Phenotype and genotype
There are two quite different questions you can ask about an organism: what alleles does it carry, and what does it actually look like. Each question has its own word.
Genotype
The alleles an organism carries for a characteristic. Written with letters: TT, Tt, tt.
Phenotype
The characteristic as it is expressed in the organism — what you can see or measure. Tall, or dwarf.
One phenotype can hide two genotypes
Three genotypes are possible for stem height, but only two phenotypes. That mismatch is the whole reason genetics needs two words instead of one.
| Genotype | Phenotype |
|---|---|
| TT | Tall |
| Tt | Tall — indistinguishable from the row above |
| tt | Dwarf |
You cannot read a genotype off a plant
Looking at a tall pea tells you it has at least one T. It cannot tell you what the second allele is. To find that out you have to cross the plant with a homozygous recessive one and look at the offspring — which is exactly what a test cross is for.
The phenotype is not the genotype alone
A pea plant with the genotype for tallness will still be stunted if it is starved of water or light. The genotype sets what is possible; the environment decides how far it is reached. That is the same distinction as the first skill's acquired characteristics, seen from the other side.
Only the genotype is inherited
A child inherits alleles, not appearances. Whatever the environment did to a parent's phenotype stays with that parent; what travels to the next generation is the genotype, and the phenotype is rebuilt from scratch in each new individual.