Prader-Willi syndrome


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Prader-Willi syndrome


Population genetics

Mendelian genetics and punnett squares

Hardy-Weinberg equilibrium

Inheritance patterns

Independent assortment of genes and linkage

Evolution and natural selection

Genetic disorders

Down syndrome (Trisomy 21)

Edwards syndrome (Trisomy 18)

Patau syndrome (Trisomy 13)

Fragile X syndrome

Huntington disease

Myotonic dystrophy

Friedreich ataxia

Turner syndrome

Klinefelter syndrome

Prader-Willi syndrome

Angelman syndrome

Beckwith-Wiedemann syndrome

Cri du chat syndrome

Williams syndrome

Alagille syndrome (NORD)


Polycystic kidney disease

Familial adenomatous polyposis

Familial hypercholesterolemia

Hereditary spherocytosis

Huntington disease

Li-Fraumeni syndrome

Marfan syndrome

Multiple endocrine neoplasia

Myotonic dystrophy


Treacher Collins syndrome

Tuberous sclerosis

von Hippel-Lindau disease


Polycystic kidney disease

Cystic fibrosis

Friedreich ataxia

Gaucher disease (NORD)

Glycogen storage disease type I

Glycogen storage disease type II (NORD)

Glycogen storage disease type III

Glycogen storage disease type IV

Glycogen storage disease type V


Mucopolysaccharide storage disease type 1 (Hurler syndrome) (NORD)

Krabbe disease


Niemann-Pick disease types A and B (NORD)

Niemann-Pick disease type C

Primary ciliary dyskinesia

Phenylketonuria (NORD)

Sickle cell disease (NORD)

Tay-Sachs disease (NORD)



Wilson disease

Fragile X syndrome

Alport syndrome

X-linked agammaglobulinemia

Fabry disease (NORD)

Glucose-6-phosphate dehydrogenase (G6PD) deficiency


Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)

Lesch-Nyhan syndrome

Muscular dystrophy

Ornithine transcarbamylase deficiency

Wiskott-Aldrich syndrome

Mitochondrial myopathy

Autosomal trisomies: Pathology review

Muscular dystrophies and mitochondrial myopathies: Pathology review

Miscellaneous genetic disorders: Pathology review


Prader-Willi syndrome


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USMLE® Step 1 questions

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High Yield Notes

4 pages


Prader-Willi syndrome

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USMLE® Step 1 style questions USMLE

of complete

A 9-month-old male infant is brought to the physician by his parents to evaluate poor feeding and growth. The infant had had problems with feeding since birth and did not improve when he was switched to formula feeding. The mother states that his sucking reflex is weak. The infant had surgery for undescended testes three months ago. His vaccinations are up to date. He is in the 3rd percentile for both height and weight. On physical exam, the child exhibits hypotonia, almond-shaped eyes, a narrow face, and a triangular mouth. He is subsequently admitted and has a nasogastric tube placed for feeding. Genetic testing reveals a microdeletion on chromosome 15q. This child is at risk of developing which of the following clinical manifestations?  

External References

First Aid









Prader-Willi syndrome p. 56


Prader-Willi syndrome p. 56


Prader-Willi syndrome p. 56


Prader-Willi syndrome p. 56

Prader-Willi syndrome

chromosome association p. 62

ghrelin in p. 342, 380

imprinting p. 56


Content Reviewers

Rishi Desai, MD, MPH


Tanner Marshall, MS

Prader-Willi syndrome is a genetic disorder that, in infancy, causes poor feeding and low muscle tone, and then in childhood, causes overeating, intellectual disability, and low sex hormones starting in childhood.

Prader-Willi syndrome happens when a handful of genes on chromosome 15 aren’t transcribed into messenger RNA and therefore aren’t expressed.

Among these are SNRPN which stands for Small Nuclear Ribonucleoprotein Polypeptide N and a cluster of snoRNAs, which stands for small nucleolar RNAs, and these genes all have protein products that modify other RNAs.

Now, normally, the copies of the genes contributed by the mother, or maternally derived genes, to this region, are silenced, or turned off, and only the genes from dad, or paternally-derived genes, get expressed.

This special genetic process is called imprinting, where only one copy of the gene gets expressed, not both.

And this differs from most genes in the genome, where both the maternal and paternal copies are expressed.

So those maternal copies in this region are imprinted and therefore silenced.

And this silencing of the maternal copies is an epigenetic process.

In the word “epigenetic”, “epi” means outside of, and “genetic” refers to the DNA sequences of A’s, C’s, G’s, and T’s.

So epigenetic silencing of a gene means turning it off while keeping the DNA sequence itself the same.

The Prader-Willi genes get turned off when methyl groups get attached to the DNA, a process that happens way back when the mother was making an egg.

Even after fertilization of the egg and all of the cell divisions it takes to make a person, that epigenetic mark remains, kind of like a reminder to keep those maternally-derived copies of the genes turned off.

Unfortunately, though, this means that if paternal copies of the genes don’t get expressed, then there aren’t any backup copies being expressed, and so no copies get expressed!

And this is what happens in Prader-Willi syndrome!

Now, there are a few ways these paternal genes wouldn’t be expressed.

The most common one is a deletion on the paternal genes spanning Prader-Willi region.

A lot of deletions also include a nearby gene called OCA2, which codes for a pigment that gives color to the eyes, the hair, and the skin.


Prader �Willi syndrome is a rare genetic disorder in which certain genes on chromosome 15 are deleted or unexpressed on the paternal chromosome. Symptoms include low muscle tone, short stature, incomplete sexual development, cognitive disabilities, behavior problems, and a chronic feeling of hunger that can lead to excessive eating and life-threatening obesity. Treatment involves a combination of genetic counseling, medical care, and behavior therapy.


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