AP Biology

## Learning Outcomes

- Define norm of reaction and explain how it relates to phenotypic plasticity.
- Distinguish genotype, phenotype, gene expression, and environmental effects.
- Interpret reaction norm graphs showing phenotype across different environments.
- Explain why the same genotype can produce different phenotypes under different environmental conditions.
- Identify genotype-by-environment interactions in experimental data.
- Design an experiment to test how an environmental factor affects phenotype.
- Avoid common exam errors, especially confusing environmental effects with mutations.

## AP Biology Syllabus

In AP Biology Unit 5: Heredity, environmental effects on phenotype connect inheritance to gene expression, development, and variation. The central exam idea is that organisms with the same genotype can show different phenotypes in different environments because environmental factors can influence gene expression and development. Unit 5 accounts for about 8–11% of the multiple-choice section, and this subtopic is commonly assessed through graph interpretation, experimental design, and short explanations.

- The same genotype may produce different phenotypes in different environments.
- Environmental factors can affect gene expression and lead to phenotypic plasticity.
- A reaction norm shows the pattern or range of phenotypes produced by one genotype across environments.
- Environmental effects usually alter phenotype without changing DNA sequence.
- Different genotypes can respond differently to the same environmental factor.
- Data may be shown as line graphs, scatterplots, bar graphs, or graphs with error bars.
- Free-response questions may ask for predictions, justifications, controls, independent variables, and dependent variables.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. What does a reaction norm show about a genotype?
2. Why can two organisms with the same genotype have different phenotypes?
3. If genetically identical plants grow taller in high light than in low light, does that mean their DNA changed? Explain.
4. On a graph of phenotype against temperature, what would a steeply sloped reaction norm suggest?
5. How could you design an experiment to test whether soil pH affects flower color?

## Introduction

A genotype is not a fixed script that produces exactly one visible outcome under all conditions. Instead, a genotype often gives an organism a range of possible phenotypes. Which phenotype appears can depend on environmental factors such as temperature, light, nutrition, pH, toxins, hormones, pathogens, stress, or resource availability.

The norm of reaction is the AP Biology idea that helps you describe this range. It connects heredity with environmental response: genes provide inherited information, while the environment can influence how that information is expressed. On the exam, this topic often appears in graph-based questions where you must compare phenotypes across different environments and explain the biological cause.

> **Key Term: genotype**
> The genetic makeup of an organism, usually described in terms of the alleles it carries.
>
> **Key Term: phenotype**
> The observable characteristics of an organism, including appearance, physiology, biochemical traits, development, and behavior.
>
> **Key Term: norm of reaction**
> The range or pattern of phenotypes that a single genotype can produce across different environmental conditions.

### How reaction norms link genotype, environment, and phenotype

A simple way to summarize this topic is:

$$
\text{Phenotype} = \text{genotype} + \text{environment} + \text{gene regulation}
$$

This is not a mathematical formula for calculating phenotype. It is a conceptual model. The genotype provides the inherited information. Environmental conditions affect how that information is used during growth, development, metabolism, and behavior. Gene regulation links the two because environmental signals can increase or decrease the expression of specific genes.

For example, a plant may carry alleles that allow tall growth, but if it receives too little light, too little water, or too few mineral nutrients, it may remain short. The alleles did not necessarily change. Instead, the environment limited the phenotype that could be produced.

> **Key Term: gene expression**
> The process by which genetic information is used to produce functional products, such as proteins or RNA molecules.

A reaction norm describes the phenotypes produced by one genotype across a range of environmental conditions. Imagine one genotype of plant grown under low, medium, and high light. If the average heights are 10 cm, 25 cm, and 40 cm, then those values form that genotype’s reaction norm for height in response to light intensity.

Some traits have a narrow reaction norm. This means the phenotype changes very little across environments. Other traits have a broad reaction norm. This means the phenotype changes greatly as the environment changes.

> **Key Term: phenotypic plasticity**
> The ability of one genotype to produce different phenotypes under different environmental conditions.

A trait showing strong phenotypic plasticity has a broad reaction norm. A trait showing little phenotypic plasticity has a narrow reaction norm. AP questions may not always use the phrase “norm of reaction,” but they may describe the same idea with phrases such as “same genotype,” “different environments,” and “different phenotypes.”

> **Test Tip:**
> If an AP question says organisms are genetically identical but show different phenotypes in different environments, the safest explanation is that environmental conditions affected gene expression or development. Do not assume the environment changed the DNA sequence.

### Reading reaction norm graphs

Reaction norms are often represented as graphs. The environmental variable is usually on the x-axis, and the phenotype is usually on the y-axis. Each line represents one genotype. The shape and slope of the line show how that genotype responds to the environment.

For example, suppose two genotypes of a plant are grown at three temperatures: 15°C, 25°C, and 35°C. The measured phenotype is average plant height.

- Genotype A is 12 cm at 15°C, 30 cm at 25°C, and 15 cm at 35°C.
- Genotype B is 20 cm at 15°C, 22 cm at 25°C, and 21 cm at 35°C.

Genotype A has a strong response to temperature. Its reaction norm changes greatly, with best growth at 25°C. Genotype B changes little across temperatures, so its reaction norm is flatter. Genotype A shows greater phenotypic plasticity for height across this temperature range.

A graph may also show that two genotypes respond differently to the same environmental change. For example, genotype A may grow taller as light intensity increases, while genotype B may show little change. This means the effect of the environment depends on the genotype.

> **Key Term: genotype-by-environment interaction**
> A situation in which different genotypes respond differently to environmental conditions.

A genotype-by-environment interaction is especially clear when reaction norm lines are not parallel. If the lines cross, the genotype with the highest phenotype in one environment may not be the highest in another environment. This matters because natural selection acts on phenotypes in particular environments. A genotype that produces the most favorable phenotype in one condition may not do so in another.

When reading a reaction norm graph, use this sequence:

1. Identify the environmental variable on the x-axis.
2. Identify the phenotype measured on the y-axis.
3. Identify which line or data series represents each genotype.
4. Describe how each genotype’s phenotype changes across environments.
5. Compare the slopes of the lines.
6. Look for nonparallel or crossing lines as evidence of genotype-by-environment interaction.
7. Use data values when possible rather than vague descriptions.

> **Exam Warning:**
> Do not write only “the environment affected the organism.” AP Biology answers need a specific link: the environmental factor affected gene expression, development, enzyme activity, hormone signaling, resource availability, or another biological process that changed the phenotype.

A strong graph interpretation answer might say: “As temperature increased from 15°C to 25°C, genotype A’s mean height increased from 12 cm to 30 cm, while genotype B changed only from 20 cm to 22 cm. This supports the claim that genotype A has greater phenotypic plasticity for height in response to temperature.”

### Mechanisms and examples of environmental effects

Environmental effects on phenotype happen through biological mechanisms. The environment does not need to rewrite DNA to change phenotype. Instead, environmental signals can alter cellular processes.

A typical pathway is:

1. An environmental factor changes, such as temperature, light, nutrition, or pH.
2. Cells detect the change through receptors, hormones, transcription factors, or changes in enzyme activity.
3. Gene expression changes; some genes are upregulated and others are downregulated.
4. Protein production changes, including enzymes, pigments, receptors, structural proteins, or hormones.
5. The organism’s phenotype changes.

One common AP-level example is nutrition and human height. Human height is influenced by many genes, but diet affects whether inherited growth potential is reached. A child with alleles associated with tall stature may not grow as tall if protein, calories, vitamins, or minerals are limited during development. In this case, nutrition affects growth by changing the resources available for cell division, protein synthesis, and bone development.

> **Key Term: multifactorial trait**
> A trait influenced by multiple genes and environmental factors.

Human height is a multifactorial trait because many genes and environmental conditions contribute to the final phenotype. On the exam, multifactorial traits are a reminder that not all variation can be explained by simple Mendelian inheritance.

Another classic example is flower color in hydrangeas. In many hydrangeas, flower color varies with soil pH because pH affects soil chemistry and pigment-related processes. Acidic conditions often produce bluer flowers, while more alkaline conditions often produce pinker flowers. The plant’s genotype matters, but the environment influences the phenotype.

Temperature can also affect development. In some reptiles, incubation temperature influences sex phenotype. Embryos with the same genetic background can develop along different pathways depending on temperature during a sensitive period. This is a clear example of environmental conditions affecting developmental outcome.

Coat color can also be environmentally influenced. Some animals change fur color seasonally in response to cues such as day length or temperature. These environmental cues can influence hormone levels and gene expression, leading to changes in pigment production or fur type.

Identical twins are useful for thinking about this topic because they usually have nearly identical genotypes. If identical twins differ in body mass, disease risk, muscle development, or other traits, those differences can reflect environmental effects such as diet, exercise, stress, illness, and life experiences. Some differences may also involve epigenetic changes.

> **Key Term: epigenetic change**
> A change in gene expression that does not involve a change in the DNA nucleotide sequence, often associated with DNA methylation or histone modification.

Epigenetic changes are relevant because they show one way the environment can affect gene expression without changing the genotype. For AP Biology, keep the distinction clear: epigenetic regulation affects how genes are expressed; mutation changes the DNA sequence.

> **Test Tip:**
> When explaining environmental effects, use the phrase “same genotype, different phenotype” if it fits the data. Then add the mechanism: “because the environmental condition changed gene expression” or “because the environment altered development.”

### Designing experiments to test reaction norms

AP Biology often tests this topic through experimental design. You may be asked to identify variables, propose a control, state a hypothesis, predict results, or justify a conclusion using data. Reaction norms are ideal for these skills because they require comparing phenotypes across environments.

A good experiment testing a reaction norm needs at least one genotype and more than one environmental condition. If you want to compare genotype-by-environment interactions, you need at least two genotypes and more than one environmental condition.

Suppose you want to test whether light intensity affects plant height in two genotypes of fast-growing plants.

A strong design would include:

- Independent variable: light intensity, such as low, medium, and high light.
- Dependent variable: plant height after a set growth period.
- Genotypes: genotype A and genotype B.
- Constants: soil type, pot size, water amount, temperature, fertilizer, growth time, and seed age.
- Control or comparison group: plants grown under standard light conditions.
- Replication: many plants per genotype per light treatment, not just one plant.
- Data display: a line graph with light intensity on the x-axis and mean plant height on the y-axis, with separate lines for each genotype.
- Statistical evidence: error bars such as 95% confidence intervals, if provided or requested.

> **Key Term: independent variable**
> The factor deliberately changed by the investigator in an experiment.
>
> **Key Term: dependent variable**
> The measured response that may change because of the independent variable.
>
> **Key Term: control group**
> A comparison group that is not exposed to the experimental treatment or is kept under standard conditions.
>
> **Question:**
> Genetically identical insects are raised at 15°C and 25°C. Insects raised at 15°C develop dark pigmentation, while insects raised at 25°C develop light pigmentation. Explain the result.

A strong answer would be: “Because the insects were genetically identical, the difference in pigmentation is likely due to the temperature difference. Temperature affected gene expression or enzyme activity involved in pigment production, causing the same genotype to produce different phenotypes.”

A weak answer would be: “The insects mutated at different temperatures.” That could happen in rare cases if a mutagen caused DNA damage, but the basic AP Biology explanation for this type of prompt is phenotypic plasticity, not mutation.

Null and alternative hypotheses may also be tested. For the plant light experiment, a null hypothesis could be: “Light intensity has no effect on mean plant height for either genotype.” An alternative hypothesis could be: “Light intensity affects mean plant height in at least one genotype.” If genotype A changes greatly across light treatments but genotype B does not, the data would reject the null hypothesis and support phenotypic plasticity in genotype A.

> **Exam Warning:**
> A single organism in each treatment is not enough for a reliable conclusion. AP experimental design answers should include repeated trials or a large sample size so that random variation is less likely to explain the result.

When writing conclusions, refer directly to the data. Instead of saying “light affected height,” say “genotype A increased from a mean height of 8 cm in low light to 28 cm in high light, while genotype B increased only from 15 cm to 17 cm.” Specific data make your explanation stronger and show that you can use evidence.

Reaction norm experiments can also include follow-up work. If plants grow differently under different pH conditions, a follow-up experiment might measure expression of pigment-producing genes or enzyme activity under each pH treatment. This links the observed phenotype to a cellular mechanism.

## Key Point Checklist

This article has covered the following key knowledge points:

- A norm of reaction shows the range or pattern of phenotypes one genotype can produce across environments.
- Phenotypic plasticity means the same genotype can produce different phenotypes under different environmental conditions.
- The environment usually affects phenotype by changing gene expression, development, physiology, or resource availability, not by changing genotype.
- Reaction norm graphs usually place the environmental factor on the x-axis and the phenotype on the y-axis.
- A steep reaction norm suggests a strong phenotypic response to the environmental factor.
- A flat reaction norm suggests little phenotypic change across the tested environments.
- Nonparallel or crossing reaction norm lines suggest a genotype-by-environment interaction.
- Environmental factors that affect phenotype include temperature, light, nutrition, pH, toxins, hormones, pathogens, and stress.
- Examples include nutrition and height, hydrangea flower color and soil pH, temperature-dependent sex determination, seasonal coat color, and differences between identical twins.
- A strong experiment includes controls, independent and dependent variables, constants, replication, and appropriate graphs.
- AP answers should use evidence from data and explain mechanisms such as gene expression or developmental change.
- Do not confuse phenotypic plasticity with mutation.

## Key Terms and Concepts

- genotype
- phenotype
- norm of reaction
- gene expression
- phenotypic plasticity
- genotype-by-environment interaction
- multifactorial trait
- epigenetic change
- independent variable
- dependent variable
- control group


Environmental effects on phenotype - Norm of reaction