A 35-year-old man with neurofibromatosis type 1 (NF1) presents to the genetics clinic with his 8-year-old daughter, who has been diagnosed with the same NF1 mutation (confirmed by genetic testing). The father has only café-au-lait macules and axillary freckling, with no neurofibromas or optic pathway involvement. His daughter, however, already has multiple cutaneous neurofibromas, bilateral Lisch nodules, and an optic pathway glioma detected on MRI. Both carry the identical truncating NF1 mutation on chromosome 17q11.2. The phenomenon illustrated by **A** in the diagram best explains why these two individuals with the same genetic mutation show such different clinical severity and organ system involvement.