Genetics is the field of science that looks at how traits (such as eye color) are passed down from parents to their children through genes. It also looks at the reasons living things are alike or different.

Genes are pieces of DNA (deoxyribonucleic acid) that have instructions on how to make the proteins the body needs to function. DNA is the genetic “blueprint” found in the center (nucleus) of each cell. Genes affect inherited traits which are passed on from a parent to a child, such as hair color, eye color, and height. They also affect whether a person is likely to develop certain diseases, such as cancer.

In humans, genes are located on 23 pairs of chromosomes. One of each chromosome pair comes from the mother, the other from the father. Each chromosome can contain hundreds or thousands of genes that are passed from the parents to the child. The genes you were born with are in every cell in your body.

Gene changes play an important role in the development of cancer. Changes in genes are called mutations. Mutations can cause cells to grow out of control, which can lead to cancer. Usually several gene changes are needed before a cell becomes cancer. Some of these changes may be inherited from a parent, but most are not. Only about 5% to 10% of all cancers are thought to have an inherited component that strongly affects a person’s risk for a certain type of cancer. To learn more about different types of cancer-related genes, please see our document, Oncogenes, Tumor Suppressor Genes, and Cancer.

Most cancers are started by gene mutations that happen sometime during a person’s lifetime — not by inherited mutations. These mutations only affect the cells that grow from the mutated cell, and not all of the cells in the person’s body. In this case, a cancerous tumor will have mutations in all of the tumor cells, but the non-cancerous cells of the body will not have the mutations. This is very different from inherited mutations, which are in every cell in the body — even the cells without cancer.

Here is some basic information about genetic testing and how it is used to learn about inherited cancer risk. It will help you understand what genetic testing is, but it won’t give you specific information or advice on genetic testing for any one disease. A doctor or a genetic counselor who knows about the diseases in your family is better able to talk with you about your risks and what you might learn from genetic testing.

What is genetic testing?

Genetic testing is the process of using medical tests to look for changes (mutations) in a person’s genes or chromosomes. Hundreds of different genetic tests are used today, and more are being developed.

Genetic testing for diseases that can be inherited

• Predictive gene testing is used to look for gene mutations that might put a person at risk of getting a disease. It is usually done in those families with a disease that may be inherited. An example is testing for changes in the BRCA1 and BRCA2 genes in a woman whose mother and sister had breast cancer.
• Carrier testing can be used to help couples learn if they carry a gene mutation for a disorder they might pass on to a child, such as cystic fibrosis, sickle-cell anemia, or Tay-Sachs disease.
• Prenatal screening can be used to diagnose some conditions in babies before they are even born, such as Down syndrome.
• Newborn screening is the most widespread form of genetic testing. Newborns are screened for a variety of inherited conditions such as phenylketonuria (PKU), cystic fibrosis, sickle cell disease, etc. The tests required vary from state to state.

These forms of genetic testing all look for gene changes that are passed from one generation to the next and are present in every cell in the body. Except for the newborn screening tests, they are used mainly for people with certain types of disease that seem to run in their families. They are not needed by most people.

Cancer-related genetic tests are mostly done for predictive reasons. They may be used:

• To see if a person has a certain gene change (mutation) known to increase the risk for a certain condition or disease (such as cancer)
• To confirm a suspected mutation in an individual or family

Genetic or DNA testing of cancer cells

There are genetic tests that look at DNA changes in cancer. These tests look for the changes only in the cancer cells that are taken from the patient. The genetic tests that are used to check cancer cells are not the same as the tests used to find out about inherited cancer risk.

Tests done on cancer cells look for DNA mutations that started in a single cell in the body and are present only in those cells that developed from the mutated cell. These tests can give information on a person’s outlook (prognosis) or even on how much he or she might benefit from certain types of treatment.

This document will not cover this type of DNA testing, which is done on tumor biopsies or cancer cells. For more about this kind of testing and its use in cancer treatment, see the information on the specific type of cancer.

The rest of this document focuses on predictive genetic testing for inherited mutations as they relate to cancer.