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Tumor-suppressor genes are a class of genes that play a crucial role in preventing the uncontrolled growth of cells, which can lead to the formation of tumors. These genes are involved in regulating the cell cycle, DNA repair, and cell death, among other important cellular processes. When functioning properly, tumor-suppressor genes act as a safeguard against the development of cancer by keeping cell division in check and promoting the repair of damaged DNA.

Each individual inherits two copies of every gene, one from the mother and one from the father. When one of the copies of a tumor-suppressor gene undergoes a mutation, it loses its normal function. However, the presence of a functional copy of the gene can still keep the cell growth under control. This is because the protein encoded by the functional copy of the gene can perform its role in suppressing uncontrolled cell division.

In genetics, a trait is considered recessive when it is only expressed when an individual has two copies of the mutated gene, while a dominant trait is expressed even if there is only one mutated gene. In the case of a mutated single copy of a tumor-suppressor gene, it is expected to behave as a recessive gene because the presence of a functional copy of the gene prevents the uncontrolled cell growth and the development of cancer. The reason behind this behavior is the mechanism through which tumor-suppressor genes work. Since they function to restrain cell division, the functional gene can still carry out this role even if one copy is mutated. Thus, both copies must be mutated for the tumor-suppressor gene to completely lose its function and result in the uncontrolled cell growth that is characteristic of cancer. This is why a mutated single copy of a tumor-suppressor gene behaves as a recessive trait.