Complete UPD. In complete UPD, both members of a homologous pair come from the one parent. It usually follows restoration of diploidy in an initially trisomic conceptus [6), a phenomenon known as “trisomy rescue”. Possible mechanisms of UPD are listed in Table 1. It should be emphasized that a minimum of two events, oc­curring independently of each other, is always necessary to produce UPD, either both meiotic, or one meiotic and the other mitotic, or both mitotic. This also holds true for cases with isolated UPD in connection with rare chromo­some aberrations such as isochromosomes or Robertsonian translocations [7].

Uniparental disomy for the entire chromosome set, “uniparental diploidy”, has a devastating effect on devel­opment. If a conceptus has lost its maternal complement, and the paternal complement doubles in an attempt to compensate, no embryo at all forms, and the chorionic villi are grossly abnormal. This is a hydatidiform mole. If an ovum attempts a parthenogenetic development, a gross­ly disorganized mass of embryonic tissue results: an ovar­ian teratoma [8].

Segmental UPD. Segmental UPD is caused by so­matic recombination, when the segments exchanged are in a region subject to imprinting and this leads to functional imbalance (Fig. 1). The only example of this causing a dysmorphic syndrome known at present is the segmental paternal UPD for 11p of some Beckwith-Wiedemann syndrome (BWS), 11p being a segment that is normally maternally imprinted. In the partial UPD cell line, this segment will now be expressing bi-allelically at distal 11p. The asymmetry of body growth in this syndrome may re­flect mosaicism of two tissue types: tissue descendant from the original genotype and tissue descendant from the cell in which the segmental UPD arose [9,10].