Uniparental Disomy

Abstract Uniparental disomy (UPD) is defined as the inheritance of both homologues of a given genome region from only one parent. In the majority of cases, a UPD concerns an entire chromosome; it may also be limited to part of a chromosome (segmental UPD) or may involve the entire genome (genome‐wide UPD: androgenetic or gynogenetic, if the two haplotypes derive from the father or the mother, respectively). The types of UPD are defined as ‘isodisomy’ (two identical copies of one of the parental alleles) or ‘heterodisomy’ (two distinct alleles derived from the same parent). A further issue relevant to uniparental disomies is mosaicism: certain uniparental disomies, for example, genome‐wide uniparental disomies, are viable only in the presence of normal biparental cells (mosaic state). A UPD may cause abnormal development if it leads to abnormal expression of imprinted genes or to reduction to homozygosity of recessive mutations. Key Concepts: Uniparental disomy explains pedigrees in conflict with mendelian inheritance. Uniparental disomy carries a risk for recessive diseases. Segmental uniparental disomy is an excellent model to explain fundamental genetic rules. Recombination errors around meiosis and early thereafter are very frequent as, among others, seen from (segmental) uniparental disomy cases.