Name two imprinting disorders.

• A. Prader-Willi Syndrome and Angelman Syndrome
• B. Klinefelter Syndrome and Turner Syndrome
• C. Down Syndrome and Patau Syndrome
• D. Beckwith-Wiedemann Syndrome and Turner Syndrome

Answer: (A)

Imprinting disorders arise when genes are expressed differently depending on whether they come from the father or the mother, due to epigenetic marks that silence one copy. The classic region is on chromosome 15q11-13. When the paternal copy in this region is lost or not expressed, Prader-Willi syndrome develops; when the maternal copy is lost or not expressed, Angelman syndrome occurs. The two share the same origin in disrupted imprinting but differ in which parent’s gene expression is missing, leading to distinct clinical features (Prader-Willi with hypotonia, hyperphagia leading to obesity, short stature and hypogonadism; Angelman with severe developmental disability, ataxia, seizures, and a characteristic happy demeanor).

Other conditions listed fall into different categories. Klinefelter and Turner are sex chromosome aneuploidies; Down and Patau are autosomal trisomies. Beckwith-Wiedemann involves imprinting as well, but it’s a separate imprinting-related overgrowth syndrome, not the paired example shown here.