Abstract
Two cases with normal karyotype and an additional signal in chromosome 22 with the 13/21 alpha satellite DNA probe have been reported in the literature. In both, one of the parents had the same marker, so an inherited polymorphism was considered. Here we describe another family case which is the first related to aneuploidy.
The patient is a 16 month old boy with Down syndrome, he is the second product of young non consanguineous parents, there is family history of a paternal cousin who was a malformed stillbirth. The G banding karyotype was 47, XY+21 and FISH with centromeric 13/21 probe showed 6 signals instead of 5. Cytogenetics analysis with G and C bands and dual FISH with 13/21 and 14/22 probes evidenced that the extra signal was in a chromosome 22, which was inherited from the father because he revealed 5 marks with the 13/21 probe and 4 with the 14/22.
This rearrangement could be explained by an inherited polymorphism, cross-hybridization or a translocation between the centromeric region of chromosomes 21 and 22. In this case, the father could be a new mutation or the product of an adyacent 1 segregation inherited from an ancestor, without an abnormal phenotype but with the risk of nondisjunction during the pairing at meiosis. In order to prove this hypothesis for genetic counseling, it is necessary to establish the parental origin of the patient's extra chromosome 21, determine the frequency of nondisjunction in the father's semen by FISH analysis and complete cytogenetic studies in other paternal relatives.
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Del Castillo, V., Ramos, S., Molina, B. et al. A possible centromeric 21/22 translocation as an alternative cause of nondisjunction in trisomy 21. Genet Med 1, 67 (1999). https://doi.org/10.1097/00125817-199901000-00103
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DOI: https://doi.org/10.1097/00125817-199901000-00103