Answer
A normal tumor suppressor gene encodes a protein that restrains cell division.
A mutant form of the protein fails to suppress cell division, but if either of the two alleles encodes normal protein, normal function will continue.
A normal oncogene encodes a regulator protein that triggers cell division, but only when an appropriate signal (growth factor) is present.
The mutant version of the oncogene product constantly sends the signal to divide, whether or not growth factors are present.
Work Step by Step
That's why mutations in tumor suppressor genes are recessive (both copies of the gene must be defective for the regulation of cell division to be defective) whereas mutations in oncogenes are dominant.
