A heterozygote (heterozygous individual) has at least half of the normal enzyme activity level, due to expression by the wild-type allele.

However, the same mutation also conveyed advantages, providing improved viability for heterozygous individuals.

Additionally, mitotic recombination can result in the expression of recessive genes in an otherwise heterozygous individual.

Haemophilia C is not completely recessive, as heterozygous individuals also show increased bleeding.

In heterozygous individuals, the allele that is expressed is the dominant.

Underdominance can also be described as homozygote advantage, wherein homozygous individuals have a higher fitness than heterozygous individuals.

Inversions can also give rise to abnormal gametes in heterozygous individuals but this effect has little importance compared to translocations.

A heterozygous individual and a person with two normal alleles (homozygous) will have a 50% chance of producing an affected child.

This situation can be important if, for example, the genetic marker is linked to a disease locus with a strong selection against heterozygous individuals.

One affected family has been identified with individuals both homozygous and heterozygous for MEN1 mutations.