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Heterozygosity is mixed inheritance with respect to a genetic trait. An individual with two sets of chromosomes ( diploid ) is heterozygous or heterozygous for this trait if a gene in these sets of chromosomes is present in two different forms (alleles). If one allele is dominant over the other allele, then the characteristic which is characterized by this allele will appear in the phenotype . If none of the alleles is dominant, there is intermediate inheritance . Then the phenotype will be a mixed type. The opposite of mixed inheritance is pure inheritance, also called homozygosity .

Loss of heterozygosity

Occasionally it can happen that despite heterozygosity of the zygote and recessivity of the characteristic an appearance occurs with a partially homozygous phenotype . This happens when an allele is lost in the course of ontogenetic development.

An example of this is retinoblastoma as a child's eye tumor, where the loss of heterozygosity (also known as loss of heterocygosity , LOH for short) is coupled with a loss of tumor suppressor genes . Since there are usually two alleles for an autosomal gene in somatic cells (exception: X inactivation ), the loss of an allele that codes for functional proteins is often only noticeable when there is a genetic defect in the other allele that is no longer caused by the second can be compensated. If one allele is defective ( 1st hit ) - this can be inherited (as in familial adenomatous polyposis [FAP], Lynch's syndrome ) or due to spontaneous mutations - the loss of the second allele (loss of the heterozygous state as 2nd . hit ) to the loss of function, for example a tumor suppressor gene. Other examples are the BRCA1 or BRCA2 gene, which are responsible for familial breast cancer, or the MSH2 gene, which has been shown to be the crucial DNA mismatch repair protein in HNPCC (hereditary non-polypous colon cancer).

See also