Genotype vs phenotype: What’s the difference?
Table of contents
Genotype is an organism’s genetic makeup, meaning the specific alleles it carries for a gene. Phenotype is what you can actually observe or measure, such as colour, height, blood type or a disease.
The short version: genotype is the instructions, and phenotype is the result.
Comparison
| Genotype | Phenotype | |
|---|---|---|
| What it is | The alleles present (AA, Aa, aa) | The observable trait (purple, tall, type A blood) |
| How you find it | DNA testing, pedigrees or test crosses | Looking at or measuring the organism |
| Inherited directly? | Yes, one allele from each parent | No. It results from the genotype plus the environment |
| Can change in a lifetime? | Essentially no | Yes (e.g. skin tanning, body weight) |
Why different genotypes can share a phenotype
Under complete dominance, one dominant allele is enough to show the trait. So AA and Aa plants with purple flowers look identical. Only aa plants have white flowers. That’s why a 1:2:1 genotype ratio becomes a 3:1 phenotype ratio.
In a dihybrid cross the effect is bigger. Nine genotypes fall into just four phenotypes in a 9:3:3:1 ratio.
How to work out a hidden genotype
If an organism shows the dominant phenotype, it could be AA or Aa. A test cross with an aa partner tells you which:
- All offspring dominant means the parent was very likely
AA. - About half recessive means the parent was
Aa.
The environment matters too
Phenotype = genotype + environment. Hydrangeas with the same genotype flower blue in acidic soil and pink in alkaline soil. Identical human twins share a genotype but can differ in height or weight because of nutrition.
Mendel’s ratios work because his pea traits are hardly affected by the environment.