Background: Mosaicism of a normal cell population and an unbalanced autosomal chromosome rearrangement is rarely seen. If the abnormal cell line contributes to a minor part of soma, the phenotype is expected to be normal.
Case report: We report a 29-year-old woman who had balance chromosomal translocation of 46,XX,t(5;21) with a two-year-old affected girl, characterized by mental retardation, dystrophia, hearing impartment, and dysphagia.
Methods and results: Cytogenetic investigation revealed a low mosaic unbalanced translocation of 46,XX,t(5;21)/ 46,XX, which was confirmed by fluorescence in situ hybridization analysis. Studying 200 metaphases and interphases of peripheral blood sample revealed 70% partial monosomy of 21q22 and partial trisomy of 5q(35.3) and 30% of normal pattern.
Conclusion: In rare cases such as this study, parents with balanced translocation with no phenotypes may lead to a mosaic unbalanced translocation with abnormal phenotypes in offspring, which underscores the need for prenatal karyotyping and genetics counseling.
Keywords: Partial monosomy 21q; Translocation t(5,21); Unbalanced autosomal chromosome translocation mosaicism.