Define tumor-suppressor genes. Why is a mutated single copy of a tumor-suppressor gene expected to behave as a recessive gene?

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Step 1: Define tumor-suppressor genes as genes that encode proteins responsible for regulating cell growth, repairing DNA damage, and ensuring cells do not divide uncontrollably, thereby preventing tumor formation.

Step 2: Explain that tumor-suppressor genes typically require both copies (alleles) to be inactivated or mutated to lose their function, which is why they are often considered recessive at the cellular level.

Step 3: Describe that when only one copy of a tumor-suppressor gene is mutated, the other normal copy can usually produce enough functional protein to maintain normal cell regulation.

Step 4: Clarify that this functional redundancy means a single mutated copy does not lead to loss of tumor-suppressor activity, so the mutated allele behaves recessively because the phenotype (loss of tumor suppression) only appears when both alleles are mutated.

Step 5: Summarize that the recessive behavior of tumor-suppressor gene mutations contrasts with dominant oncogenes, where a single mutated copy can promote cancer development.

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Tumor-Suppressor Genes

Tumor-suppressor genes are genes that regulate cell growth and division, preventing uncontrolled proliferation. They act as cellular brakes by repairing DNA damage or initiating apoptosis, thereby protecting against cancer development.

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09:09

Mapping Genes

Recessive Nature of Tumor-Suppressor Gene Mutations

A mutation in a single copy of a tumor-suppressor gene usually does not cause loss of function because the other normal allele can compensate. Therefore, the mutated allele behaves recessively, requiring both copies to be inactivated for tumorigenesis.

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04:46

Cancer Mutations

Two-Hit Hypothesis

The two-hit hypothesis explains that both alleles of a tumor-suppressor gene must be mutated or inactivated to lose function. The first 'hit' is often inherited or acquired, and the second 'hit' occurs somatically, leading to cancer progression.

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