The penetrance of the Fu allele of the Fused (Axin1) gene was strongly dependent on its parental
origin. For example, when maternally inherited approximately 33% of heterozygotes had a fused
skeletal phenotype compared to 81% when paternally inherited (Reed
SC, 1937). The Fu mutation of Axin is attributable to the insertion of an intracisternal A
particle (IAP) transposon between exons 6 and 7 of Axin1. IAP elements are known to be more
methylated during spermatogenesis than oogenesis (Vasicek TJ
et al, 1997).