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As we know that tumor suppressor gene causes cancer only when both the alleles are recessive in nature.But in case of oncogenes if only one allele is dominant it can cause cancer.Why in case of oncogenes to become a cause of cancer both allele aren't recessive?

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A tumor suppressor is an essential gene that regulates the cell cycle at different checkpoints. If one allele is lost due to a mutation then the function of that gene can be carried out by the other allele (There might be a less expression of that gene because of copy number reduction but it is not necessary in all cases- cases where there are feedback controls).

An oncogene is a gene that promotes cell division. Different kinds of mutations will have different kinds of effect.

  • Mutation leading to loss/reduction of function: Nothing is likely to happen unless both alleles are affected.
  • Mutation leading to hyperactivity: In this case one rogue allele will always remain active leading to uncontrolled growth stimulation.

So, it is the second type of mutation that you are referring to as in the case of Ras. Note that any mutation leading to formation of a toxic or hyperactive protein (rogue allele), will show genetic dominance. Also see the case of Huntington's disease.

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