Genomic imprinting (Prader-Willi vs Angelman)
What it means
Same locus (15q11–13), opposite parent-of-origin. Prader-Willi (loss of paternal contribution): neonatal hypotonia/poor feeding → later hyperphagia, obesity, hypogonadism, short stature, mild learning difficulty. Angelman (loss of maternal contribution): severe learning difficulty, ataxic 'puppet' gait, seizures, inappropriate laughter, minimal speech.
Worked example
Hypotonic neonate who later becomes hyperphagic and obese → Prader-Willi (paternal 15q deletion / maternal uniparental disomy).
In the exam
A floppy neonate with poor feeding later develops insatiable hyperphagia, obesity and hypogonadism; the defect is loss of the paternally-expressed 15q11–13 region.
What settles it
Prader-Willi = lost PATERNAL gene (hyperphagia/obesity); Angelman = lost MATERNAL gene (seizures, ataxia, laughter).
Classically confused with
Autosomal recessive
Source: StatPearls