Pedigree — Uniparental Disomy Producing Prader-Willi Syndrome (Maternal UPD15)

A 3-year-old boy is referred to the pediatric endocrinology clinic with a history of severe neonatal hypotonia, poor feeding in infancy, and now presents with rapid weight gain, hyperphagia, short stature, and cryptorchidism. His parents are phenotypically normal with no family history of similar disease. Chromosome analysis shows a normal 46,XY karyotype. The pedigree pattern shown in the diagram is marked as **A**, representing the inheritance mechanism in this case. Methylation analysis of the SNRPN locus shows only the maternal (methylated) pattern with no paternal (unmethylated) signal. Which of the following best describes the genetic mechanism illustrated by **A** in this pedigree?