Individuals with Beckwith-Wiedemann syndrome (BWS) develop sporadic tumors due to a mutation in an imprintable gene, p57, on chromosome 11. Normally, p57 regulates the cell cycle. The imprint on p57 is such that only the maternal allele is expressed. A heterozygous normal couple has two children with BWS and one normal child. How is it possible that both parents have the mutant p57 allele to pass along, but these parents do not themselves show the cancerous phenotype