FIGURE 24-17 Role of spontaneous somatic mutation in retinoblastoma. This disease is marked by retinal tumors that arise from cells carrying two mutant RB⁻ alleles. (a) In hereditary (familial) retinoblastoma, a child inherits a normal RB⁺ allele from one parent and a mutant RB⁻ allele from the other parent. When the second normal allele is lost in a heterozygous somatic retinal cell, a cell is generated that lacks any Rb gene function. (b) In sporadic retinoblastoma, a child inherits two normal RB⁺ alleles. Two separate Rb loss events must occur in a particular retinal cell to produce a cell lacking all Rb function.