Final answer:
A family shares a genetic risk of developing a specific disease if the disease is caused by germ-line mutations which are heritable and have occurred in an ancestor.
Step-by-step explanation:
A family can share a genetic risk of developing a specific disease if the disease is caused by germ-line mutations.
Germline mutations are those that occur in the reproductive cells (gametes) and are heritable, which means they can be passed down to offspring. Diseases like Tay-Sach's disease, cystic fibrosis, hemophilia, sickle-cell anemia, and increased risks for certain cancers (like those associated with mutations of the BRCA2 gene) are examples of disorders that can result from germline mutations. If a germ-line mutation in one of the genes implicated in the disease occurred in an ancestor, the mutation can be inherited by descendants and cause the disease or increase the risk of developing the disease in the family.
Germ-line mutations, which occur in the cells that give rise to eggs or sperm, can be inherited and passed on to offspring. This means that if a germ-line mutation occurs in one of the genes implicated in a specific disease, the family members can share a genetic risk of developing that disease. For example, if a germ-line mutation in a gene related to breast cancer occurred in an ancestor, it can increase the likelihood of other family members having the disease.
On the other hand, somatic cell mutations occur in other cells of the body, such as skin cells, and are not passed on to offspring. These mutations may lead to problems like cancer if many of them accumulate in a somatic cell.