contribute small effects, and nutrition or illness can also change growth. A researcher studying this trait needs large samples and appropriate controls; a family story alone cannot separate genetic and environmental contributions. An allele linked to greater average height is not a precise prediction for one person. This is the bridge from monohybrid problems to polygenic inheritance: many genotypes produce overlapping phenotypes. The specification asks students to explain variation, not to infer destiny from a single DNA variant.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.21 understand how some phenotypes are affected by multiple alleles for the same gene, or by polygenic inheritance, as well as the environment, and how polygenic inheritance can give rise to phenotypes that show continuous variation
Use the science
Why would one allele usually be insufficient to predict an individual’s exact height?
Explore a possible response
Other loci and environmental influences also contribute, so a single allele does not specify an exact outcome.
Explore the source
- NHGRI: Polygenic TraitNational Human Genome Research Institute
What identical twins reveal, and do not reveal
3C · Interactions Between Genes and the EnvironmentIdentical twins share much of their DNA sequence, yet their phenotypes can differ. A study of monozygotic twin pairs found differences in DNA methylation and histone acetylation, especially…Specification 3.20(i) · 3.20(ii)Read & use the scienceClose article
Why this matters
Identical twins share much of their DNA sequence, yet their phenotypes can differ. A study of monozygotic twin pairs found differences in DNA methylation and histone acetylation, especially among older pairs. This gives researchers a way to investigate gene regulation alongside environment. It does not prove that every difference between twins was caused by a particular experience or epigenetic mark. A strong scientific explanation separates a measured association from a demonstrated mechanism. For Unit 2, the example connects genotype–environment interaction to epigenetic modification while keeping the distinction between DNA sequence and gene activity clear.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.20(i) understand how phenotype is the result of an interaction between genotype and the environment
- 3.20(ii) know how epigenetic modification, including DNA methylation and histone modification, can alter the activation of certain genes
Use the science
If two twins differ in a trait and in methylation, does that prove methylation caused the trait difference?
Explore a possible response
No. The observations are associated; other environmental or biological factors may affect both. A causal test needs additional evidence.