Sotos Syndrome (NSD1)

A 3-year-old boy born to unaffected parents presents with overgrowth, macrocephaly with frontal bossing, sparse frontoparietal hair, downslanting palpebral fissures, and mild learning difficulties. Genetic testing confirms a heterozygous NSD1 mutation. The inheritance pattern shown in the pedigree is marked as **A** — autosomal dominant with de novo mutation, unaffected parents, and no consanguinity. Which of the following best explains the recurrence risk for the parents' future children?