# Environmental Effects on Phenotype

> AP Biology · CED Unit 5 Heredity
> Source: https://www.owlsprep.com/study/ap-biology-u5-environmental-effects-on-phenotype/

This module explains how an organism’s phenotype arises from interactions between its inherited genotype and environmental conditions, covering core concepts tested on the AP Biology exam including norm of reaction, plasticity, epigenetics, and sex-influenced traits.

**Prerequisites:** [Genotype vs phenotype distinction from Mendelian genetics](https://www.owlsprep.com/study/ap-biology-mendelian-genetics/); [Enzyme structure and function relationships](https://www.owlsprep.com/study/ap-biology-enzyme-structure-function/); [Basic epigenetic mechanisms of gene regulation](https://www.owlsprep.com/study/ap-biology-epigenetic-gene-regulation/)

## Learning objectives

- Explain how phenotype arises from genotype-environment interaction
- Interpret norm of reaction graphs and identify G×E interaction
- Describe examples of environmental effects including temperature-sensitive alleles and epigenetics
- Distinguish between sex-influenced, sex-limited, and X-linked traits

## Norm of Reaction

The norm of reaction describes the full range of phenotypes that a single, fixed genotype can produce across different environmental conditions. It quantifies how sensitive a genotype’s phenotype is to environmental variation. Non-plastic traits have a flat norm of reaction (phenotype does not change across environments), while plastic traits have a non-zero slope, indicating phenotype changes with environment. Different genotypes often have different norms of reaction, creating genotype-environment (G×E) interaction.

**Norm of Reaction** — The range of phenotypes produced by a single genotype across different environmental conditions

**Worked example:** A researcher studies three genetically distinct clones of duckweed (each clone has identical genotype) grown at five different nitrogen concentrations. 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. Each clone is a separate genotype, so each has its own norm of reaction plotted against nitrogen (the 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, the most common expected answer for graph interpretation questions.

## 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, it is called polyphenism. A common example of temperature-dependent plasticity comes from temperature-sensitive alleles, where the protein product is only active at a specific temperature range due to temperature effects on protein folding.

**Phenotypic Plasticity** — The ability of a single genotype to produce different phenotypes across different environmental conditions

**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. Original back fur is light because the back is 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 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.

## Nutrient Effects and Transgenerational Epigenetics

Nutrient availability is a major environmental modifier of phenotype, especially during development. For example, human height is ~80% heritable, but severe childhood malnutrition can stunt adult growth far below an individual’s genetic potential. Epigenetic effects are another key class of environmental effects: environment alters gene expression without changing the underlying DNA sequence, and these changes can sometimes be passed to offspring.

**Epigenetic Modification** — A change in gene expression that does not alter the underlying nucleotide sequence of DNA

**Worked example:** Two inbred mice have identical genotypes for the agouti viable yellow ($A^{vy}$) 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. The AP exam frequently tests the distinction between genetic changes (mutation, altered sequence) and epigenetic changes (altered expression, sequence unchanged).

## Sex-Influenced and Sex-Limited Traits

An individual’s biological 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 expressed differently in males and females due to hormone differences. Sex-limited traits are autosomal traits only expressed in one sex, because they are tied to sex-specific reproductive functions.

**Sex-Influenced Trait** — An autosomal trait that is expressed differently in males and females due to differences in hormonal environment

**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 $1\ BB : 2\ Bb : 1\ bb$.
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 gene location.

## Common pitfalls

- **Wrong:** Claiming phenotypic differences between identical twins must be genetic because they share the same genotype
  - Why it fails: Students forget that identical twins share a genotype but experience different environmental conditions over development, driving phenotypic differences via plasticity and epigenetic changes
  - Correct: 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:** 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 it fails: Students mix up population-level variation across genotypes with genotype-specific responses to environmental change
  - Correct: 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:** Stating that all epigenetic changes are heritable across multiple generations
  - Why it fails: Textbooks highlight rare examples of transgenerational epigenetic inheritance, leading students to assume all epigenetic changes are passed to offspring
  - Correct: 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:** Classifying pattern baldness as an X-linked trait because it is much more common in males
  - Why it fails: Students associate male-biased traits with X-linkage, but pattern baldness is a classic example of an autosomal sex-influenced trait
  - Correct: Always check whether the trait is described as autosomal or X-linked; male-biased expression alone is not sufficient evidence for X-linkage
- **Wrong:** Claiming that environmental effects on phenotype mean the trait is not heritable
  - Why it fails: Students see a large environmental effect and assume genetics do not matter, but most traits are the product of both
  - Correct: Unless the trait is explicitly non-plastic (e.g. ABO blood type), always explicitly state that phenotype arises from genotype-environment interaction

## Cheatsheet

| Term | Key Definition | Example |
| --- | --- | --- |
| Norm of Reaction | Phenotype range for 1 genotype across environments | Duckweed biomass vs nitrogen |
| Phenotypic Plasticity | Single genotype produces multiple phenotypes | Siamese cat temperature-sensitive fur |
| G×E Interaction | Different genotypes respond differently to environment | Three duckweed clones have unique responses |
| Epigenetic Change | Altered gene expression, no DNA sequence change | Agouti mouse phenotype from maternal diet |
| Sex-Influenced Trait | Autosomal trait expressed differently by sex | Pattern baldness (B dominant in males) |
| Sex-Limited Trait | Autosomal trait only expressed in one sex | Milk production in dairy cattle |

## What's next

Understanding environmental effects on phenotype connects basic Mendelian heredity to evolutionary processes, as genotype-environment interactions generate the phenotypic variation that natural selection acts upon. This concept also helps explain why genetically identical individuals can have dramatically different traits, a key observation for studies of heritability and complex trait genetics. Mastery of this topic is critical for experimental analysis questions on the AP Biology exam, which often require interpreting graphs of norm of reaction or connecting phenotype to gene expression. Explore these related topics to build on your knowledge:

- [Unit 5 Heredity Overview](https://www.owlsprep.com/study/ap-biology-u5-overview/)

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