Uniparental disomy
Uniparental disomy (UPD) occurs when a person receives two copies of a chromosome, or of part of a chromosome, from one parent and no copy from the other parent. 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). Uniparental disomy may have clinical relevance for several reasons. For example, either isodisomy or heterodisomy can disrupt parent-specific genomic imprinting, resulting in imprinting disorders . Additionally, isodisomy leads to large blocks of homozygosity, which may lead to the uncovering of recessive genes, a similar phenomenon seen in inbred children of consanguineous partners. UPD has been found to occur in about 1 in 2,000 births. Pathophysiology. UPD can occur as a random event during the formation of egg cells or sperm cells or may happen in early fetal development. It can also occur during trisomic rescue. Phenotype. Most occurrences of UPD result in no phenotypical anomalies. 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. UPD should be suspected in an individual manifesting a recessive disorder where only one parent is a carrier. Uniparental inheritance of imprinted genes can also result in phenotypical anomalies. Although few imprinted genes have been identified, uniparental inheritance of an imprinted gene can result in the loss of gene function, which can lead to delayed development, mental retardation, or other medical problems. UPD has rarely been studied prospectively, with most reports focusing on either known conditions or incidental findings. It has been proposed that the incidence may not be as low  as believed, rather it may be under-reported. All chromosomes. Genome wide UPD, also called uniparental diploidy, is when all chromosomes are inherited from one parent. Only in mosaic form can this phenomenon be compatible with life. As of 2017, there have only been 18 reported cases of genome wide UPD. History. The first clinical case of UPD was reported in 1988 and involved a girl with cystic fibrosis and short stature who carried two copies of maternal chromosome 7. Since 1991, out of the 47 possible disomies, 29 have been identified among individuals ascertained for medical reasons. This includes  chromosomes 2, 5–11, 13–16, 21 and 22. "This article incorporates public domain text from The U.S. National Library of Medicine"