In diploid organisms, heterozygosity is the property of having different alleles for a particular locus on homologous chromosomes. Organisms that are heterozygous for any particular locus are termed heterozygotes. Because two different alleles are present in the cell, the term 'heterozygote' may be considered synonymous with 'genetic hybrid'. Heterozygosity is the opposite of homozygosity, where two identical alleles for a given locus are carried on both homologous chromosomes.

In heterozygotes where there is a simple dominance relationship, one of the two alleles is dominant and overrides the phenotypic effect of the other, which is said to be recessive. In such a scenario, the phenotype of the heterozygote is identical to that of the dominant-homozygote. However, in most cases the pattern of dominance is more complex and the heterozygote will display its own phenotype, distinguishable from both of the homozygous phenotypes.

A heterozygous parent will distribute the alleles for the locus at which it is heterozygous randomly to its gametes, such that an offspring may inherit either of the two different alleles at random.