Heterozygous, Homozygous, Genotype, and Phenotype

One locus, two allele copies

In a typical diploid organism, a gene locus is represented by two inherited allele copies, one from each parent. A notation such as $AA$, $Aa$, or $aa$ records the genotype at that locus. It does not describe the organism's entire genome.

• Homozygous means the two allele symbols are the same, as in $AA$ or $aa$.
• Heterozygous means the two allele symbols differ, as in $Aa$.

This classification says whether the pair matches. It does not, by itself, say what trait will be observed.

Genotype and phenotype answer different questions

Genotype describes inherited allele composition. Phenotype describes an observable or measurable outcome produced by genotype together with biological and environmental context.

Under a simple complete-dominance model, $AA$ and $Aa$ can share a phenotype even though their genotypes differ. In codominance, both allele products can contribute visibly in the heterozygote. In incomplete dominance, the heterozygous phenotype can differ from either homozygote. Therefore “heterozygous” is not a phenotype label.

Dominance is a relationship, not a synonym for frequency or strength

Calling one allele dominant means that its associated phenotype is expressed in a specified heterozygous comparison. It does not mean that the allele is more common, more beneficial, or biologically stronger. The relationship must always be tied to a particular trait and comparison.

When the word carrier applies

In medical and trait genetics, a carrier can have one variant allele associated with a recessive or sex-linked condition while lacking the corresponding condition or trait. That label adds information about the allele's association and inheritance context. A heterozygous genotype is a structural description; carrier status is an interpretation tied to a particular variant and trait. The terms can overlap, but they are not interchangeable.

Use the vocabulary in the right order

For an unfamiliar genetics problem:

1. Identify the locus and the two allele copies.
2. Describe the genotype as homozygous or heterozygous.
3. Apply the stated allele relationship to predict phenotype.
4. Use “carrier” only if the disease or trait context supports that label.