All types of cancer involve genes. Cancer begins when cells grow uncontrollably. This growth is controlled or not controlled by instructions from the genes. When genes are damaged, or mutated, these instructions may be incorrect or incomplete and lead to tumor formation.

All aspects of the genetic risks of cancer begin in the biology of the body. All cells have chromosomes, the structures that contain genetic information that enable cells to function and reproduce. In humans, most cells contain 46 chromosomes in 23 pairs: 22 pairs of autosomal chromosomes and one pair of sex chromosomes. In these chromosome pairs, one chromosome comes from the mother and one from the father. The autosomes have similar information regardless of the parent and produce the coding for eye or hair color, height and all other aspects of human development.

Egg and sperm cells contain only 23 chromosomes: 22 autosomes and 1 sex chromosome, either X or Y. Each person receives an X chromosome from the mother's egg. A father's sperm may contribute an X or a Y chromosome. When the father contributes an X chromosome, the child has two X chromosomes and is a girl. If the father contributes a Y chromosome, the child has one X and one Y chromosome and is a boy.

Each chromosome contains DNA, the complex substance that contains genetic information and provides the instructions for all processes in the cells. An area of DNA that contains instructions for specific proteins is called a gene, the basic unit of heredity. The entire collection of genes in humans is called the human genome. Researchers estimate that there are 20,000 to 25,000 genes in the human genome. Genes have specific locations on chromosomes. For example, researchers estimate that chromosome 13 contains between 300 and 700 genes. In one section of chromosome 13, missing areas, or deletions, in a gene called RB1 may be responsible for some cases of retinoblastoma, an eye cancer that usually affects children.

Like chromosomes, genes also occur in pairs. Genes may be dominant or recessive. If a gene is dominant, one mutated copy of the gene can affect its expression. It need only be inherited from one parent. Many cancer-predisposing mutations involve this type. When a gene is recessive, a person needs both copies of the mutated gene (one from each parent) to express the condition. The exception involves the X chromosome. A male carrying mutation that affects the X chromosome will have the mutation condition because males only have one X chromosome. A female carrying only one X mutation will be unaffected but will be a carrier, a person who has one recessive gene for a disease.

The body has two main types of genes that influence cancer risk:

• Oncogenes are acquired or hereditary mutations that cause normal cells to grow out of control. In normal function, these genes direct cell growth. When their DNA is damaged or mutated, they stay “turned on” and tell a cell to continue to grow without stopping. In such situations, the oncogene may promote the growth of a tumor.  

• Tumor suppressor genes are normal, helpful genes that control cell division, repair damaged DNA and destroy cells that cannot be repaired. Acquired or hereditary mutations can inactivate tumor suppressor genes, which means they do not stop cell growth and a tumor may develop. Inherited abnormalities of tumor suppressor genes cause many of the hereditary cancers.

Researchers have identified more than 100 oncogenes and about 30 tumor suppressor genes and continue their attempts to discover more of these genes. Scientists are only beginning to understand the complexity of the genetic aspects of cancer. For example, one of the identified tumor suppressor genes is known as p53, In normal function, the p53 protein binds with DNA and stimulates another gene to produce a different protein (called p21), which stops cell division. If p53 is damaged, all the remaining steps in the process cannot occur and p21 is not produced to stop cell division. The cells grow uncontrollably. The p53 gene mutation has been associated with several cancers. including bladder cancer and breast cancer. When one entire copy of the p53 gene is not functioning, the person is predisposed to several cancers, a condition known as Li-Fraumeni syndrome.

Most people may carry 5 to 10 gene mutations in their cells, according to the American Academy of Family Physicians. These mutations may cause no problems. Diseases such as cancer may occur if a person has a dominant disease gene, two recessive genes for the disease, or mutations that interact with each other or the environment.

Even in cancers that can be linked to an environmental conditions, such as exposure to tobacco smoke or radiation, the environment damages the tissue and genetic material to initiate the tumor growth. The genetic risk associated with cancers lies in the inheritance or mutation of genes that make a person more likely to develop a certain type of cancer. In this sense, all cancers are genetic. Some types of cancer can occur in families with above-average frequency. However, this does not mean a hereditary mutation is involved. Genetic mutations can harm, have no effect on or even help a person’s health. The cells usually but not always recognize and repair acquired mutations.

Recently, researchers have completed a “map” of the genes associated with breast and colon cancer tumors. After analyzing more than 13,000 genes from tumors, the study showed that each type of cancer had about 100 different genes with mutations. Continuing research to identify which mutations affect cancer may provide information about effective treatment and new types of therapies to repair the gene damage or stop the progress of the cancer.