Loss of hetrozygosity associated with

Retinoblastoma (Rb) is a rare form of cancer that rapidly develops from the immature cells of a retina, the light-detecting tissue of the eye. It is the most common malignant cancer of the eye in children, and it is almost exclusively found in young children. Though most children survive this cancer, they may lose their vision in the affected eye(s) or need to have the eye removed. Almost half of children with retinoblastoma have a hereditary genetic defect associated with retinoblastoma. In other cases, it is caused by a congenital mutation in the chromosome 13 gene, Loss of heterozygosity (LOH) is a gross chromosomal event that results in loss of the entire gene and the surrounding chromosomal region. Most diploid cells, for example human somatic cells, contain two copies of the genome, one from each parent (chromosome pair); each copy contains approximately 3 billion bases (adenine (A), guanine (G), cytosine (C) or thymine (T)). For the majority of positions in the genome the base present is consistent between individuals, however a small percentage may contain different bases (usually one of two; for instance, 'A' or 'G') and these positions are called 'single nucleotide polymorphisms' or 'SNPs'. When the genomic copies derived from each parent have different bases for these polymorphic regions (SNPs) the region is said to be heterozygous. Most of the chromosomes within somatic cells of individuals are paired, allowing for SNP locations to be potentially heterozygous. However, one parental copy of a region can sometimes be lost, which results in the region having just one copy. The single copy cannot be heterozygous at SNP locations and therefore the region shows loss of heterozygosity (LOH). Loss of heterozygosity due to loss of one parental copy in a region is also called hemizygosity in that region.