Heterozygote cells are diploid or polyploid and have different alleles at a locus (position) on homologous chromosomes. When an organism is referred to as a heterozygote, or being heterozygous for a specific gene, it means that the organism carries two different versions of that gene on the two corresponding chromosomes. Heterozygosity refers to both the state of being a heterozygote, but more commonly in population genetics to the fraction of individuals in a population that are heterozygous for that locus.

The effect of heterozygosity depends on the protein product coded for by the gene in question. In some cases a mixture of normal and abnormal proteins is produced. In other cases, the abnormal allele may produce no protein at all. If the proteins are enzymes, the less-functional enzyme molecules usually have no effect and the person is an asymptomatic carrier. If the proteins are structural, such as those that comprise skin or bone matrix, a mixture of normal or abnormal products usually produces abnormal skin or bone, and the resulting abnormality is termed a dominant trait or disease. An abnormal gene that produces no protein may not affect the person at all if the other chromosome has a normal copy of the gene; this would be referred to as a recessive disease.

To describe how a trait inherits (Main article: Mendelian inheritance), the dominant trait is indicated with a capital and the recessive with a lower case character. The colour of Mendel's peas are often indicated as PP for the dominant homozygote pink flower and pp for the recessive homozygote white flower. When these are crossed, the F1 or second generation will be heterozygote for the colour of the flower and this will be indicated with Pp. All F1 plants will flower pink.

Some traits are neither dominant nor recessive but are intermediary: the red carnation crossed with a white carnation will give you heterozygote pink carnations.

See also

• homozygote
• dominant gene
• recessive gene