For the retinal cancer retinoblastoma, the inheritance of one mutated copy of RB1 from one of the parents is often referred to as a mutation that produces a 'dominant predisposition to cancer.' This means that the first mutation does not produce cancer but makes it very likely that cancer will develop.

Using RB1⁺ for the normal wild-type allele and RB1⁻ for the mutant allele, identify the genotype of a cell in a retinoblastoma tumor.

Key Concepts

Dominant Inheritance

Dominant inheritance refers to a pattern where a single copy of a mutated gene is sufficient to express a trait or condition. In the case of retinoblastoma, inheriting one mutated RB1 gene (RB1⁻) from a parent increases the likelihood of developing cancer, even though the individual may not show symptoms until a second mutation occurs.

RB1 Gene Function

The RB1 gene encodes a protein that is crucial for regulating the cell cycle and preventing uncontrolled cell growth. When this gene is mutated, its ability to control cell division is compromised, leading to an increased risk of tumor formation, particularly in retinal cells, which can result in retinoblastoma.

Genotype of Tumor Cells

In retinoblastoma tumors, the genotype typically reflects the loss of function of both RB1 alleles. A common genotype for tumor cells would be RB1⁻/RB1⁻, indicating that both copies of the RB1 gene are mutated, which is necessary for the development of the cancerous phenotype in the retinal cells.