Sample Essay on Uniparental Disomy
Uniparental Disomy [UPD] is a mechanism that occurs as a result of the interference of the expression of imprinted genes. UPD occurs when a child inherits two copies of a chromosome from one parent and fails to acquire another copy of the same chromosome from the other parent. This error on the division of genes occurs during the formation of egg or sperm cells through meiosis. When the problem causing UPD occurs during meiosis, both chromosomes from a single parent are transmitted and heterodisomy occurs. When the error causing UPD occurs during meiosis II or as a postzygotic event, a single parent homology is transferred to the offspring in duplicate; isodisomy occurs.
Meiotic recombination events in the context of Uniparental Disomy often result to a mixture of heterodisomy and isodisomy. The UPD can either involve a whole chromosome or only a segment. For mosaicism of UPD to occur, there must also be combination with either chromosomally normal or abnormal cell lines. Whenever Uniparental Disomy takes place, the imbalance of the maternal set against paternal genetic formation of the involved chromosome can be linked to the clinical symptoms found in the affected child.
Notably, UPD does not invariably impart an abnormal clinical phenotype. Isodisomy can occur in disease due to a falling allele at any location, heterodisomy is not likely to occur in an abnormal clinical phenotype unless there is an involved chromosome or segment that includes imprinted genes. These imprinted genes illustrate differential expression in regard to the parent of origin. Disorders that result from UPD of imprinted genes do not occur because of the defect in the imprinting process itself, but due to unbalanced parental contribution of the normally imprinted alleles that result in changed expression of imprinted genes.
Uniparental Disomy has always been described for many but not all chromosomes. There are other clinical syndromes that are linked to UPD have been described only for a few chromosomes include Prader-Willi Syndrome [PWS] due to UPD 15, Russell-silver syndrome [RSS] UPD7, Transient neonatal disease due to UPD 6 and UPD 14 and Angelman Syndrome SA occurs due to UPD 15. Karyotype is significant in understanding the UPD formation.
Uniparental Disomy is not that easy to detect or diagnose. This is because it cannot be identified by gross cytogenetic analysis. For the identification to be possible, it requires DNA-based analysis using multiple polymorphic markers spanning the chromosomes of choice. For the analysis to be successful, specimens from both parents and the child or fetus must be obtained.
Uniparental Disomy is useful for patients with mosaicism, Robertsonia translocations or confined placental mosaicism. It also helps in evaluation of patients who present features of disorders associated with UPD like Russell-Silver Syndrome among others. The first clinical case of Uniparental Disomy was reported in 1988. The case involved a girl with cystic fibrosis and an extraordinary short stature who carried two copies of maternal chromosome 7. Out of the 47 disomies formed in 1991, 29 have been identified among individuals determined for medical purposes. The studies were based on chromosomes 2, 5-11, 13-16, 21 and 22.
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