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D3.2.6—Phenotypic plasticity

Phenotypic plasticity occurs when the same genotype produces different phenotypes under different environmental conditions in inheritance problems in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.6
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1A
Typical marks1

Recent exam appearances

November 2025Paper1A ["HL"] · TZ332[ 1 ]D3.2.6—Phenotypic plasticity

Coverage 2025–2025 · Updated 16 Jul 2026

Phenotypic Plasticity Lets One Genotype Respond to Different Environments

Phenotypic plasticity is the capacity of one genotype to develop different phenotypes in different environments by changing patterns of gene expression.

environmental cue → signalling pathway → transcription or translation changes → different proteins, cell activity or growth → phenotype better matched to the experienced environment

Plastic response Genetic evolution
genotype is unchanged within the individual allele frequencies change across generations
can be reversible during a lifetime inherited population change is not reversed by one individual's environment
response range itself can be inherited selection acts on heritable differences

A suntan or training response is acquired plasticity; it does not mean the altered phenotype is encoded as a new allele and passed directly to offspring.

Inheritance Predictions Move from Chromosomes to Alleles to Phenotypes

homologous chromosomes carry two alleles in a diploid parent → meiosis separates one allele into each gamete → fertilization restores a two-allele genotype → allele relationship and environment shape phenotype → offspring data test the prediction

For any genetic cross:

  • define allele symbols and their relationship
  • write parental genotypes, not only phenotypes
  • derive gametes before drawing the grid
  • combine gametes, then translate genotype to phenotype
  • state probabilities and the assumptions behind them
If the simple 3:1 expectation fails First question to ask
heterozygote has a third phenotype incomplete dominance?
both products appear codominance?
phenotype changes with conditions environmental or plastic response?
more than two alleles occur in the population multiple-allele locus such as ABO?

Many Small Genetic Effects and Environment Produce a Phenotypic Range

In polygenic inheritance, several loci contribute to one characteristic. Different combinations of small allele effects generate many genotypic values rather than a few Mendelian phenotype classes.

many contributing genotypes + nutrition, health, temperature or other environmental effects + genotype-specific responses to those conditions → overlapping phenotypes across a continuous range

Trait Genetic contribution Environmental contribution
height many growth-related loci nutrition, disease and developmental conditions
body mass metabolism and appetite loci diet, activity and health
skin pigmentation several melanin-related loci ultraviolet exposure changes melanin production

Do not confuse multiple genes contributing to one trait with multiple alleles at one gene locus. These are different levels of genetic variation.

Phenotypic plasticity

[ 1 ]

Scientists incubated larvae of the moth Utetheisa ornatrix at either 15∘C15^{\circ} \mathrm{C} or 22∘C22^{\circ} \mathrm{C} until they hatched. They found the hatched moths had different wing colour patterns due to phenotypic plasticity.

Moth from larvae incubated at \(15^{\circ

Moth from larvae incubated at \(22^{\circ

Which of the following explains the observed differences in wing colour?

Concept essentials

  • One genotype can produce different phenotypes in different environments.
  • Phenotypic plasticity changes expression without changing DNA sequence.
  • Environmental effects can complicate genotype-to-phenotype predictions.