Fig. 5.

Possible mechanisms of uniparental disomy (UPD) formation. Dark gray and light gray are maternal nonhomologous chromosomes, white is paternal chromosomes. The examples of UPD are shown for maternal chromosomes, however paternal UPD can occur by the same mechanism. Where mosaicism is shown as a possible outcome, there is a possibility of the presence of trisomic cells in the placenta or the fetus for ( A ), ( B ), ( C ), and ( F1) , in the case of segmental isodisomy ( F2 ), mosaicism is present for a biparental cell line. ( A , B , C ) Trisomy can result from meiosis I or II nondisjunction in a parent with a normal karyotype. Rescue can occur by complete loss of one of the parental chromosomes or formation of a residual small supernumary marker chromosome. Trisomy rescue and UPD can also occur if the egg or sperm carries a balanced translocation and nondisjunction occurs. ( C ) Oocyte carriers of a homologous Robertsonian translocation or isochromosome. ( D ) Monosomic fertilized zygote has to undergo immediate endoreduplication of monosomic chromosomes, otherwise the embryo won't survive. ( E ) Gamete complementation is a very rare event as it assumes nondisjunction of the same chromosome in both oocyte and sperm. ( F1 ) Two sequential mitotic nondisjunction events can lead to isodisomy. ( F2 ) Segmental UPD can result from homologous chromatid exchange during mitosis, resulting in segmental loss of heterozygosity and mosaicism for a UPD cell line and a biparental cell line. (This figure was adapted with modifications from Yamazawa K, Ogata T, Ferguson-Smith AC. 2010). ⁶³