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Answers to Concept Checks

1. Retinoblastoma results from at least two separate genetic defects, both of which are necessary for cancer to develop. In sporadic cases, two successive mutations must occur in a single cell, which is unlikely and therefore typically affects only one eye. In people who have inherited one of the two required mutations, every cell contains this mutation, and so a single additional mutation is all that is required for cancer to develop. Given the millions of cells in each eye, there is a high probability that the second mutation will occur in at least one cell of each eye, producing tumors in both eyes and the inheritance of this type of retinoblastoma.
2. 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.
3. d
4. a
5. c
6. b
7. d

WORKED PROBLEMS

Problem

In some cancer cells, a specific gene has become duplicated many times. Is this gene likely to be an oncogene or a tumor-suppressor gene? Explain your reasoning.

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Solution Strategy

What information is required in your answer to the problem?

Whether the gene is likely to be an oncogene or tumor-suppressor gene and why.

What information is provided to solve the problem?

In cancer cells, the gene has been amplified many times.

For help with this problem, review:

Oncogenes and Tumor-Suppressor Genes in Section 23.2.

Solution Steps

The gene is likely to be an oncogene. Oncogenes stimulate cell proliferation and act in a dominant manner. Therefore, extra copies of an oncogene will result in cell proliferation and cancer. Tumor-suppressor genes, on the other hand, suppress cell proliferation and act in a recessive manner; a single copy of a tumor-suppressor gene is sufficient to prevent cell proliferation. Therefore, extra copies of the tumor-suppressor gene will not lead to cancer.

Recall: An oncogene is an accelerator to cell division, while a tumor-suppressor gene is a brake.