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Prader-Willi syndrome
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Prader-Willi syndrome

Prader-Willi syndrome is a complex genetic condition characterized by weak muscle tone and feeding difficulties in infancy, progressing to extreme hunger and obesity in childhood, often accompanied by intellectual impairment and behavioral problems.

Overview

Prader-Willi syndrome is a complex genetic condition that affects many parts of the body. In infancy, this condition is characterized by weak muscle tone (hypotonia), feeding difficulties, poor growth, and delayed development. Beginning in childhood, affected individuals develop an extreme hunger, which leads to chronic overeating (hyperphagia) and obesity. Some people with Prader-Willi syndrome, particularly those with obesity, also develop type 2 diabetes (the most common form of diabetes).

People with Prader-Willi syndrome typically have mild to moderate intellectual impairment and learning disabilities. Behavioral problems are common, including temper outbursts, stubbornness, and compulsive behavior such as picking at the skin. Sleep abnormalities can also occur. Additional features of this condition include distinctive facial features such as a , almond-shaped eyes, and a triangular mouth; short stature; and and . Some people with Prader-Willi syndrome have unusually fair skin and . Both affected males and affected females have underdeveloped genitals. Puberty is delayed or incomplete, and most affected individuals are unable to have children (infertile).

Frequency

Prader-Willi syndrome affects an estimated 1 in 10,000 to 30,000 people worldwide.

Causes

Prader-Willi syndrome is caused by the loss of function of genes in chromosome 15. People normally inherit one copy of this chromosome from each parent. Some genes are turned on (active) only on the copy that is inherited from a person's father (the paternal copy). This parent-specific gene activity results from a process called genomic imprinting.

Most cases of Prader-Willi syndrome (about 70 percent) occur when a segment of the paternal chromosome 15 is deleted in each cell. People with this chromosomal change are missing certain critical genes in this region because the genes on the paternal copy have been deleted, and the genes on the maternal copy are turned off (inactive). In another 25 percent of cases, a person with Prader-Willi syndrome has two copies of chromosome 15 inherited from his or her mother (maternal copies) instead of one copy from each parent. This situation is called maternal uniparental disomy. Rarely, Prader-Willi syndrome can also be caused by a chromosomal rearrangement called a translocation, or by a genetic alteration or other change that abnormally turns off (inactivates) genes on the paternal chromosome 15.

It appears likely that the characteristic features of Prader-Willi syndrome result from the loss of function of several genes on chromosome 15. Among these are genes that provide instructions for making molecules called small nucleolar RNAs (snoRNAs). These molecules have a variety of functions, including helping to regulate other types of RNA molecules. (RNA molecules play essential roles in producing proteins and in other cell activities.) Studies suggest that the loss of a particular group of snoRNA genes, known as the SNORD116 cluster, may play a major role in causing the signs and symptoms of Prader-Willi syndrome. However, it is unknown how a missing SNORD116 cluster could contribute to intellectual disability, behavioral problems, and the physical features of the disorder.

In some people with Prader-Willi syndrome, the loss of a gene called OCA2 is associated with fair skin and light-colored hair. The OCA2 gene is located on the segment of chromosome 15 that is often deleted in people with this disorder. However, loss of the OCA2 gene does not cause the other signs and symptoms of Prader-Willi syndrome. The protein produced from this gene helps determine the coloring (pigmentation) of the skin, hair, and eyes.

Researchers are studying other genes on chromosome 15 that may also be related to the major signs and symptoms of this condition.

Associated Genes and Chromosomes

  • OCA2
  • chromosome 15

Inheritance

Most cases of Prader-Willi syndrome are not inherited, particularly those caused by a deletion in the paternal chromosome 15 or by maternal uniparental disomy. These genetic changes occur as random events during the formation of reproductive cells (eggs and sperm) or in early embryonic development. Affected people typically have no history of the disorder in their family.

Rarely, a genetic change responsible for Prader-Willi syndrome can be inherited. For example, it is possible for a genetic change that abnormally turns off genes on the paternal chromosome 15 to be passed from one generation to the next.

Other Names

  • Prader-Labhart-Willi syndrome
  • PWS
  • Willi-Prader syndrome

Frequently Asked Questions

Managing hyperphagia is a lifelong challenge and requires strict environmental control. This includes locking food pantries and refrigerators, establishing consistent meal and snack schedules, and using smaller portion sizes. A low-calorie, nutrient-dense diet is essential. Regular physical activity, behavioral therapy, and close monitoring by a healthcare team are also critical to prevent obesity and associated complications like type 2 diabetes.
multidisciplinary approach is most effective. This includes special education services, speech therapy for communication challenges, and occupational therapy for adaptive skills. Behavioral therapy and consistent routines are crucial for managing common issues like temper outbursts, stubbornness, and compulsive behaviors. Support groups and counseling for families can also provide valuable strategies and emotional support.
Prader-Willi syndrome is a genetic condition, but it is usually not inherited in the typical sense. Most cases (about 70%) occur when a segment of chromosome 15 from the father is missing. Other causes include inheriting two copies of chromosome 15 from the mother (maternal uniparental disomy) or a defect in the imprinting process. In most cases, the risk of having another child with Prader-Willi syndrome is very low (less than 1%). However, if a parent has a rare chromosomal rearrangement, the risk can be higher. Genetic counseling is strongly recommended to determine the specific cause and assess recurrence risk for future pregnancies.
In infancy, the primary focus is on ensuring adequate nutrition due to weak muscle tone (hypotonia) and feeding difficulties. This often involves specialized feeding techniques, high-calorie formulas, and sometimes feeding tubes. Physical therapy is crucial to improve muscle tone, strength, and motor development, while occupational therapy can help with feeding skills and sensory integration. Early intervention services are vital for supporting overall development.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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