Genotype, phenotype, dominance and phenotypic plasticitySpec D3.2.3, D3.2.4, D3.2.5, D3.2.6
In short
Genotype is the combination of alleles inherited by an organism, and phenotype is the observable traits resulting from genotype and environmental factors. A dominant allele has the same effect in one or two copies, so homozygous-dominant and heterozygous individuals look alike. Phenotypic plasticity is the capacity to develop traits suited to the environment by varying gene expression.
- Gene
- A heritable factor that occupies a specific position (locus) on a chromosome and influences a characteristic.
- Allele
- One of the alternative forms of a gene, differing in base sequence by one or a few bases.
- Genotype
- The combination of alleles of a gene (or genes) inherited by an organism.
- Homozygous
- Having two identical alleles of a gene, for example AA or aa.
- Heterozygous
- Having two different alleles of a gene, for example Aa.
- Phenotype
- The observable traits of an organism, resulting from its genotype and environmental factors.
| Cause | Examples |
|---|---|
| Genotype only | ABO blood group; inherited conditions such as haemophilia |
| Environment only | Scars, tattoos, the language a person speaks |
| Interaction of genotype and environment | Height and body mass (genes plus nutrition); skin colour (genes plus exposure to sunlight) |
Dominant and recessive alleles
A dominant allele usually codes for a functional protein, often an enzyme. One copy produces enough of it for the trait to appear, so homozygous dominant (AA) and heterozygous (Aa) individuals have the same phenotype. A recessive allele often codes for a non-functional protein, or none, so its effect only shows when there is no dominant allele (aa).
Phenotypic plasticity
Phenotypic plasticity is the capacity to develop traits suited to the environment an organism experiences, by varying patterns of gene expression. The genotype does not change, and the changes may be reversible during the individual's lifetime.
- Human skin tans in strong sunlight as melanin production increases, then fades in winter.
- Skeletal muscles grow larger with regular training and shrink when training stops.
- Water crowfoot (Ranunculus aquatilis) grows finely divided leaves under water and broad leaves above it, from the same genotype.
Phenotypic plasticity is not mutation or evolution. The alleles stay the same; only which genes are expressed, and how much, changes.
Written and checked against the IB Biology HL specification · Updated October 2026