Genetic Testing: What You Need to Know

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Genes, mutations, and cancer risk

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.

Genes are pieces of DNA (deoxyribonucleic acid) inside each of our cells that instruct them how to make the proteins the body needs to function. DNA is the genetic “blueprint” found in each cell. Genes affect inherited traits 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 long strands of DNA called chromosomes. One of each chromosome pair comes from the mother, and 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.

Changes to these genes, called mutations, play an important role in the development of cancer. Mutations can cause a cell to make (or not make) proteins that affect how it grows and divides into new cells. Certain 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. To learn more about different types of cancer-related genes, please see our document called Genes and Cancer.

Some gene changes that lead to cancer may be inherited from a parent, but most are not. Only about 5% to 10% of all cancers are thought to be related to an inherited gene change that strongly affects a person’s risk for a certain type of cancer.

Most cancers start because of gene mutations that happen sometime during a person’s lifetime. Sometimes these gene changes have an outside cause, such as exposure to sunlight or tobacco. But gene changes can also just be random events that sometimes happen inside a cell, without an obvious cause.

These types of acquired (as opposed to inherited) mutations only affect the cells that grow from the mutated cell. They do not affect all the cells in the person’s body. This means all the cancer cells will have the mutations, but the normal, non-cancerous cells of the body will not have them. This is very different from inherited mutations, which are in every cell in the body – even the cells without cancer.


Last Medical Review: 10/18/2013
Last Revised: 10/18/2013