[1] UPD can be the result of heterodisomy, in which a pair of non-identical chromosomes are inherited from one parent (an earlier stage meiosis I error) or isodisomy, in which a single chromosome from one parent is duplicated (a later stage meiosis II error).
[citation needed] However, if the UPD-causing event happened during meiosis II, the genotype may include identical copies of the uniparental chromosome (isodisomy), leading to the manifestation of rare recessive disorders.
[citation needed] UPD has rarely been studied prospectively, with most reports focusing on either known conditions or incidental findings.
[9] Genome wide UPD, also called uniparental diploidy, is when all chromosomes are inherited from one parent.
[11][12] Eight years later in 1988, the first clinical case of UPD was reported and involved a girl with cystic fibrosis and short stature who carried two copies of maternal chromosome 7.