D3.2.4—Phenotype
A phenotype is the observable characteristic produced by genotype, environment and their interaction in an organism in inheritance problems in inheritance problems.
- Syllabus
- First assessment 2025
- Objective
- D3.2.4
- Level
- HL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2013
Most common paperPaper2
Typical marks2
Common command terms
- Identify
Recent exam appearances
SessionPaperQuestionMarksFocus
May 2013Paper2 ["HL"] · TZ22(a)(iii)[ 2 ]D3.2.4—Phenotype
Coverage 2013–2013 · Updated 16 Jul 2026
Phenotype Is the Measured Outcome, Not the Genotype Itself
A phenotype is an observable or measurable characteristic produced by the genotype, the environment, or an interaction between them.
| Main cause | Example | Why |
|---|---|---|
| genotype | ABO blood group | inherited alleles determine A and B antigens |
| environment | an acquired scar | injury changes tissue without changing inherited alleles |
| genotype × environment | adult height | growth potential interacts with nutrition and health |
The same phenotype can come from different genotypes, and one genotype can produce different phenotypes in different environments. Phenotype alone therefore does not always identify genotype.
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.
Phenotype exam focus
[ 2 ]
Identify the phenotypes of each part of the phenotypic ratio.
| Ratio | Phenotypes |
|---|---|
| 9 | |
| 3 | |
| 3 | |
| 1 |
Concept essentials
- Phenotype means the expressed or observable characteristic.
- A phenotype can reflect both genotype and environmental influence.
- Inheritance data often use phenotype counts to infer genotypes.