GeneticTreeBy Haley Fuoco

Unfortunately, most of us know someone whose life has been touched by cancer in some way. In some families, multiple people have received cancer diagnoses, which could be a sign of hereditary cancer. Sometimes, cancer diagnoses appear every other generation, leading to a misconception that cancer can skip generations. Although it may look like certain cancers only show up in every other generation, this might not be the case. In medical genetics the term reduced penetrance describes genetic disorders that seemingly skip generations.

Penetrance is the measurement of how often a certain trait occurs given a certain genetic marker or vice versa. Some disorders show complete penetrance, which means all individuals with a certain genetic marker will express that specific disorder. An example of a genetic disorder with complete penetrance is neurofibromatosis type 1, where everyone with the genetic marker for this disorder will show symptoms of this disorder. However, some disorders show reduced penetrance.

Reduced penetrance occurs when an individual has a genetic marker for a disorder but may never develop that disorder. Reduced penetrance is often seen in hereditary cancers, specifically those related to theBRCA1 and BRCA2 genes. There is no guarantee that individuals with BRCA1 or BRCA2 mutations will develop cancer, but they do have a higher chance to develop cancer compared to those without a cancer mutation. Some individuals with a mutation in their BRCA1 or BRCA2 genes may develop cancer during their lifetimes, but some will never develop cancer. This is exactly why it may seem like cancer skips generations in a family. While the underlying cause of reduced penetrance is still unknown, researchers believe that a combination of genetic, environmental, and lifestyle factors all play a role.

Reduced penetrance can also complicate the interpretation of family health history, but a genetic counselor can help. To start, collect your family’s health history as far back as possible. This might be a challenge for some families but collect what you can and share it with a healthcare provider. The Sarnoff Center website provides helpful tools to help individuals document family health history and provides information on how to asses your risk. The team, and specifically the genetic counselor at the Sarnoff Center, is available to answer any questions or connect you to more resources related to cancer genetics.