The two main genetic abnormalities that result in PWS are a chromosome 15q11—q13 deletion on the chromosome 15 of paternal origin and a chromosome 15 maternal disomy (see Chapter 1). The two other much rarer subtypes are a chromosomal re-arrangement (e.g. a translocation between chromosomes involving a break at 15q11—q13), and an imprinting centre defect on chromosome 15 (e.g. mutation or sub-microscopic deletion) that results in the equivalent of a disomy, in that both alleles of the imprinted genes have the same imprint (maternal in the case of PWS). In all four, the phenomenon of gender specific genomic imprinting is crucially important. PWS is among one of the best examples in humans where the expression of an allele of a gene depends upon whether it is inherited from the mother or father. In PWS, it is established that the gene or genes, whose absence of expression results in the syndrome, are only fully expressed when inherited from the father, but are quite normally imprinted (switched off) when inherited from the mother. It is proposed that genomic imprinting arose as a result of differential maternal/paternal evolutionary pressures. In the case of such genes, in order for normal development to occur, the non-imprinted gene has to be present and transcriptionally active since the imprinted allele from the other parent, although present, is silenced and non-functional.