Prader-Willi is caused by the deletion of a gene on chromosome 15. The majority of patients have a deletion of the father's DNA in this region. The remaining patients frequently have two copies of the mother's chromosome 15. The maternal copy of this gene is turned off in all people. When there is a deletion of the father's DNA (approximately 70% of patients), the disease occurs. This is because the patient is left with only the inactive, maternal copy. Signs of Prader-Willi may be seen at birth. New infants with the condition are often small and very floppy (hypotonic). Male infants may have undescended testicles. The growing child exhibits slow mental and delayed motor development, increasing obesity, and characteristically small hands and feet. Rapid weight gain may occur during the first few years because the patient develops uncontrollable hunger which leads to morbid obesity.
Mental development is slow, and the IQ seldom exceeds 80. However, children with Prader-Willi generally are very happy, smile frequently, and are pleasant to be around.
Affected children have an intense craving for food and will do almost anything to get it. This results in uncontrollable weight gain. Morbid obesity (the degree of obesity that seriously affects health) may lead to respiratory failure with hypoxia (low blood oxygen levels), cor pulmonale (right-sided heart failure), and death.
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