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Basic genetics and patterns of inheritance
Published in Hung N. Winn, Frank A. Chervenak, Roberto Romero, Clinical Maternal-Fetal Medicine Online, 2021
One frequent cause of structural alteration is a translocation. There are two basic types of translocations. A reciprocal translocation occurs when two chromosomes break and the pieces exchange places (Fig. 10). The resulting abnormal chromosomes are called “derivative chromosomes.” If the reciprocal translocation is balanced, the individual is usually normal. Translocations such as these are found in approximately 1 in 500 normal individuals in the general population and are usually never detected. However, if a parent carries a balanced translocation and passes only one of his or her derivative chromosomes to an offspring, along with the other normal chromosomes, the child will have an unbalanced chromosome complement and will likely have multiple problems as a result (Fig. 11). Inheritance of a derivative chromosome in this situation causes a partial duplication (partial trisomy) of one chromosome and a partial deletion (partial monosomy) of another chromosome.
Molecular Approaches Towards the Isolation of Pediatric Cancer Predisposition Genes
Published in John T. Kemshead, Pediatric Tumors: Immunological and Molecular Markers, 2020
Genetic predisposition to Rb can also result from chromosome rearrangements32–34 such as translocations, inversions, and insertions. The chromosome rearrangement is usually carried in a “balanced” form by the parent who is unaffected; inheritance of the unbalanced form confers the predisposition because of a net loss of material from ql4. Figure 8 shows an apparently balanced chromosome translocation from an Rb patient. The ESD levels from red blood cells from this patient were normal suggesting that the breakpoint in this case has occurred within the coding sequence of the Rb gene or its control elements. In two other cases, the translocation has involved the relocation of 13ql4 onto the X chromosome.35,36 The apparent random inactivation of the derivative chromosome responsible for tumor development represents a functional, rather than a physical, deletion of the Rb locus.
Preimplantation Genetic Testing for Structural Rearrangements
Published in Darren K. Griffin, Gary L. Harton, Preimplantation Genetic Testing, 2020
Robertsonian translocations (RobT) result from breakage of two acrocentric chromosomes (13, 14, 15, 21, and 22) and subsequent fusion of their long arms to form one derivative chromosome (Figure 4.1b). The short arms are lost, and the total chromosome number is reduced to 45. Since short arms have only repetitive sequences coding nucleolar organizer (NOR) genes, their absence is not associated with any phenotypic consequences. The incidence of RobT is 1 in 1085 births [3]. During meiosis I, derivative and normal chromosomes pair as a “trivalent.” There are three segregation modes: alternate, adjacent, and 3:0. In alternate segregation, the derivative chromosome segregates into one pole and both normal chromosomes segregate to the other pole, producing a carrier or a normal gamete, respectively. In adjacent segregation, the derivative chromosome segregates together with one of the normal chromosomes, and the other normal chromosome segregates to the other pole producing either disomic and/or nullisomic gametes, leading to trisomic/monosomic embryos. The 3:0 segregation is very rare and produces double disomy/nullisomy in gametes and double trisomy/monosomy in embryos (Figure 4.5). Sperm karyotyping and FISH studies demonstrate that the frequency of abnormal gametes ranges from 3% to 40% (reviewed [25]). The natural tendency of the trivalent having a cis configuration [28] might have an influence on the high rate of normal/balanced gametes via increasing the probability of alternate segregation.
Abnormal chromosomes identification using chromosomal microarray
Published in Journal of Obstetrics and Gynaecology, 2022
Yunfang Shi, Xiaozhou Li, Duan Ju, Yan Li, Xiuling Zhang, Ying Zhang
Indeed, there are discordant results between karyotype analysis and CMA, which require further postnatal investigation. For case 6, karyotype indicated a structural abnormality on chromosome X, while CMA detected an 80.0 Mb deletion in Xq13.3q28 not consistent with karyotype analysis. The couples refused further examination and were lost in the follow-up. On this basis, we could not accurately diagnose the abnormality of chromosome X. In a previous study, Zhang et al. (2018) investigated a derivative X chromosome using karyotype analysis, FISH and SNP-array, which showed unbalanced derivative X chromosome with complex inversion, translocation and deletion, notably exhibiting a pericentric inversion between Xp22.3 and Xq28, as well as fragment translocation between Yq11.21 and q11.23 except the SRY gene. Different genetic testing methods have their advantages, disadvantages and applications. Therefore, it is recommended that the incorporation of multiple genetic techniques, such as karyotype, FISH, QF-PCR, MLPA or CMA, is essential for prenatal diagnosis, especially in the presence of an uncommon chromosome aberration (Liu et al. 2019). This may accurately reveal the nature, sources and manifestations of the derivative chromosome abnormalities and avoid the birth of children with defects.
CGH Array and Karyotype as Complementary Tools in Prenatal Diagnosis: Prenatal Diagnosis of a 4q Derivative Chromosome from Maternal 4q;11q Translocation
Published in Fetal and Pediatric Pathology, 2018
Cristina Gonzalez, Miriam Gutierrez Serrano, Carmen Barbancho Lopez, Taida Garcia-Riaño, Vanesa Barea Calero, Rebeca Moreno Perea, Begoña Rodriguez Mogollón, Amelia Queipo Rojas, Ana Garcia Climent, Fernando Cava Valenciano
Karyotyping permits a reliable study of all chromosomic structural alteration, such as inversion, balanced translocation, triploidies and low mosaicism that CMA cannot detect. Furthermore, karyotyping helps to characterize CMA findings like derivative chromosome or extra marker chromosomes, which are essential for helping to understand the mechanism of the alteration, and to facilitate the familiar genetic counseling for futures pregnancies. CMA is faster than karyotyping, and this technique can be the first diagnostic step until we obtain the karyotype results approximately one week later. Once we apply a sampling technique that implies a risk for abortion, it is worth performing all needed techniques to obtain the most comprehensive diagnosis. To undertake techniques simultaneously allows a more comprehensive diagnosis in a more timely manner. The detection of the familial chromosomal translocation may direct the investigation of other family members and permit to plan future pregnancies.
A retrospective exploratory study of fetal genetic invasive procedures at a University Hospital
Published in Journal of Obstetrics and Gynaecology, 2018
Chitra Andrew, Teena Koshy, Shivani Gopal, Solomon Franklin Durairaj Paul
Both the chromosome structural aberrations detected were apparently balanced translocations which were ascertained to be inherited from one of the parents. The rearrangements observed were a Robertsonian translocation between chromosome 13 and 14, a Robertsonian translocation between chromosome 15 and 21 and a translocation between the short arms of chromosome 3 and 20. The unbalanced chromosome rearrangement observed was a derivative chromosome Y which appeared to be microdeleted. The parental karyotyping in this case was normal. For the 11 (4%) foetuses with cardiac anomalies, FISH for the 22q11 microdeletion and 9% tested positive for the 22q11.2 microdeletion syndrome.