Heredity 101

We all know the phrase "it runs in the family." From an obvious family resemblance to a not-so-obvious inherited trait such as the shape of our earlobes, much of who we are physically comes from our DNA—and our DNA comes from the DNA of our two biological parents.

The Human Genome Project estimates that we have between 20,000 and 25,000 genes. The genes in our DNA are like a tool kit, used by different cells in different ways. Most cells use only a few of the many, many possible functions our genes have.

All of Your Cells Contain Your DNA* 

DNA diagram

* Except for red blood cells

Genes Can Change

All cancers are caused by mutations—harmful changes in a person’s DNA. 

Changes in a person’s DNA happen regularly, but most of these changes are not mutations and do not cause cancer.  The changes usually occur by chance.  For example, when cells divide to produce new cells, a mistake can happen that introduces a tiny change.  Fortunately, these changes are usually repaired by your body.

Sometimes, however, the change does not get repaired and this change is passed on when the cell divides to make new cells.  If the change is a mutation that interferes with a critical function, such as regulating cell growth or DNA repair, the mutation can make the cells more cancer-like. If enough mutations accumulate in a cell, it can lead to cancer.

Mutations Sometimes Lead to Cancer

Gene mutation diagram

Shown above, a series of random mutations creates cancer cells. Mutations can also be inherited—part of your genetic makeup from birth.

For cancer, two kinds of gene mutations are important:

  • Random—the mistakes that occur in DNA by chance and get repeated. These mistakes are more likely to happen as you get older. Being exposed to radiation or certain chemicals can also make genetic changes more likely
  • Inherited—the genetic changes that a person is born with, passed on by the person's father or mother or both

Most cancers develop from random mutations, but up to 10% come from genetic changes passed on by a parent. Certain inherited genetic mutations can make it much more likely that a person carrying the mutation will get cancer—and get cancer earlier.

But, please note, sometimes a family will appear to have a hereditary cancer when what they really share is a common living or working environment that exposes them to radiation or chemicals that can cause mutations. Your doctor can help you learn more.

If you do carry an inherited mutation that has been linked to cancer, knowing your risk can help you and your doctor make better, more informed decisions about your health, possibly before cancer has a chance to develop.

“All cancer is genetic, in that it is triggered by altered genes. Genes that control the orderly replication of cells become damaged, allowing the cells to reproduce without restraint” — National Cancer Institute, National Institutes of Health

Family History

When talking about your genetic heritage, the only family history that matters is the history of relatives with whom you have a direct biological connection. For example, if your father has a brother, he is your uncle—a biological second-degree relative. If he marries a woman, she becomes your aunt, but is not a biological relative. If she has a child, however, that child is your first cousin—a biological third-degree relative.

Which Relatives Are Important When Thinking About Hereditary Risk?

First-degree Relatives Second-degree Relatives> Third-degree Relatives
Biological mother, biological father, full biological sibling, or biological child (stepparents, stepchildren, or half siblings are not first-degree relatives.) Biological grandparents, biological aunts and uncles, biological nieces and nephews, half siblings, or biological grandchildren Biological first cousins, great-grandparents, great-aunts and great-uncles


People with a family health history of a particular disease can take action to reduce their risk of developing the disease. To learn more about your biological inheritance, you'll want to talk to your relatives, gather information, and fill out a pedigree—a family tree. If you find that certain illnesses, such as breast or ovarian cancer, have appeared in more than one relative, you'll be ready to talk with your doctor and investigate further.

To help assess whether you might be a candidate for testing, you can take the Hereditary Cancer Quiz.  This quiz can help you get the information you need to discuss your risk of cancer with your healthcare provider and ask for further evaluation. If you take the quiz and find red flags in your own history or your family history, you may benefit from hereditary cancer testing.

For more information about hereditary cancer testing and the hereditary cancer testing process, please visit

How a Hereditary Condition Can Be Passed on

Condition Affecting Members of a Family
How gene mutations spread in a family

How Risk Can Vary by Population

Just as an increased risk of cancer can run in families, some inherited conditions are more likely to occur among particular population groups. People in these groups may have inherited a certain genetic makeup from the ancestors they share. If the genes that are common to a group include a genetic mutation that can cause a specific disease, that disease can occur more frequently in the group. Hereditary breast and ovarian cancer (HBOC) syndrome, for example, is more common in people of Ashkenazi, or Eastern European Jewish, ancestry.

 For more information about hereditary cancer testing and the hereditary cancer testing process, please visit