Heterozygous

A diploid organism is heterozygous at a gene locus when its cells contain two different alleles (one wild-type allele and one mutant allele) of a gene.^[3]

The cell or organism is called a heterozygote specifically for the allele in question, and therefore, heterozygosity refers to a specific genotype.

Heterozygous genotypes are represented by an uppercase letter (representing the dominant/wild-type allele) and a lowercase letter (representing the recessive/mutant allele), as in "Rr" or "Ss". Alternatively, a heterozygote for gene "R" is assumed to be "Rr". The uppercase letter is usually written first.

If the trait in question is determined by simple (complete) dominance, a heterozygote will express only the trait coded by the dominant allele, and the trait coded by the recessive allele will not be present.

In more complex dominance schemes the results of heterozygosity can be more complex.

A heterozygous genotype can have a higher relative fitness than either the homozygous dominant or homozygous recessive genotype – this is called a heterozygote advantage.