Pedigree — Triplet Repeat Expansion (Fragile X Syndrome FMR1 CGG > 200)

A 5-year-old boy presents with moderate intellectual disability (IQ 52), autism spectrum features (poor eye contact, hand-flapping), a long narrow face with prominent ears, and macroorchidism. His maternal grandfather has late-onset progressive cerebellar ataxia and tremor beginning at age 58. His mother had premature menopause at age 38. The structure marked **A** in the pedigree diagram represents the molecular mechanism underlying this family's presentation. Which of the following BEST describes the genetic principle demonstrated by this inheritance pattern?