Final answer:
Tumor suppressor genes need both alleles to be mutated for tumors to form, acting as brakes in the cell cycle and preventing uncontrolled cell growth; the most studied example is the p53 gene.
Step-by-step explanation:
Since tumor suppressor genes are responsible for coding the proteins that prevent uncontrolled cell division and growth, both copies of the gene must be inactive or mutated for a tumor to form. Tumor suppressor genes, like p53, function as the brakes for the cell cycle, halting cell division to prevent cancer. When both alleles of a tumor suppressor gene are mutated, essential functions such as DNA repair, cell cycle progression control, and apoptosis are compromised. This disruption can lead to cellular changes that accumulate over time, resulting in unchecked cell proliferation that may develop into cancerous tumors. Even though there are cases where one mutated tumor suppressor gene can exert a dominant negative effect on the normal allele, typically, both need to be affected for complete loss of function.