Final answer:
A genetic counselor uses a pedigree to determine the likelihood of BRCA gene mutations being present in Rebecca's family, which are linked to an increased risk of breast and ovarian cancers and more prevalent among Ashkenazi Jews. Rebecca's mother not having cancer despite her lineage suggests variable inheritance of these dominant mutations.
Step-by-step explanation:
To determine which genetic marker is linked to hereditary breast cancer in Rebecca's family, a genetic counselor creates a pedigree using her family's medical history. The pedigree charts which relatives on her mother's side developed breast and ovarian cancers, and their ages at diagnosis. Given that these relatives include her grandmother, aunt, and uncle, and considering her Ashkenazi Jewish heritage, the counselor posits that mutations in the BRCA1 and BRCA2 genes might be prevalent in the family. These genes, found on chromosomes 17 and 13, increase the risk of cancer when mutated and are more common among Ashkenazi Jews. Rebecca's mother not having these mutations may be due to the fact that they do not have a 100% inheritance rate, and not all carriers will develop cancer.
Rebecca's ethnicity is significant because BRCA gene mutations are known to be more prevalent in certain ethnic groups, such as Ashkenazi Jews. The increased frequency of these mutations in certain populations can be due to a variety of factors, including founder effects and genetic drift. Understanding this prevalence is important for assessing risk and informing medical decisions, such as more frequent and earlier screenings or preventative surgeries.