Mutations may arise throughout an organism's life cycle. Typically, sporadic meiotic mutations give rise to individuals with all their germinal and somatic cells bearing the mutant gene. These mutations may be amorphs (with full penetrance and expressivity) or hypomorphs (with reduced penetrance and expressivity). Mutational mosaicism, however, involves the origin of mutations occurring during mitosis, whether in the parent at some stage prior to reproductive maturity or in the offspring at some time following fertilization. The phenotypic expression and transmission of these new mutations are dependent on the proportion of cells bearing the mutant gene as well as the location of these cells in somatic and/or germinal tissues. Mutational mosaicism was used as a developmental model to analyze 1,500 sporadic and 179 familial cases of retinoblastoma from the world literature. This model provided an interpretation for the origin, onset, and transmissibility of the sporadic unilateral retinoblastoma cases, which represent over 60% of all retinoblastoma patients. The model also permits a reclassification of all transmissible types of retinoblastoma; based on this classification, more accurate risk figures for genetic counseling can be offered. In addition, mutational mosaicism can be extended as a model to other autosomal dominant and X-linked mutations.
