Our bodies are made of many tiny building blocks called cells. Inside each of those cells there is a set of genes that control how the cells grow and work. When we start life, we inherit two sets of genes – one from our mother and one from our father. These genes contain the instructions that our bodies use to determine many things including how tall we are or the color of our eyes. Genes also contain instructions for many things inside of us that we cannot see, such as how our bones are formed or how our heart works.
A gene is a long stretch of chemical code that our body knows how to read, like letters in an instruction manual. If one or more of the letters in that instruction manual are changed, it is like a spelling mistake, and the body can’t read the instructions anymore. The technical term for this is “mutation,” meaning there is a change to the usual genetic code that stops the body from being able to use the instructions from that gene. If there is a mutation in an important gene controlling cell growth and repair, this increases the risk of developing cancer.
The genes that are involved in hereditary cancer are genes that help control how fast the cells of our body grow. Some of these genes are called “tumor suppressor genes” because they give instructions for repairing damage to our bodies, and they protect us from developing cancer. In families with hereditary cancer, one of these important tumor suppressor genes is not working properly.
Every cell of our body has two copies of each of these tumor suppressor genes, one inherited from our mother, and one from our father. In families with hereditary cancer, there is a mutation in one of these important genes that is being passed down from generation to generation. In most cases, the mutation is in just one of the two copies of a gene. Having this mutation does not directly cause cancer, because there is a second copy of the gene from the other parent that is still working properly. However, if we follow people who have one mutation through their lives, we see that they develop more cancers than the average person does because they have less protection against cancer.
An inherited mutation that increases cancer risk can come from either the mother’s or the father’s family. The pattern of inheritance for most forms of hereditary cancer is called “dominant.” This means that a mutation in just one copy of a gene increases the risk of cancer. When a parent with mutation has children, each child inherits just one copy of each gene, because they inherit a second copy from the other parent. So, each child of a parent with a mutation has a 50% chance of inheriting the gene with the mutation and a 50% chance of not inheriting the gene with the mutation. Therefore, even in families with hereditary cancer, about half of the family members did not inherit the mutation that is causing cancer, and their risk of cancer is similar to the average person.