![]() Chimeras of the same sex would have a normal phenotype and thus are usually only discovered by chance, such as through paternity testing, blood group testing, or tissue donor testing prior to organ transplantation. Most chimeras remain undetected, and thus the actual incidence is unknown. Partial chimerism results from feto-fetal transfusion between dizygotic twins (twin chimera) or feto-maternal transplacental exchange (microchimera), while whole-body chimerism arises as the result of the union of two or more different zygotes into one body (fusion chimera) ( Sawai et al., 1994 Boklage, 2006 Madan, 2020). The latter can be categorized into two groups, partial chimerism and whole-body chimerism. Acquired chimerism occurs later in life through transfusion or transplantation, while constitutional chimerism is a condition that an individual is born with. Chimerism has rarely been reported and is far less common than mosaicism. While mosaicism results from a mitotic error in a single zygote, chimerism develops from the fusion of two or more separate zygotes. The difference between chimerism and mosaicism depends on the number of zygotes involved in the developmental process. The mechanisms underlying chimera in our case was likely fertilization two spermatozoa, one with an ovum and the other with the second polar body.Ĭhimerism and mosaicism are rare phenomena in human. The extra chromosome 21 was paternally derived and meiosis I nondisjunction likely occurred during spermatogenesis. SNP microarray and STR analyses revealed a single maternal and double paternal genetic contribution to the patient for the majority of the markers, including the chromosome 21 markers. Four years later, the percentage of trisomy 21 cells had decreased to approximately 6%. Cytogenetic analyses of the patient’s peripheral blood revealed approximately 17% of a 47,XY,+21 lineage by G-banding karyotype analysis, 13%–17% by FISH analyses of uncultured peripheral blood, and 10%–15% by SNP microarray analysis. We performed short tandem repeat (STR) and single nucleotide polymorphism (SNP) microarray analyses to attempt to identify the mechanism underlying the chimerism in this patient and the origin of the extra chromosome 21. Growth and developmental milestones were within normal ranges. For the case, a physical examination at the age of 1 year revealed ambiguous genitalia with no features of Down syndrome or other malformations. Herein we describe an additional case with a chi 47,XY,+21/46,XX karyotype. Only three non-twin cases carrying both trisomy 21 and a normal karyotype have been reported, including two cases with a chi 47,XY,+21/46,XX karyotype and a case with a chi 47,XX,+21/46,XY karyotype. Most reported cases have a chi 46,XX/46,XY karyotype. 6Integrative Computational Bioscience Center (ICBS), Mahidol University, Nakhon Pathom, ThailandĬhimerism is a very rare genetic finding in human.5Chakri Naruebodindra Medical Institute, Faculty of Medicine Ramathibodi Hospital, Mahidol University, Samut Prakan, Thailand.4Program in Translational Medicine, Faculty of Medicine Ramathibodi Hospital, Mahidol University, Bangkok, Thailand.3Department of Pediatrics, Faculty of Medicine, Prince of Songkla University, Songkhla, Thailand.2Genomic Medicine Center, Faculty of Medicine, Prince of Songkla University, Songkhla, Thailand.1Department of Pathology, Faculty of Medicine, Prince of Songkla University, Songkhla, Thailand.Chariyawan Charalsawadi 1,2, Somchit Jaruratanasirikul 3, Areerat Hnoonual 1,2, Aussanai Chantarapong 1, Pornsiri Sangmanee 1, Sasipong Trongnit 1, Natini Jinawath 4,5,6 and Pornprot Limprasert 1,2*
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