Complex sentence: Willi syndrome (PWS) is a genetic disorder in which some of seven genes on chromosome 15 (q 11--13) are deleted or unexpressed (chromosome 15q partial deletion) on the paternal chromosome.
Split into simpler sentences: Willi syndrome (PWS) is a genetic disorder due to a loss of function of some genes on chromosome 15. It is due to some of seven genes on chromosome 15 (q 11--13) being deleted or unexpressed (chromosome 15q partial deletion) on the paternal chromosome.