Definition · Plain-language
Genotype
A genotype is the genetic make-up of an organism — the particular set of alleles it carries for a trait.
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The genetic make-up
A genotype is the combination of alleles an organism carries for a given trait — its genetic instructions, inherited one allele from each parent. It is the underlying genetic information, as opposed to the phenotype, which is the visible result. For example, an organism’s genotype for eye colour might be two alleles for brown, or one for brown and one for blue. The genotype is fixed at fertilisation and stays the same throughout life, even though the environment can affect how it shows up.
Homozygous and heterozygous
For a trait controlled by a pair of alleles, an organism’s genotype is described as homozygous or heterozygous. Homozygous means the two alleles are the same — either two dominant (BB) or two recessive (bb). Heterozygous means the two alleles are different, one dominant and one recessive (Bb). This distinction matters because a heterozygous organism carries a hidden recessive allele that it can pass on, even though only the dominant trait shows. The terms describe the make-up of the genotype, not what the organism looks like.
How it is written
Genotypes are written using letters for the alleles, with a capital for the dominant allele and a lower-case letter for the recessive one — for example T for tall and t for short. The three possible genotypes are then TT, Tt and tt. Working out the genotypes of offspring from their parents is the basis of genetic crosses, often drawn in a grid called a Punnett square. This lets you predict the proportions of different genotypes, and so the likely traits, in the next generation.
Key facts
At a glance
- Definition: the set of alleles an organism carries for a trait
- Inherited: one allele from each parent, fixed at fertilisation
- Homozygous: two identical alleles (BB or bb)
- Heterozygous: two different alleles (Bb)
- Written as: letters, e.g. TT, Tt or tt
- Versus phenotype: the genotype is the genes; the phenotype is the visible trait
Common misconceptions
What people often get wrong
Often heard: Genotype and phenotype mean the same thing.
Actually: They are different. The genotype is the set of alleles an organism carries — its genetic make-up. The phenotype is the visible trait that results from the genotype and the environment. The genotype is the cause; the phenotype is the outcome.
Often heard: You can always tell an organism’s genotype just by looking at it.
Actually: Often you cannot. Because a dominant allele masks a recessive one, an organism showing the dominant trait could be homozygous (BB) or heterozygous (Bb). A genetic cross or DNA test is usually needed to be sure.
Often heard: An organism’s genotype changes during its life.
Actually: The genotype is fixed at fertilisation and stays the same. The environment can change the phenotype — how the genes show up — but it does not alter the alleles in the genotype itself.
Going deeper
