What type of gene can become activated by a mutation occurring in just one allele?

A proto-oncogene can become activated by a mutation occurring in just one allele because it typically functions in a dominant manner. When a mutation occurs in one copy of a proto-oncogene, it can lead to the production of a protein that promotes cell growth and division in an uncontrolled manner, contributing to cancer development. This is why even a single mutation can be sufficient to drive the oncogenic process.

In contrast, tumor suppressor genes usually require both alleles to be inactivated to contribute to tumor formation, as they typically function through a recessive mechanism. DNA repair genes, while crucial for maintaining genomic integrity, don't directly lead to uncontrolled proliferation by themselves; rather, they prevent the accumulation of mutations that can lead to cancer. Oncogenes are the mutated forms of proto-oncogenes but generally refer to the activated state of these genes after mutation, thus supporting the definition of proto-oncogenes in this context.