Alleles, Genotypes, and Phenotypes

Three terms that unlock genetics

Allele, genotype, and phenotype are easy to mix up because they describe connected ideas.

The simple version is:

• an allele is a version of a gene
• a genotype is the allele combination
• a phenotype is the observed result

What is an allele?

An allele is a version of a gene.

For a gene at a particular chromosome location, different DNA sequence versions may exist in a population. Those versions are alleles.

In many beginner examples, alleles are written as letters such as A and a. That notation is a teaching tool, not the literal DNA sequence.

What is a genotype?

A genotype is the genetic makeup for a trait or gene.

For a single gene in a diploid organism, a genotype often includes two alleles: one inherited from each parent.

If the two alleles are the same, the genotype is homozygous. If they are different, it is heterozygous.

What is a phenotype?

A phenotype is an observable or measurable trait.

That can mean a visible feature, such as flower color, but it can also mean something molecular or physiological, such as enzyme activity, blood type, or disease symptoms.

Phenotype is shaped by genotype, but it is not always determined by genotype alone.

Dominant and recessive

In simple dominance, one allele can mask the effect of another in a heterozygote.

The allele that shows in the heterozygote is called dominant. The allele masked in the heterozygote is called recessive.

This does not mean the dominant allele is better, stronger in every sense, or more common in a population. It only describes how the trait appears in that genotype.

A simple example

Imagine a plant gene with two alleles:

• P for purple flowers
• p for white flowers

If P is dominant, then:

• PP plants have purple flowers
• Pp plants have purple flowers
• pp plants have white flowers

The genotypes PP and Pp are different, but the phenotype is the same in this simple example.

Beyond simple examples

Real genetics is often more complicated.

Traits can involve many genes, incomplete dominance, codominance, environmental influence, gene regulation, or developmental timing. Beginner examples are useful maps, but they are not the whole landscape.

The key idea

Alleles are versions, genotype is the combination, and phenotype is the outcome you can observe or measure.