Humans have 23 paired chromosomes, each pair containing one chromosome from the person’s mother (maternal contribution) and one from their father (paternal contribution).

In PWS the affected chromosome identified is the paternal chromosome 15. The precise area affected is a section on the long arm of the chromosome, namely q11-13. Research is continuing within this region but currently four genes - SNRPN [Glenn 2003, Reed 1994] IPW [Wevrick 1994] ZNF127 and FNZ1276 [Glenn 1997] and two expressed sequence tags-PAR1 and PAR5 [Sutcliffe 1995] have been found to be expressed only from the paternally inherited chromosome, and therefore may be involved in the pathogenesis of PWS.

The absence of the paternal copy of 15q11-13 can occur in a number of ways. 

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	Interstitial Deletion
	Maternal Uniparental Disomy
	Imprinting Centre Defect

Theories have suggested that differences in the genetic cause of PWS can lead to variations in phenotype. Rogan et al [Rogan 1998] described two PWS patients who exhibited maternal uniparental disomy of chromosome 15 and but who expressed chromosome 15 alleles which are normally silent in most PWS cases. They found these patients had a relaxation of imprinting at specific loci and therefore an expression of maternal genes which led to a milder phenotype. These patients lacked the hyperphagia common in PWS and consequently had normal or decreased body weight.