Describing an organism or cell in which the alleles at a given locus on homologous chromosomes are different. An allele that is fully dominant with respect to its recessive counterpart at the same locus will solely determine the nature of the corresponding trait (i.e. the phenotype), whereas alleles displaying incomplete dominance will result in an intermediate phenotype. Heterozygous individuals (i.e. heterozygotes) having greater reproductive fitness compared with either homozygote are said to have heterozygote advantage. Heterozygous organisms do not breed true. Compare hemizygous; homozygous.