Proto-oncogenes are genes that control normal cellular functions. When mutated, proto-oncogenes become oncogenes that stimulate cell proliferation. They tend to be dominant in their action. Tumor-suppressor genes normally inhibit cell proliferation; when mutated, they allow cells to proliferate. Tumor-suppressor genes tend to be recessive in their action. Individual organisms that are heterozygous for tumor-suppressor genes are often predisposed to cancer.
-Why are oncogenes usually dominant in the their action, whereas tumor-suppressor genes are recessive?

Question

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The Answer is Oncogenes have a stimulatory effect on cell proliferation. Mutations in oncogenes are usually dominant because a mutation in a single copy of the gene is usually sufficient to produce a stimulatory effect. Tumor-suppressor genes inhibit cell proliferation. Mutations in tumor-suppressor genes are generally recessive, because both copies must be mutated to remove all inhibition.

Proto-Oncogenes Are Genes That Control Normal Cellular Functions