Cancer happens when mutations build up in DNA. Most of these mutations happen in genes that control cell copying and the ability to repair DNA. A few cancers are caused by germline (inherited) mutations in these genes. Germline mutations can be passed down from parents to children. Cancers caused by germline mutations are called inherited or familial cancers.
A hereditary cancer syndrome is a tendency to develop tumors caused by germline (inherited) mutations, especially at younger ages. Most hereditary cancer syndromes are inherited in an autosomal dominant way. Autosomal changes are not related to a person’s sex. Males and females are at equal risk for inheriting the mutation. Dominant means a person only needs to inherit the mutation from 1 (one) parent to inherit the disease. Everyone in the family who inherits the mutation will have an increased chance of developing cancer. Scientists call this complete penetrance.
Sometimes a person who inherits a dominant gene does not express the trait. They still have the gene and can pass it down to their children, but they do not have the disease or other condition caused by the gene. When doctors or genetic counselors know someone has a gene, but do not see the expression of the trait, they call this incomplete penetrance.