An organism is said to be heterozygous for a Mendelian factor if it possesses two different alleles for the factor. This is in contrast to a homozygous organism, which has two identical alleles.

For example, if a dominant allele is represented by $\textrm{A}$ and its corresponding recessive allele is represented by $\textrm{a}$, then a heterozygous organism has genotype $\textrm{Aa}$ and a homozygous organism has genotype $\textrm{AA}$ (homozygous dominant) or $\textrm{aa}$ ((homozygous recessive)).