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

Tumor-suppressor genes are genes that encode proteins which help regulate cell growth and division. These genes play a crucial role in preventing the formation of tumors by controlling cell proliferation, repairing DNA damage, and promoting programmed cell death (apoptosis). When functioning correctly, tumor-suppressor genes act as a protective mechanism against the formation of cancerous cells.

In genetics, a gene can be dominant or recessive, depending on its interaction with other genes when inherited from each parent. A dominant gene is one that is always expressed when present, regardless of the accompanying gene, while a recessive gene is only expressed when both copies are present. For example, if a gene is dominant, only one copy is needed for the trait to be expressed; if the gene is recessive, two copies are needed for the trait to be expressed.

The reason a mutation in a single copy of a tumor-suppressor gene is expected to behave as a recessive gene is due to the concept of haploinsufficiency. Haploinsufficiency occurs when a single functional copy of a gene is not sufficient to maintain the biological processes associated with that gene. In the case of a tumor-suppressor gene, one mutated copy and one functional copy can still produce enough active tumor-suppressor protein to maintain normal cell regulation and suppress tumor formation. Therefore, the mutation is not expressed, as the non-mutated, functional copy is sufficient for the cell's needs. However, if both copies of the tumor-suppressor gene are mutated, the cell lacks functional tumor-suppressor proteins, leading to uncontrolled cell growth and increased risk of tumor formation. In this situation, the mutations in the gene are expressed, resulting in the recessive nature of the single copy mutation in tumor-suppressor genes.