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

Verified step by step guidance

1. Understand the role of tumor-suppressor genes: These genes are responsible for regulating cell growth and division, ensuring that cells do not proliferate uncontrollably.
2. Recognize the function of tumor-suppressor genes: They act as a brake on cell division, repair DNA mistakes, or tell cells when to die (a process known as apoptosis).
3. Identify the impact of mutations: A mutation in a tumor-suppressor gene can lead to a loss of function, meaning the gene can no longer regulate cell growth effectively.
4. Explain the concept of recessiveness: For a tumor-suppressor gene to lose its function completely, both copies (alleles) of the gene must be mutated. This is because the presence of one normal (wild-type) allele can often compensate for the mutated one, maintaining some level of normal function.
5. Conclude why a single mutated copy behaves recessively: Since the normal allele can still perform the gene's function, the mutation in one allele does not manifest in a dominant manner, thus behaving as a recessive trait.

Key Concepts

Tumor-Suppressor Genes

Tumor-suppressor genes are segments of DNA that encode proteins responsible for regulating cell growth and division. They act as a safeguard against uncontrolled cell proliferation, which can lead to tumor formation. When functioning properly, these genes help maintain genomic stability and prevent the development of cancer.

Recessive Alleles

A recessive allele is a variant of a gene that does not manifest its trait in the presence of a dominant allele. For tumor-suppressor genes, both copies (one from each parent) must be mutated or inactivated for the cancerous phenotype to occur. This is because the remaining functional copy can still produce enough protein to suppress tumor growth, illustrating the recessive nature of these mutations.

Two-Hit Hypothesis

The two-hit hypothesis, proposed by Alfred Knudson, explains how mutations in tumor-suppressor genes lead to cancer. It posits that both alleles of a tumor-suppressor gene must be inactivated for tumorigenesis to occur. The first 'hit' can be a hereditary mutation, while the second is often acquired, emphasizing the need for both copies to be affected for the recessive behavior to manifest in cancer development.