Pedigree — Founder Effect: Ashkenazi Jewish BRCA1 185delAG Hereditary Breast-Ovarian Cancer

A 38-year-old Ashkenazi Jewish woman presents with a history of bilateral breast cancer diagnosed at ages 32 and 35 years, both triple-negative histology. Her mother had ovarian cancer at age 41, and her maternal grandmother had breast cancer at age 48. Genetic testing confirms a pathogenic BRCA1 mutation. The pedigree pattern shown in the diagram, marked **A**, illustrates which population-genetics mechanism that explains the high frequency of this specific mutation in her ancestry?