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

For example, if a dominant allele is represented by $\textrm{A}$ and its corresponding recessive allele is represented by $\textrm{a}$, then a homozygous organism has genotype either $\textrm{AA}$ (homozygous dominant) or $\textrm{aa}$ ((homozygous recessive)). Note that a homozygous organism can therefore have different phenotypes, as the former organism will display the trait corresponding to the dominant allele, and the latter will display the one corresponding to the recessive allele. A heterozygous organism for this factor must have genotype $\textrm{Aa}$.