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.

Verified step by step guidance

1. Understand that retinoblastoma is caused by mutations in the RB1 gene, where RB1⁺ represents the normal (wild-type) allele and RB1⁻ represents the mutant allele.
2. Recognize that individuals with a dominant predisposition inherit one mutated allele (RB1⁻) but cancer develops only after a second mutation occurs in the other allele within a cell.
3. Recall the 'two-hit hypothesis' which states that both copies of the RB1 gene must be inactivated (mutated) in a single cell for the tumor to form.
4. Therefore, the genotype of a cell in a retinoblastoma tumor will have both alleles mutated, meaning it will be homozygous mutant: ${\mathrm{RB1}}_{-}{\mathrm{RB1}}_{-}$.
5. Summarize that the tumor cell genotype is $RB1^\{-\}RB1^\{-\}$, indicating loss of function of both RB1 alleles, which leads to uncontrolled cell growth and cancer.

Key Concepts

Tumor Suppressor Genes and the RB1 Gene

Tumor suppressor genes like RB1 produce proteins that regulate cell growth and prevent cancer. The RB1 gene encodes a protein that controls the cell cycle, and mutations in RB1 can lead to loss of this control, promoting tumor development. Both copies of the gene typically need to be inactivated for cancer to occur.

Knudson's Two-Hit Hypothesis

This hypothesis explains that two mutations ('hits') are required to inactivate both alleles of a tumor suppressor gene for cancer to develop. In hereditary retinoblastoma, one mutated allele (RB1⁻) is inherited, and a second somatic mutation inactivates the other allele, leading to tumor formation.

Genotype of Retinoblastoma Tumor Cells

In retinoblastoma tumor cells, both RB1 alleles are mutated or inactivated (RB1⁻/RB1⁻), resulting in loss of tumor suppressor function. Although the inherited mutation is dominant in predisposition, the tumor cells themselves are homozygous or functionally null for RB1, driving cancer progression.