9-angelman's syndrome

- according to traditional mendelian principles , the parental origin of a mutant gene is irrelevant for the expression of the phenotype

- exceptions to this rule are the PRADER WILLI SYNDROME AND THE ANGELMAN SYNDROME.

-deletions in the prader willi syndrome occur exclusively in the paternal chromosome 15

- deletions at the same site in the maternal chromosome 15 causes the angelman syndrome ( mental retardation , seizures , ataxia and hypotonia )

- features of the prader willi syndrome are obesity, diminished fetal activity , hypotonia , mental retardation , short stature and hypogonadotropic hypogonadism.

- THESE SYNDROMES MAY ALSO RESULT FROM UNIPARENTAL DISOMY . IN THIS CASE THE SYNDROMES ARE NOT CAUSED BY DELETIONS ON CHROMOSOME 15 BUT BY THE INHERITENCE OF EITHER TWO MATERNAL CHROMOSOMES ( ANGELMAN SYNDROME ) OR TWO PATERNAL CHROMOSOMES ( PRADER WILLI SYNDROME ).