Loss of heterozygosity

Loss of heterozygosity (LOH) in a cell represents the loss of one parent's contribution to part of the cell's genome. LOH can arise via several pathways, including deletion, gene conversion, mitotic recombination and chromosome loss. The latter event is sometimes followed by duplication of the remaining chromosome.

In cancer
It is a common occurrence in cancer, where it indicates the absence of tumor suppressor gene in the lost region. However, many people go healthy with such a loss, because there still is one functional gene left on the other chromosome of the chromosome pair. However, the remaining copy of the tumor suppressor gene can be inactivated by a point mutation, leaving no tumor suppressor gene to protect the body.

Retinoblastoma
The classical example of such a loss of protecting genes is hereditary retinoblastoma, in which one parent's contribution of the tumor suppressor Rb1 is flawed. Although most cells will have a functional second copy, chance loss of heterozygosity events in individual cells almost invariably lead to the development of this retinal cancer in the young child.

Detection
Loss of heterozygosity can be identified in cancers by noting the presence of heterozygosity at a genetic locus in an organism's germline DNA, and the absence of heterozygosity at that locus in the cancer cells. This is often done using polymorphic markers, such as microsatellites or single nucleotide polymorphisms, for which the two parents contributed different alleles.