Chapter 5 Cancer and benign tumours/Fig 5.30.jpg

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Inheritance of retinoblastoma. The retinoblastoma gene lies on the long arm of chromosome 13. (A) The sporadic form of retinoblastoma occurs in individuals who have two normal Rb genes. Loss of both alleles is required before a tumour will develop, therefore the risk of this is very small and the tumour, which is unilateral, often occurs in older children. (B) In contrast, in patients with familial retinoblastoma one Rb allele is mutated in the germ line, and therefore in every cell in the body. Only one mutation in the remaining normal Rb gene is required for a tumour to develop. Accordingly, this almost invariably occurs, and most patients develop bilateral tumours at an early age. Unfortunately, they also develop a variety of other tumours in later life.