Prader–Willi Syndrome and Angelman Syndrome in Cousins from a Family with a Translocation between Chromosomes 6 and 15

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Abstract
PRADER—WILLI syndrome represents the most common form of genetic obesity and is associated with mental retardation, short stature, sexual infantilism, and hypotonia1 2 3 4 5 6 ( Table 1 ). In about 60 percent of affected persons a microscopically visible interstitial deletion in chromosome 15 (band ql2) is observed,2 , 3 , 7 8 9 10 and in up to 75 percent deletions can be found at the molecular level.11 122 13 14 DNA studies with polymorphic markers have indicated that in Prader-Willi syndrome the aberrant chromosome 15 is always of paternal origin,11 , 12 suggesting that the two parental chromosomes are differently imprinte1515 and that the presence of a gene or genes on the paternally derived chromosome is necessary to prevent the syndrome. This suggestion was corroborated by the recent finding that in 20 to 30 percent of the cases, Prader—Willi syndrome results from the inheritance of both copies of chromosome 15 from the mother (uniparental disomy), with the consequent absence of a paternal chromosome 15.16