Heterozygosity is the heterozygosity with respect to a genetic trait. An individual having two sets of chromosomes ( diploid) is heterozygous for this trait if a gene in these two sets of chromosomes in various forms is present ( alleles ). Is that an allele over the other allele is dominant, then the embossed by this allele trait will occur in the phenotype. If none of the alleles are dominant, the phenotype will be a mixed type. The opposite of heterozygosity is the homozygosity, also called homozygosity.

Loss of heterozygosity

It may happen that, in spite of heterozygous recessive phenotype (or the feature ) occurs. This occurs when one allele is lost. For example, when a child's eye tumor ( retinoblastoma ). Loss of heterozygosity (also called loss of heterocygosity, short LOH) usually leads to the loss of tumor suppressor genes. Such a loss occurs, however, only if two conditions are fulfilled, since a gene always determined by two different alleles and thus a potentially diseased or defective allele can be compensated by the healthy. If this is a defective allele - this can genetically (such as familial adenomatous polyposis in [ FAP ] or Lynch syndrome ) or by point mutations caused his ( the so-called 1st Hit) - represents the loss of the second allele ( second hit ) the loss of the heterozygous state is (LOH ) and thus leads to loss of function of the tumor suppressor. Examples are the BRCA1 and BRCA2 genes, which are responsible for the familial breast cancer, or the MSH2 gene, which has proven to be the ultimate DNA mismatch repair protein in the HNPCC ( hereditary nonpolyposis colon cancer - not ).

Compound heterozygote

The characteristic value is not due to homozygosity of the same mutation. There are rather different mutations in both copies of the same gene. Examples: phenylketonuria, cystic fibrosis ( see also complex heterozygosity ).