A mutation of a tumour suppressor gene can result in the formation of a tumour. Explain how. Not all mutations to the gene result in a tumour. Explain how.

To answer this two-part question students must be able to understand two concepts: what a gene can do; and what constitutes a gene.

For the first part, a mutation in the genetic code of the tumour suppresor gene can inactive the gene. Inactivation of the gene will prevent the protein produced from slowing down cell division. An increased rate in cell division can become too fast for the body and cause a tumour.
If a mutation does not cause a tumour then this can be attributed to the genetic code being degenerate. (An alternative answer could be that the mutation is within an intron or an untranslated region of the gene).