A 2-year-old boy born to unaffected parents presents with short stature. On examination, he has a relatively large head with frontal bossing, a depressed nasal bridge, and notably short upper arms and thighs compared to his forearms and lower legs. The structure marked **A** in the diagram shows disproportionate rhizomelic shortening of the proximal limbs. Which of the following genetic mechanisms best explains this clinical presentation?