Investigation of two cases of paternal disomy 13 suggests timing of isochromosome formation and mechanisms leading to uniparental disomy

Genomic study via chromosomal microarray analysis in a group of Romanian patients with obesity and developmental disability/intellectual disability

Background Obesity with developmental disability/intellectual disability (DD/ID) is the most common association in syndromic obesity. Genomic analysis studies have allowed the decipherment of disease aetiology, both in cases of syndromic obesity as well as in cases of isolated or syndromic DD/ID. However, more data are needed to further elucidate the link between the two. The aim of this pangenomic study was to use single nucleotide polymorphism (SNP) array technology to determine the copy number variant (CNV) type and frequency associated with both obesity and DD/ID. Methods Thirty-six patients were recruited from the Clinical Emergency Hospital for Children, in Cluj-Napoca, Romania during the period 2015-2017. The main inclusion criterion was a diagnosis that included both obesity and DD/ID. Genomic analysis via SNP array technology was performed. Results Out of the 36 patients, 12 (33%) presented CNVs with a higher degree of pathogenicity (A group) and 24 (66%) presented benign CNVs (B group). The SNP array results for the A group were as follows: pathogenic CNVs in 8/12 patients (67%); variants of unknown significance (VOUS) in 2/12 patients (16%); and uniparental disomy (UPD) in 2/12 patients (16%). Conclusions Some of these CNVs have already been observed in patients with both obesity and DD/ID, but the others were noticed only in DD/ID patients and have not been described until now in association with obesity.

Chromosomal instability in mammalian pre-implantation embryos: potential causes, detection methods, and clinical consequences

Formation of a totipotent blastocyst capable of implantation is one of the first major milestones in early mammalian embryogenesis, but less than half of in vitro fertilized embryos from most mammals will progress to this stage of development. Whole chromosomal abnormalities, or aneuploidy, are key determinants of whether human embryos will arrest or reach the blastocyst stage. Depending on the type of chromosomal abnormality, however, certain embryos still form blastocysts and may be morphologically indistinguishable from chromosomally normal embryos. Despite the implementation of pre-implantation genetic screening and other advanced in vitro fertilization (IVF) techniques, the identification of aneuploid embryos remains complicated by high rates of mosaicism, atypical cell division, cellular fragmentation, sub-chromosomal instability, and micro-/multi-nucleation. Moreover, several of these processes occur in vivo following natural human conception, suggesting that they are not simply a consequence of culture conditions. Recent technological achievements in genetic, epigenetic, chromosomal, and non-invasive imaging have provided additional embryo assessment approaches, particularly at the single-cell level, and clinical trials investigating their efficacy are continuing to emerge. In this review, we summarize the potential mechanisms by which aneuploidy may arise, the various detection methods, and the technical advances (such as time-lapse imaging, "-omic" profiling, and next-generation sequencing) that have assisted in obtaining this data. We also discuss the possibility of aneuploidy resolution in embryos via various corrective mechanisms, including multi-polar divisions, fragment resorption, endoreduplication, and blastomere exclusion, and conclude by examining the potential implications of these findings for IVF success and human fecundity.

Genome-wide uniparental disomy screen in human discarded morphologically abnormal embryos

Uniparental disomy (UPD) has been shown to be rare in human normal blastocysts, but its frequency in discarded morphologically abnormal embryos and its relevance to embryonic self-correction of aneuploid remains unknown. The aim of this study was to detect UPD in discarded morphologically abnormal embryos. Both discarded morphologically abnormal embryos, including zero-pronuclear zygotes (0PN), one-pronuclear zygotes (1PN), three-pronuclear zygotes (3PN) and 2PN embryos scored as low development potential were cultured into blastocysts then underwent trophectoderm biopsy. Genome-wide UPD screening of the trophectoderm of 241 discarded morphologically abnormal embryo sourced blastocysts showed that UPD occurred in nine embryos. Five embryos exhibited UPDs with euploid chromosomes, and four displayed UPDs with chromosomal aneuploid. The percentage of UPDs among the morphologically abnormal sourced blastocysts was 3.73%, which is significant higher than the percentage observed in normal blastocysts. The frequency of UPD in 3PN-sourced blastocysts was 7.69%, which is significantly higher than that in normal blastocysts. This study provides the first systematic genome-wide profile of UPD in discarded morphologically abnormal embryos. Our results indicated that UPD may be a common phenomenon in discarded morphologically abnormal embryos and may be relevant to human embryonic self-correction.

Recessive congenital methemoglobinemia caused by a rare mechanism: maternal uniparental heterodisomy with segmental isodisomy of a chromosome 22

Recessive congenital methemoglobinemia (RCM) is a very rare disorder caused by NADH-cytochrome b5 reductase (cb5r) deficiency. Two distinct clinical forms, types I and II, caused by cb5r deficiency have been recognized. In type I, the enzyme deficiency is restricted only to erythrocytes with cyanosis being the only major symptom. In contrast, in type II, the enzyme deficiency is generalized to all tissues and associated with neurological impairment, mental and growth retardation and reduced life expectancy, in addition to cyanosis. Recently, we conducted a study on an 11-year-old boy with cb5r deficiency type I. The mutational analysis of the CYB5R3 gene revealed that the boy is homozygous for L72P mutation. Surprisingly, his mother is heterozygous for this L72P mutant, but not his father. Thirteen microsatellite markers of chromosome 22 were selected to analyze the origins of the patient's chromosome 22. The result showed that both of the chromosome 22(s) of this patient came from the maternal side (uniparental heterodisomy of chromosome 22 with segmental isodisomy). This is the first case report of a patient with cb5r deficiency type I resulting from uniparental disomy and also discloses an alternate mechanism whereby this enzymatic disorder can be derived from a single parent.

Monozygotic twins discordant for trisomy 13

Monozygotic twins with discordant karyotypes are rare. We report a case of monozygotic twins discordant for trisomy 13 by amniocyte karyotypes. Ultrasound revealed multiple congenital anomalies in Twin A (47,XY,+13), none in Twin B (46,XY), and monochorionic-diamniotic placentation. Zygosity testing performed both prenatally and after birth supported monozygosity. Twin A died in the neontal period. Twin B survived and had normal physical examination, but peripheral blood karyotype revealed 20% mosaicism for trisomy 13. Monochorionic-diamniontic placentation with vascular anastomoses was confirmed by pathological examination. In this paper, we discuss the various mechanisms by which monozygotic twins may have discordant karyotypes. The surviving twin, structurally and developmentally normal at 6 months of age, will be monitored for potential complications of uniparental disomy of chromosome 13 and trisomy 13 mosaicism.

Isochromosome 13 in a patient with childhood-onset schizophrenia, ADHD, and motor tic disorder

BACKGROUND

A small percentage of all cases of schizophrenia have a childhood onset. The impact on the individual and family can be devastating. We report the results of genetic analyses from a patient with onset of visual hallucinations at 5 years, and a subsequent diagnosis at 9 years of schizophrenia, attention deficit hyperactivity disorder (ADHD) with hyperactivity and impulsivity, and chronic motor tic disorder.

RESULTS

Karyotypic analysis found 45,XX,i(13)(q10) in all cells examined. Alpha satellite FISH of isochromosome 13 revealed a large unsplit centromeric region, interpreted as two centromeres separated by minimal or undetectable short-arm material or as a single monocentric centromere, indicating that the isochromosome likely formed post-zygotically by a short arm U-type or centromeric exchange. Characterization of chromosome 13 simple tandem repeats and Affymetrix whole-genome 6.0 SNP array hybridization found homozygosity for all markers, and the presence of only a single paternal allele in informative markers, consistent with an isodisomic isochromosome of paternal origin. Analysis of two chromosome 13 schizophrenia candidate genes, D-amino acid oxidase activator (DAOA) and 5-hydroxytryptamine (serotonin) receptor 2A (5-HTR2A), failed to identify non-synonymous coding mutations but did identify homozygous risk polymorphisms.

CONCLUSIONS

We report a female patient with childhood-onset schizophrenia, ADHD, and motor tic disorder associated with an isodisomic isochromosome 13 of paternal origin and a 45,XX,i(13)(q10q10) karyotype. We examined two potential mechanisms to explain chromosome 13 involvement in the patient's pathology, including reduction to homozygosity of a paternal mutation and reduction to homozygosity of a paternal copy number variation, but were unable to identify any overtly pathogenic abnormality. Future studies may consider whether epigenetic mechanisms resulting from uniparental disomy (UPD) and the lack of chromosome 13 maternal alleles lead to the patient's features.

The consequences of uniparental disomy and copy number neutral loss-of-heterozygosity during human development and cancer

UPD (uniparental disomy) describes the inheritance of a pair of chromosomes from only one parent. Mechanisms that lead to UPD include trisomy rescue, gamete complementation, monosomy rescue and somatic recombination. Most of these mechanisms can involve aberrant chromosomes, particularly isochromosomes and Robertsonian translocations. In the last decade, the number of UPD cases reported in the literature has increased exponentially. This is partly due to the advances in genomic technologies that have allowed for high-resolution SNP (single nucleotide polymorphism) studies, which have complemented traditional methods relying on polymorphic microsatellite markers. In this review, we discuss aberrant cellular mechanisms leading to UPD and their impact on gene expression. Special emphasis is placed on the unmasking of mutant recessive alleles and the disruption of imprinted gene dosage, which give rise to specific and recurrent imprinting phenotypes. Finally, we discuss how copy number maps determined from SNP array datasets have helped identify not only deletions and duplications but also recurrent copy number neutral regions of loss-of-heterozygosity, which have been reported in many cancer types and that may constitute an important driving force in cancer. These tiny regions of UPD also alter imprinted gene dosage, which may have cumulative tumourgenic effects in addition to that of unmasking homozygous cancer-associated mutations.

Nonmosaic balanced homologous translocations of major clinical significance: some may be mosaic

The main mechanism proposed for formation of homologous translocations/isochromosomes is mitotic and if this occurs in a chromosomally normal conceptus, mosaicism would be expected to be seen. The lack of mosaicism in published cases of rearrangement (REA) of mitotic origin might be explained by under-detection due to the low level mosaicism for a normal line. Recently it was reported that sex-specific centromere instability in early embryogenesis leads to a female prevalence among individuals with mosaicism for pericentromeric rearrangements. To determine whether carriers of apparent non-mosaic homologous REA could be mosaics for a normal cell line, the sex ratio (male to female ratio) among carriers of balanced and unbalanced homologous translocations/isochromosomes was studied. This ratio was determined to establish if there is a female predominance similar to that seen in carriers of REA with mosaicism. In reviewing the literature, a female prevalence among fetuses with balanced homologous REA and among carriers of unbalanced homologous REA detected prenatally, postnatally and in miscarriages was found. Overall, there were 48 males and 72 females in the collected sample, and this ratio differed significantly from the expected sex ratio of 1.06 (P = 0.0075). There is not a male prevalence among miscarried fetuses, there is no evidence of selection against males in the collected material of this study. The analysis of sex ratios in different variants of trisomy 13 with respect to ascertainment (prenatal diagnosis, miscarriages, liveborn) also does not support an intrauterine selection against males as a cause of a female prevalence among carriers of homologous REA. Thus the data presented in this paper suggests that a proportion of the carriers of balanced homologous REA may have mosaicism for a normal line. Since low level mosaicism for a normal line in a translocation carrier would alter his/her reproductive options, it can be recommended that molecular polymorphic analysis be applied to these cases. This would allow those resulting from meiotic formation to be distinguished from those resulting from postzygotic formation. This latter mechanism may indicate the presence of a mosaicism for a normal line, making further intensive karyotypic analysis advisable. However, additional studies of healthy carriers of homologous REA of chromosome 14 or 15 should not be done. (c) 2007 Wiley-Liss, Inc.

Identification of a novel chromosome region, 13q21.33-q22.2, for susceptibility genes in familial chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most prevalent form of leukemia in adults in western countries. A genome scan of CLL-prone families revealed a lod score of one in band 13q22.1. To investigate this finding, we selected 6 CLL families consisting of 63 individuals (CLL affected, n = 19; unaffected, n = 44) for fine mapping of a 23-megabase region in 13q14.2-q22.2. Interphase fluorescence in situ hybridization (FISH) revealed 13q14 deletion in 85% (11/13) of CLL patients. Four CLL families shared a 3.68-Mb minimal region in 13q21.33-q22.2. Two asymptomatic siblings who shared the 13q21.33-q22.2 at-risk haplotype exhibited CD5+ monoclonal B-cell lymphocytosis (MBL) on flow cytometry. One of these individuals also had a 13q14 deletion by FISH. These 2 individuals with MBL shared the at-risk haplotype with their CLL-affected relatives, providing further evidence of the relationship between CLL and MBL, as well as of the biologic significance of this novel region. Using direct DNA sequencing analysis, we screened 13 genes for mutations, but no frameshift or nonsense mutations were detected. Our studies revealed that 11 of the 13 genes in the candidate region were expressed in immune tissues, supporting their functional relevance in investigations of familial CLL. In conclusion, we identified a novel candidate region that may predispose to familial CLL.

Paternal uniparental isodisomy of the entire chromosome 3 revealed in a person with no apparent phenotypic disorders

Uniparental disomy (UPD) is a rare genetic abnormality. During a whole genome linkage study we identified a case of paternal uniparental isodisomy 3 serendipitously. This is the first ascertained human paternal UPD for chromosome 3 (UPD3pat). The finding of this paternal UPD case of the entire chromosome 3 with no apparent phenotypic disorders suggests that there are no paternal imprinted genes causing rare genetic disorders on chromosome 3.

Prenatal search for UPD 14 and UPD 15 in 83 cases of familial and de novo heterologous Robertsonian translocations

OBJECTIVES

The presence in the conceptus of a Robertsonian translocation predisposes to UPD formation, mainly by post-zygotic events of chromosome abnormality rescue. This is due to the increased risk of generating aneuploid zygotes because the rearranged chromosome and the respective homologues are prone to non-disjunction errors. Given this, carriers and karyotypically normal individuals conceived from a parent with a Robertsonian translocation are at risk for UPD. Abnormal phenotypes due to an imprinting effect have been found to be associated with UPD 14 and 15. The aim of the study was to refine, at the time of prenatal diagnosis, the risk for UPD 14 and 15 in a population with Robertsonian translocations involving these chromosomes.

METHODS

Sixty-five cases of familial and de novo heterologous Robertsonian translocations involving chromosomes 14 and 15 and 18 fetuses with a normal karyotype, but conceived by a Robertsonian translocation carrier were prenatally studied to investigate the presence of UPD for chromosomes 14 and 15.

RESULTS

Of the 65 Robertsonian translocation carriers, one fetus with a de novo der(14;21) showed maternal UPD 14. None of the 18 fetuses with a normal karyotype had UPD.

CONCLUSION

Our data, combined with other previous prenatal investigations provide a general risk estimate for UPD 14 and 15 of 0.6%. Nevertheless, combining our data and those previously reported, all three fetuses with UPD had a de novo Robertsonian translocation, thus suggesting a risk of UPD formation of about 3% for this specific group of translocation carriers.

Uniparental disomy (UPD) other than 15: Phenotypes and bibliography updated

Uniparental disomy (UPD) describes the inheritance of a pair of chromosomes from only one parent. The concept was introduced in Medical Genetics by Engel (1980); Am J Med Genet 6:137-143. Aside UPD 15, which is the most frequent one, up to now (February 2005) 197 cases with whole chromosome maternal UPD other than 15 (124 X heterodisomy, 59 X isodisomy, and 14 cases without information of the mode of UPD) and 68 cases with whole chromosome paternal UPD other than 15 (13 X heterdisomy, 53 X isodisomy, and 2 cases without information of the mode of UPD) have been reported. In this review we discuss briefly the problems associated with UPD and provide a comprehensive clinical summary with a bibliography for each UPD other than 15 as a guide for genetic counseling.

A child with Angelman syndrome and trisomy 13 findings due to associated paternal UPD 15 and segmental UPD 13

A child with Angelman syndrome, cutis aplasia, cleft palate, and congenital microform cleft lip, born to a father with a Robertsonian translocation 13;15 is described. Molecular studies using polymorphic markers on chromosomes 15 and 13 showed paternal uniparental disomy (UPD) 15 and segmental UPD 13.

Under-ascertainment of mosaic carriers of balanced homologous acrocentric translocations and isochromosomes

Acrocentric rearrangements are the most common chromosome abnormalities in humans. Carriers of homologous acrocentric rearrangements (Robertsonian translocations (ROBs) between homologous chromosomes and isochromosomes) are at very high risk of having multiple spontaneous abortions and chromosomally abnormal offspring. Parents of fetuses and children with unbalanced homologous acrocentric rearrangements are rarely found to be carriers or mosaic for the same rearrangement. Even though recurrent miscarriages may indicate a carrier parent, carriers are rarely identified. Comparison of non-chromosome 21 homologous rearrangements to rea(21q21q) culled from the literature revealed a 7-fold decrease in the number of mosaic cases among the parents of non-rea(21q21q) offspring. This under-ascertainment in parents may be due to low level mosaicism confined to the gonads, a true biological difference between chromosome 21 rearrangements and other homologous acrocentric rearrangements, or simply to the lack of rigorous clinical investigation of the parental karyotypes to uncover mosaicism. We recommend that polymorphic marker analysis be applied to apparently de novo acrocentric rearrangements to distinguish those resulting from biparental postzygotic formation from those resulting from meiotic formation; the latter of which may indicate a potential carrier parent. Parental chromosomal constitutions could then be screened in a large number of cells and in more than one tissue type to identify mosaicism. Identification of mosaicism allows for accurate genetic counseling and discussion of reproductive options. However, given that mosaicism may be restricted to the gonads, prenatal testing is likely to be desired by the family whether or not mosaicism is found.

The role of imprinted genes in fetal growth

Genomic imprinting is the phenomenon by which one of the two alleles of a subset of genes is preferentially expressed according to its parental origin. This pattern of inheritance is different from the more frequent mode of Mendelian inheritance, which is not influenced by the parental origin of the allele. The idea that imprinted genes can affect fetal growth is becoming increasingly intriguing as it has been shown that most imprinted genes are expressed in the placenta and some play a role in regulating the interactions between its fetal and maternal interfaces. This article considers genomic imprinting by reviewing recent findings of alterations in fetal growth related to different types of genetic changes affecting the expression of imprinted genes. Among the genetic anomalies, the uniparental disomy (UPD) defines the inheritance of both homologous chromosomes from only one parent. UPDs of a number of chromosomes have been described in association with effects on the phenotype. We reviewed cases of UPD reported till now with particular reference to those associated to growth alterations.

Identification of uniparental disomy in phenotypically abnormal carriers of isochromosomes or Robertsonian translocations

Carriers of either homologous or non-homologous acrocentric rearrangements are at an increased risk for aneuploidy, and, thus, for uniparental disomy (UPD). Abnormal phenotypes due to genomic imprinting are associated with UPD for the acrocentric chromosomes 14 and 15. The purpose of this study was to determine the prevalence of UPD in a population with acrocentric rearrangements (either an isochromosome or a Robertsonian translocation) and abnormal phenotypes. Fifty individuals were studied. Of the 50 rearrangements, two were homologous rearrangements and both showed UPD. Forty-eight were non-homologous Robertsonian translocations, of which two showed UPD. This study demonstrates that UPD explains the abnormal phenotypes in some balanced carriers of acrocentric rearrangements. Our results and the large number of case reports in the literature suggest that patients with abnormal phenotypes and acrocentric rearrangements of chromosomes 14 or 15 should be tested for UPD.

Complex and segmental uniparental disomy (UPD): review and lessons from rare chromosomal complements

OBJECTIVE

To review all cases with segmental and/or complex uniparental disomy (UPD), to study aetiology and mechanisms of formation, and to draw conclusions.

DESIGN

Searching published reports in Medline.

RESULTS

The survey found at least nine cases with segmental UPD and a normal karyotype, 22 cases with UPD of a whole chromosome and a simple or a non-homologous Robertsonian translocation, eight cases with UPD and two isochromosomes, one of the short arm and one of the long arm of a non-acrocentric chromosome, 39 cases with UPD and an isochromosome of the long arm of two homologous acrocentric chromosomes, one case of UPD and an isochromosome 8 associated with a homozygous del(8)(p23.3pter), and 21 cases with UPD of a whole or parts of a chromosome associated with a complex karyotype. Segmental UPD is formed by somatic recombination (isodisomy) or by trisomy rescue. In the latter mechanism, a meiosis I error is associated with meiotic recombination and an additional somatic exchange between two non-uniparental chromatids. Subsequently, the chromatid that originated from the disomic gamete is lost (iso- and heterodisomy). In cases of UPD associated with one isochromosome of the short arm and one isochromosome of the long arm of a non-acrocentric chromosome and in cases of UPD associated with a true isochromosome of an acrocentric chromosome, mitotic complementation is assumed. This term describes the formation by misdivision at the centromere during an early mitosis of a monosomic zygote. In cases of UPD associated with an additional marker chromosome, either mitotic formation of the marker chromosome in a trisomic zygote or fertilisation of a gamete with a marker chromosome formed in meiosis by a disomic gamete or by a normal gamete and subsequent duplication are possible.

CONCLUSIONS

Research in the field of segmental and/or complex UPD may help to explain undiagnosed non-Mendelian disorders, to recognise hotspots for meiotic and mitotic recombinations, and to show that chromosomal segregation is more complex than previously thought. It may also be helpful to map autosomal recessively inherited genes, genes/regions of genomic imprinting, and dysmorphic phenotypes. Last but not least it would improve genetic counselling.

Prenatal diagnosis of a homologous Robertsonian translocation involving chromosome 15

We report the prenatal diagnosis of a fetus with a de novo Robertsonian translocation: 45,XY,der(15;15)(q10;q10). Although Robertsonian translocations are common chromosomal rearrangements, those involving homologous chromosomes are infrequent. Since chromosome 15 is imprinted, uniparental disomy (UPD) is a concern when chromosomal rearrangements involving chromosome 15 are identified. In the present case, UPD studies showed normal biparental inheritance. In contrast to the fact that most homologous acrocentric rearrangements are isochromosomes, these results indicate postzygotic formation of a Robertsonian translocation between biparentally inherited chromosomes 15.

Molecular mechanisms for constitutional chromosomal rearrangements in humans

Cytogenetic imbalance in the newborn is a frequent cause of mental retardation and birth defects. Although aneuploidy accounts for the majority of imbalance, structural aberrations contribute to a significant fraction of recognized chromosomal anomalies. This review describes the major classes of constitutional, structural cytogenetic abnormalities and recent studies that explore the molecular mechanisms that bring about their de novo occurrence. Genomic features flanking the sites of recombination may result in susceptibility to chromosomal rearrangement. One such substrate for recombination is low-copy region-specific repeats. The identification of genome architectural features conferring susceptibility to rearrangements has been accomplished using methods that enable investigation of regions of the genome that are too small to be visualized by traditional cytogenetics and too large to be resolved by conventional gel electrophoresis. These investigations resulted in the identification of previously unrecognized structural cytogenetic anomalies, which are associated with genetic syndromes and allowed for the molecular basis of some chromosomal rearrangements to be delineated.

Identification of uniparental disomy following prenatal detection of Robertsonian translocations and isochromosomes

Rearrangements of the acrocentric chromosomes (Robertsonian translocations and isochromosomes) are associated with an increased risk of aneuploidy. Given this, and the large number of reported cases of uniparental disomy (UPD) associated with an acrocentric rearrangement, carriers are presumed to be at risk for UPD. However, an accurate risk estimate for UPD associated with these rearrangements is lacking. A total of 174 prenatally identified acrocentric rearrangements, including both Robertsonian translocations and isochromosomes, were studied prospectively to identify UPD for the chromosomes involved in the rearrangements. The overall goal of the study was to provide an estimate of the risk of UPD associated with nonhomologous Robertsonian translocations and homologous acrocentric rearrangements. Of the 168 nonhomologous Robertsonian translocations studied, one showed UPD for chromosome 13, providing a risk estimate of 0.6%. Four of the six homologous acrocentric rearrangements showed UPD, providing a risk estimate of 66%. These cases have also allowed delineation of the mechanisms involved in producing UPD unique to Robertsonian translocations. Given the relatively high risk for UPD in prenatally identified Robertsonian translocations and isochromosomes, UPD testing should be considered, especially for cases involving the acrocentric chromosomes 14 and 15, in which UPD is associated with adverse clinical outcomes.

Mechanisms leading to uniparental disomy and their clinical consequences

Uniparental disomy (UPD) refers to the situation in which both copies of a chromosome pair have originated from one parent. In humans, it can result in clinical conditions by producing either homozygosity for recessive mutations or aberrant patterns of imprinting. Furthermore, UPD is frequently found in conjunction with mosaicism for a chromosomally abnormal cell line, which can also contribute to phenotypic abnormalities. Investigations into the mechanisms by which UPD may arise have helped to expand our general awareness of the impact of chromosomal abnormalities and chromosomal mosaicism in normal human development. Specifically, it appears that errors in the transmission of a chromosome from parent to gamete and during early somatic cell divisions are remarkably common but that embryo and cell selection during early embryogenesis help to ensure the presence of a numerically balanced chromosome complement in the developing fetus. UPD is also likely to occur within a portion of cells in all individuals simply as a consequence of somatic recombination occurring during mitotic cell divisions. This can be an important step in cancer development as well as a contributing factor to other late onset diseases. This review summarizes mechanisms by which UPD may arise and their associated clinical consequences.