Partial Trisomy of Chromosome 9 Inherited by Adjacent Segregation 2 of a Balanced Reciprocal Translocation in the Mother: Rare Mechanism in Chromosomal Abnormalities.

Background: Reciprocal translocations are one of the most common structural chromosomal rearrangements in humans, they are due to the exchange of chromosome material between two nonhomologous chromosomes.If a person carries a balanced reciprocal translocation, meiosis may produce normal gametes, as well as gametes with either balanced or unbalanced abnormalities, depending on the segregation pattern during gametogenesis. All types of segregations can occur, although with different percentages. The adjacent-2 segregation is the less common one and it often leads to non-viable products in many species. Case presentation: We report a case of an 18 months-old female patient, who was referred to the medical genetics' consultation for psychomotor retardation and facial dysmorphism. A postnatal constitutional karyotype was performed in our laboratory on a peripheral blood sample followed by a parental karyotype. The results revealed an additional derivative chromosome 9 in the patient resulting from an adjacent-2 segregation in the mother. Conclusion: This observation highlights the possible occurrence of rare mechanisms of segregation during meiosis in families with a reciprocal translocation. Investigating these events is crucial for both diagnosis and genetic counseling.