A 4-year-old boy presents with mild developmental delay and some dysmorphic features including a single palmar crease and mild cardiac murmur. His phenotype is notably less severe than typical Down syndrome. Karyotype analysis reveals a mixture of two cell lines: some cells with 46,XY and others with 47,XY,+21. The condition marked **D** in the diagram represents this clinical presentation. Which of the following best explains why this patient's phenotype is milder than classic trisomy 21?