Name two imprinting disorders affecting neurodevelopment and their genetic basis.

• A. Prader-Willi syndrome results from maternal deletion; Angelman from paternal deletion.
• B. Prader-Willi syndrome (paternal 15q11-q13 deletion or paternal UPD) and Angelman syndrome (maternal 15q11-q13 deletion or maternal UPD).
• C. Angelman syndrome is caused by paternal deletion.
• D. Both disorders are caused by mutations in the same gene that are always inherited from the same parent.

Answer: (B)

Genomic imprinting in the 15q11-q13 region means that gene expression depends on which parent the chromosome is inherited from. Some genes are active only from the paternal copy, others only from the maternal copy. Prader-Willi syndrome arises when the paternal copy of 15q11-q13 is missing or nonfunctional—most commonly a deletion on the paternal chromosome or paternal uniparental disomy, in which both copies come from the father and no paternal gene expression is available (the maternal copy is silenced in this region). Angelman syndrome arises when the maternal copy is missing or nonfunctional—most often a deletion on the maternal chromosome or maternal uniparental disomy, leading to loss of maternal expression of UBE3A in the brain while the paternal copy remains silenced. Therefore, the two imprinting disorders affecting neurodevelopment are Prader-Willi (paternal 15q11-q13 deletion or paternal UPD) and Angelman (maternal 15q11-q13 deletion or maternal UPD). The other statements misstate which parent contributes the active copy, or imply changes in the same gene or inheritance pattern that don’t fit imprinting biology.