Environmental Effects on Phenotype — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Genotype-environment interaction to produce phenotype, including norm of reaction, phenotypic plasticity, temperature-sensitive alleles, nutrient-dependent traits, epigenetic effects, and sex-influenced trait expression as specified in the AP Biology CED.

You should already know: The difference between genotype and phenotype from basic Mendelian genetics. The central dogma of molecular biology and enzyme structure-function relationships. The basic mechanism of epigenetic modification of gene expression.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Environmental Effects on Phenotype?

Environmental Effects on Phenotype describes the core biological principle that an organism’s observable traits (phenotype) are the product of interactions between its inherited genotype and the surrounding biotic and abiotic environment. Unlike the simplified Mendelian examples that assume constant environmental conditions, nearly all traits are influenced to some degree by environment, even highly heritable traits. This topic is part of AP Biology CED Unit 5 (Heredity) and accounts for approximately 3-5% of total exam score weight. Concepts from this topic appear in both multiple-choice questions (MCQ), typically as graph interpretation or concept application, and free-response questions (FRQ), often as part of experimental analysis connecting genetics to evolution or gene regulation. Variation in phenotypic responses to environment creates the raw material for natural selection, making this topic a critical link between heredity and evolutionary change.

2. Norm of Reaction

The norm of reaction is defined as the full range of phenotypic phenotypes that a single, fixed genotype can produce across different environmental conditions. This concept quantifies how sensitive a genotype’s phenotype is to environmental variation. For non-plastic traits, the norm of reaction is a flat line: phenotype does not change across environments. The classic example of a non-plastic trait is ABO blood type in humans: genotype is fixed, and no common environmental factor alters blood type phenotype. For plastic traits, the norm of reaction has a non-zero slope, meaning phenotype changes as environment changes. Different genotypes often have different norms of reaction, which means they respond differently to the same environmental change. This interaction between genotype and environment (called G×E interaction) is a major source of phenotypic variation in natural populations. Norm of reaction is almost always tested in AP exams as graph interpretation, where you will be asked to compare responses of different genotypes across an environmental gradient.

Worked Example

A researcher studies three genetically distinct clones of duckweed (an aquatic plant, each clone has identical genotype) grown at five different nitrogen concentrations. Data shows that Clone A increases biomass linearly as nitrogen increases, Clone B’s biomass plateaus at medium nitrogen, and Clone C’s biomass decreases above medium nitrogen. (a) What does this data indicate about the effect of nitrogen and genotype on duckweed biomass? (b) Is biomass a plastic trait for these clones?

1. First, recall that each clone is a separate genotype, so each has its own norm of reaction plotted against nitrogen (environmental gradient).
2. All clones change biomass as nitrogen concentration changes, so nitrogen environment has a significant causal effect on biomass phenotype. Each clone has a unique response pattern, so genotype also affects biomass, via genetic differences in the norm of reaction.
3. A plastic trait is defined as a trait where a single genotype produces different phenotypes across different environments. All three clones change phenotype across nitrogen concentrations, so biomass is plastic for all three.
4. The data confirms that biomass phenotype is the product of G×E interaction, not solely genotype or environment.

Exam tip: On the AP exam, always look for differences in slope or shape between genotype lines on a norm of reaction graph. Different shapes are direct evidence of G×E interaction, which is the most common answer to "what does this graph show" questions.

3. Phenotypic Plasticity and Temperature-Sensitive Alleles

Phenotypic plasticity is the ability of a single genotype to produce different phenotypes in response to different environmental conditions. When the plastic response produces discrete, distinct phenotypes (rather than a continuous range of traits) it is called a polyphenism. One of the most common forms of plasticity in poikilothermic (cold-blooded) organisms is temperature-sensitive allele expression, where the protein product of an allele is only active at a specific temperature range, due to effects of temperature on protein folding. For example, the tyrosinase allele that produces dark coat color in Siamese cats codes for an enzyme that is unstable and inactive at the warm core body temperature of a cat, but folds correctly and is active at the cooler temperatures of extremities (ears, paws, tail). This produces the characteristic dark-extremity, light-body phenotype of Siamese cats. Other examples include temperature-dependent sex determination in many reptiles: incubation temperature of the egg determines the sex of the offspring via effects on gene expression during development.

Worked Example

Siamese cats homozygous for the temperature-sensitive tyrosinase allele $c^h{c}^h$ have dark extremities and light body fur. A researcher shaved a patch of fur from the warm back of a Siamese cat, then taped a cold pack to the shaved area for two weeks while fur regrew. Predict the color of the new fur and explain your reasoning.

1. The $c^h$ allele produces a tyrosinase enzyme required for melanin (dark pigment) production that is only active at temperatures below the cat’s core body temperature (~37°C).
2. The original back fur is light because the back sits close to the warm body core, so tyrosinase is inactive and no melanin is produced.
3. The cold pack lowers the temperature of the shaved skin region below the activation threshold for tyrosinase.
4. Active tyrosinase will catalyze melanin production in the newly growing fur, so the regrown fur in the cold-treated patch will be dark, matching the extremities.

Exam tip: Always connect temperature-sensitive phenotype to enzyme structure and function. AP exam questions almost always expect you to explicitly link temperature to protein folding and activity, not just state the allele is temperature-sensitive.

4. Nutrient and Transgenerational Epigenetic Effects

Nutrient availability is a major environmental factor that shapes phenotype, especially during development. For example, human height is ~80% heritable, but severe childhood malnutrition can lead to stunted adult height far below an individual’s genetic potential. Epigenetic effects are another key class of environmental effects, where environment alters gene expression without changing the underlying DNA sequence. These changes can persist for the life of the organism, and in some cases are passed to offspring. A classic model example is the agouti viable yellow ($A^{vy}$) allele in mice: the allele causes yellow fur and obesity when active, but the activity of the allele is regulated by DNA methylation (an epigenetic modification) that depends on maternal diet during pregnancy. Dams fed a diet high in methyl donors (folate, vitamin B12) produce pups with heavy methylation of the $A^{vy}$ promoter, which silences the allele, resulting in brown, lean pups. Dams fed a low-methyl diet produce pups with low methylation, active $A^{vy}$, and yellow, obese pups, even though the pups have identical $A^{vy}$ genotypes.

Worked Example

Two inbred mice have identical genotypes for the $A^{vy}$ allele. One is brown and lean, the other is yellow and obese. Explain how this difference can occur, what environmental factor causes it, and what molecular mechanism is involved.

1. Epigenetic modifications alter gene expression without changing the underlying DNA sequence, so identical genotypes can produce different phenotypes if they have different epigenetic marks.
2. The difference arises from the maternal diet during pregnancy: the mother of the brown lean mouse had a diet high in methyl donors, while the mother of the yellow obese mouse had a low-methyl diet.
3. Methyl donors from the diet are used to add methyl groups to the promoter region of the $A^{vy}$ allele. High methylation silences the overactive $A^{vy}$ allele, while low methylation leaves it active.
4. Active $A^{vy}$ causes yellow fur and increased appetite leading to obesity, while silenced $A^{vy}$ produces brown fur and normal weight, explaining the difference between the two genetically identical mice.

Exam tip: Remember that epigenetic changes do not alter DNA sequence, only gene expression. AP exam frequently tests the distinction between genetic changes (mutation, altered sequence) and epigenetic changes (altered expression, sequence unchanged).

5. Sex as an Internal Environmental Factor

An individual’s sex creates a unique internal hormonal environment that interacts with autosomal genotype to produce phenotype, leading to two classes of traits: sex-influenced and sex-limited. Sex-influenced traits are autosomal traits that are expressed differently in males and females due to differences in hormone levels. For example, pattern baldness in humans is an autosomal trait where the allele for early balding is dominant in males (only one copy needed for expression) but recessive in females (two copies required, and expression is much milder) due to the effect of testosterone. Sex-limited traits are autosomal traits that are only expressed in one sex, because the trait is tied to sex-specific reproduction. For example, milk production in dairy cows is only expressed in lactating females; bulls carry the genes for milk production but never express them because their hormonal environment does not support lactation.

Worked Example

Pattern baldness is an autosomal sex-influenced trait, where the baldness allele (B) is dominant in males and recessive in females. A heterozygous male (Bb) marries a heterozygous female (Bb). What is the probability that their female child will have pattern baldness?

1. This is an autosomal trait, so inheritance follows standard Mendelian segregation for the B and b alleles. A cross between Bb × Bb produces offspring genotypes in the ratio $1BB:2Bb:1bb$.
2. For female offspring, the B allele is recessive, so only homozygous BB individuals will express pattern baldness. Bb females will not express the trait.
3. The probability of a BB genotype from this cross is $1/4=25\%$, and all BB females will express the trait.
4. The probability that their female child will have pattern baldness is $25\%$, or $1/4$.

Exam tip: Do not confuse sex-influenced autosomal traits with X-linked traits. Male-biased expression does not automatically mean the trait is carried on the X chromosome; always check what the question states about the location of the gene.

6. Common Pitfalls (and how to avoid them)

• Wrong move: Claiming that phenotypic differences between identical twins must be genetic because they have the same genotype. Why: Students forget that identical twins share genotype but experience different environmental conditions over development, which drive phenotypic differences via plasticity and epigenetic changes. Correct move: When given a set of identical genotypes, attribute any phenotypic variation to environmental effects, unless the question explicitly provides evidence of new genetic mutations.
• Wrong move: Defining norm of reaction as the range of phenotypes in a population across different genotypes, rather than the range for one genotype across environments. Why: Students mix up population-level variation across genotypes with genotype-specific response to environmental change. Correct move: On any norm of reaction question, remember each line on the graph corresponds to one genotype, and the range of y-values for that line is its norm of reaction.
• Wrong move: Stating that all epigenetic changes are heritable across multiple generations. Why: Textbooks often highlight rare examples of transgenerational epigenetic inheritance, leading students to assume all epigenetic changes are passed to offspring. Correct move: Only state an epigenetic change is heritable if the question explicitly gives evidence of inheritance; default to saying it alters phenotype in the exposed individual.
• Wrong move: Classifying pattern baldness as an X-linked trait because it is much more common in males. Why: Students associate male-biased traits with X-linking, but pattern baldness is a classic example of an autosomal sex-influenced trait. Correct move: Always check whether the trait is described as autosomal or X-linked; male-biased expression is not sufficient evidence for X-linkage.
• Wrong move: Claiming that environmental effects on phenotype mean the trait is not heritable. Why: Students see a large environmental effect and assume genetics do not matter, but most traits are the product of both. Correct move: Unless the trait is explicitly non-plastic (e.g. blood type), always explicitly state that phenotype arises from genotype-environment interaction.

7. Practice Questions (AP Biology Style)

Question 1 (Multiple Choice)

A researcher studies the effect of soil pH on flower color in hydrangeas. Three hydrangea cuttings (all from the same parent plant, so identical genotype) are grown in soils of pH 4.5, 5.5, and 6.5. The flowers are blue at pH 4.5, purple at pH 5.5, and pink at pH 6.5. Which of the following conclusions is best supported by this data? A) Flower color in hydrangeas is determined solely by environment, with no genetic component B) The norm of reaction for hydrangea flower color for this genotype is discontinuous across pH values C) Flower color in hydrangeas is a plastic trait resulting from interaction between genotype and environment D) Flower color is a polyphenism because it shows three discrete colors across different pH treatments

Worked Solution: The experiment uses identical genotypes grown in different environments, so any change in flower color is an environmental effect acting on the same genotype. Option A is incorrect because the experiment does not test for genetic variation, and the response to pH is genetically determined. Option B is incorrect because the three pH values are discrete experimental treatments, not a reflection of the underlying norm of reaction, which is continuous. Option D is incorrect because polyphenisms produce discrete phenotypes regardless of environmental gradient, while hydrangea flower color changes continuously with pH. The only correct conclusion is that flower color is plastic, produced by G×E interaction. Correct answer: C

Question 2 (Free Response)

Scientists studying the effect of temperature on the yield of three inbred (genetically uniform) corn varieties collected the following data:

(a) Draw the norm of reaction for each variety (label each line clearly). (2 points) (b) Identify which variety has the highest plasticity for yield. Justify your answer. (2 points) (c) Predict which variety would be best suited for a region with temperatures that regularly vary from 15°C to 35°C. Explain the connection between phenotypic plasticity and fitness in this environment. (3 points)

Worked Solution: (a) Plot temperature (°C) on the x-axis (range 15-35) and yield (bu/acre) on the y-axis (range 10-90). Draw: (1) Variety 1: a line with positive slope from (15, 20) to (35, 90); (2) Variety 2: a line with negative slope from (15, 60) to (35, 10); (3) Variety 3: a hump-shaped line peaking at (25, 60) from (15, 30) to (35, 40). 1 point per correctly drawn, labeled line (2 total). (b) Phenotypic plasticity is the range of phenotypes a single genotype produces across environments. Variety 1 has a yield range of $90-20=70$ bu/acre, Variety 2 has a range of 50 bu/acre, and Variety 3 has a range of 30 bu/acre. A larger range means higher plasticity, so Variety 1 has the highest plasticity. (c) Variety 3 is best suited. It maintains a yield between 30 and 60 bu/acre across the entire temperature range, so even with large temperature fluctuations, it always produces a moderate to high yield. Adaptive plasticity maintains fitness across variable environments: Varieties 1 and 2 have very low yields at one end of the temperature range, so they will have low fitness when temperatures reach that extreme. Variability in the local environment favors genotypes that maintain consistent yield across the range of experienced conditions.

Question 3 (Application / Real-World Style)

Human birth weight is a trait influenced by both genotype and maternal nutrition during pregnancy. A study in a low-income country found that the average birth weight of babies born to mothers with severe malnutrition during the first trimester was 2.4 kg, compared to 3.3 kg for babies born to mothers with adequate nutrition, even after controlling for genetic ancestry. The heritability of birth weight in this population is 0.6, meaning 60% of the variation in birth weight within the population is due to genetic differences between individuals. Explain why the population-level difference in average birth weight between the two groups is environmental, even though birth weight is highly heritable. What does this mean for public health interventions?

Worked Solution: Heritability measures the proportion of phenotypic variation within a group of individuals with shared environment that is due to genetic variation between individuals. It does not describe the proportion of a trait that is genetic in an individual, nor does it explain differences in average phenotype between groups exposed to different environments. The 0.6 heritability means 60% of the variation in birth weight among individuals with similar nutrition is genetic, but the difference between the malnourished and well-nourished groups is caused by the difference in maternal nutritional environment, not genetics (the study controlled for genetic ancestry). This means public health interventions that provide adequate nutrition to pregnant women will successfully increase the average birth weight in the population, even though birth weight is highly heritable. Low average birth weight caused by malnutrition can be addressed by improving nutrition, regardless of the genetic contribution to variation within the population.

8. Quick Reference Cheatsheet

9. What's Next

After mastering environmental effects on phenotype, you will next explore other exceptions to simple Mendelian inheritance in non-Mendelian genetics, then progress to the mechanisms of evolution in Unit 6. This topic is an essential prerequisite for understanding the sources of phenotypic variation within populations, which is the raw material for natural selection. Without grasping how environment interacts with genotype to produce phenotype, you cannot explain why phenotypic variation exists even among genetically related individuals, or how adaptive evolution proceeds in changing environments. This topic also connects broadly to gene regulation, ecological interactions, and animal behavior across the rest of the AP Biology course.

Non-Mendelian Inheritance Sources of Genetic Variation Natural Selection Population Genetics